diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1fc429d..75b1ba3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -197,6 +197,13 @@
   takes `--f32`; it had kept printing "f32" after the default changed.
 
 ### Interop
+- **h5py could not open chunked datasets we wrote with a chunk dimension
+  from 65 536 to 16 777 215.** A version-4 layout must store its chunk
+  dimensions in the fewest bytes that hold the largest (3 for 70 000);
+  the writer rounded 3 up to 4, and HDF5 2.0.0 (h5py 3.16) refuses that
+  ("stored chunk dimension encoding length does not match value calculated
+  from chunk dimensions"). Newer libhdf5 and clawhdf5 read those files; new
+  files use the exact width. Test: `we_write_chunk_dimensions_in_the_fewest_bytes`.
 - **Conformance sweep in the repo** (`conformance/`, report in
   `CONFORMANCE.md`). `conformance/run.sh` fetches eight public HDF5 corpora
   pinned by commit (libhdf5's test files, the HDF Group's CVE reproducers,
@@ -296,6 +303,84 @@
 - CI keeps zlib-ng building and tested; the arm64 job no longer needs cmake.
 
 ### Correctness
+- **Corrupt files libhdf5 refuses are now refused instead of read.** On the
+  HDF Group's CVE reproducers, 18 objects that libhdf5 (HDF5 2.0, through
+  h5py) refuses to open were read by clawhdf5, some as wrong data (a chunk
+  dimension of 0 read as all fill values; chunks read at offsets off the
+  chunk grid). The
+  parser now makes libhdf5's checks, with libhdf5's error text:
+  - object headers (`FormatError::InvalidObjectHeader`): every message of a
+    v1 chunk is read and more than the prefix's count is refused (the rest
+    used to be dropped); v1 message sizes must be multiples of 8 and a v1
+    chunk cannot end in a gap; a message running past its chunk is an error
+    (it used to end the chunk quietly); contradictory message flags; a
+    message of a class that cannot be shared flagged shareable; a
+    reference-count message in a v1 header; malformed continuation,
+    reference-count and modification-time messages; unknown v2 header
+    flags.
+  - datatypes (`FormatError::InvalidDatatype`): size 0; integer bits outside
+    the type; float exponent/mantissa outside the type, empty or
+    overlapping; a compound with no members, a member outside the compound,
+    a duplicate name or overlapping members; an enum whose size differs from
+    its base type's or with an empty name; array rank over 32 or a zero
+    dimension; an opaque tag length that is not a multiple of 8; in a
+    version-1 (unchecksummed) header, a numeric type that leaves more than
+    half its bits unused (`Datatype::parse_in_header`,
+    `Datatype::check_unused_bits`). A v1/v2 float's class bit 6 was read as
+    VAX byte order; libhdf5 ignores it before version 3, and so does this.
+    The overlap check measures each earlier member by its stored size, as
+    libhdf5 does, so a variable-length member (4 + offset size + 4 bytes)
+    in a file with 4-byte offsets does not overlap the member after it.
+  - chunked layouts (`FormatError::InvalidChunkDimensions`): a zero chunk
+    dimension, a chunk rank that does not match the dataspace, a chunk of
+    4 GiB or more indexed by a v1 B-tree (layout version 3 or earlier;
+    0x80000000-sized chunks hung the reader — layout versions 4 and 5 allow
+    larger chunks, and HDF5 2.0 writes them), an element size in the
+    layout that differs from the datatype's stored size (the chunks were
+    laid out with the wrong element size), and v1 B-tree
+    chunk keys whose offsets are not multiples of the chunk dimensions,
+    including the keys that only bound a node
+    (`chunked_read::collect_chunk_info_checked`).
+  - truncated files (`FormatError::TruncatedFile`, `Superblock::data_end`):
+    a file shorter than the end of file its superblock records is refused
+    ("truncated file"), and nothing past that end is read. Every reader
+    does this: `File`, `LazyFile` and `MmapFile`, and in `clawhdf5-io`
+    `NativeVol` (at `open`, and on read for `from_bytes`),
+    `AsyncHDF5File` and `MpiVol` (the MPI path is not built in CI: it
+    needs an MPI installation).
+  - the writer: `FileWriter::finish()` / `FileBuilder::finish()` refuse a
+    datatype the reader would refuse (`FormatError::SerializationError`,
+    "datatype cannot be written: ..."), such as a compound with a repeated
+    field name or no fields, or an enum member with an empty name
+    (`CompoundTypeBuilder` and `EnumTypeBuilder` build them without
+    complaint). These were never valid HDF5 — h5py refuses them — and
+    clawhdf5 wrote them, which made files it could not read back.
+
+  Checks newer libhdf5 releases make but HDF5 2.0 does not (bit-field
+  offsets, the variable-length kind, array sizes) are left out, so files
+  h5py opens still open. Two libhdf5 checks are skipped on purpose because
+  clawhdf5 up to v2.7.0 wrote files that fail them without being wrong:
+  the sign bit of every float at position 63, and a size-0 string type for
+  an empty-string attribute (new fixtures written by v2.7.0 guard this).
+  Conformance: 569 -> 571 ok (h5stat_err_refcount.h5,
+  h5clear_fsm_persist_less.h5), and 17 of the 18 CVE objects now fail as in
+  libhdf5 (see `docs/known-issues.md` for the one left), as do 10 files
+  h5py refuses as truncated. Tests:
+  `header_validation_interop.rs` (h5py writes, the test damages a copy, both
+  libraries must refuse it), `legacy_writer_files.rs`, and unit tests next
+  to each check. **Breaking (format crate):** `FormatError` gained
+  `InvalidObjectHeader`, `InvalidDatatype`, `InvalidChunkDimensions` and
+  `TruncatedFile`; an exhaustive `match` on it needs the new arms.
+- **Chunked datasets whose chunk dimensions take 3, 5, 6 or 7 bytes did not
+  open.** A version-4 layout (`libver="latest"`) stores each chunk dimension
+  in the fewest bytes that hold the largest one, so a chunk dimension from
+  65 536 to 16 777 215 (e.g. h5py `chunks=(70000,)`) takes 3 bytes; only 1, 2,
+  4 and 8 were read, and the rest failed with `UnexpectedEof`. Widths 1-8 are
+  read now, and 0 or more than 8 is refused as libhdf5 refuses it. A width
+  larger than needed is accepted: HDF5 2.0.0 refuses one ("stored chunk
+  dimension encoding length does not match"), but libhdf5 since
+  HDFGroup/hdf5@e124c36 (2026-06-05) reads it, and clawhdf5 itself wrote such
+  layouts.
 - `clawhdf5-format` VDS: variable-length and reference data from a source in
   another file is refused. Those elements are global-heap IDs and object
   addresses in the source file; copied into the virtual dataset they would
diff --git a/conformance/probe/src/main.rs b/conformance/probe/src/main.rs
index 1298cf0..87c660d 100644
--- a/conformance/probe/src/main.rs
+++ b/conformance/probe/src/main.rs
@@ -309,11 +309,19 @@ impl<'a> Ctx<'a> {
         }
     }
 
+    fn read_named_datatype(&self, h: &ObjectHeader) -> Result<(), String> {
+        let dtb = self
+            .payload(h, MessageType::Datatype)?
+            .ok_or("MissingMessage(Datatype)")?;
+        Datatype::parse_in_header(&dtb, h.version).map_err(e)?;
+        Ok(())
+    }
+
     fn read_dataset(&self, h: &ObjectHeader, rec: &mut Map<String, Value>) -> Result<(), String> {
         let dtb = self
             .payload(h, MessageType::Datatype)?
             .ok_or("MissingMessage(Datatype)")?;
-        let (dt, _) = Datatype::parse(&dtb).map_err(e)?;
+        let (dt, _) = Datatype::parse_in_header(&dtb, h.version).map_err(e)?;
         rec.insert("dtype".into(), Value::String(dtype_str(&dt)));
         let dsb = self
             .payload(h, MessageType::Dataspace)?
@@ -716,6 +724,17 @@ fn main() {
             return;
         }
     };
+    // libhdf5 refuses a truncated file and reads nothing past the recorded
+    // end of file.
+    let base = (data.len() - hdf5.len()) as u64;
+    let hdf5 = match sb.data_end(base, data.len() as u64) {
+        Ok(end) => &hdf5[..end as usize],
+        Err(err) => {
+            top.insert("open_error".into(), Value::String(e(err)));
+            println!("{}", Value::Object(top));
+            return;
+        }
+    };
     top.insert("superblock_version".into(), json!(sb.version));
     let ctx = Ctx {
         data: hdf5,
@@ -778,6 +797,13 @@ fn main() {
         {
             rec.insert("error".into(), Value::String(msg));
         }
+        // Opening a committed datatype decodes it (h5py's `f[name]` fails on
+        // one libhdf5 cannot decode), so decode it here too.
+        if kind == "datatype"
+            && let Err(msg) = guarded(|| ctx.read_named_datatype(&h))
+        {
+            rec.insert("error".into(), Value::String(msg));
+        }
         if kind != "datatype" {
             match guarded(|| ctx.attrs(&h)) {
                 Ok(m) => {
diff --git a/crates/clawhdf5-format/src/attribute.rs b/crates/clawhdf5-format/src/attribute.rs
index c844c3c..11bde3e 100644
--- a/crates/clawhdf5-format/src/attribute.rs
+++ b/crates/clawhdf5-format/src/attribute.rs
@@ -362,6 +362,18 @@ fn extract_name(bytes: &[u8]) -> String {
     String::from_utf8_lossy(&bytes[..end]).into_owned()
 }
 
+/// An attribute's datatype gets libhdf5's extra check for a header without
+/// a checksum (see [`Datatype::check_unused_bits`]).
+fn check_in_header(
+    attr: AttributeMessage,
+    header: &ObjectHeader,
+) -> Result<AttributeMessage, FormatError> {
+    if header.version == 1 {
+        attr.datatype.check_unused_bits()?;
+    }
+    Ok(attr)
+}
+
 /// Extract all attribute messages from an object header.
 pub fn extract_attributes(
     header: &ObjectHeader,
@@ -371,7 +383,7 @@ pub fn extract_attributes(
     for msg in &header.messages {
         if msg.msg_type == MessageType::Attribute {
             let attr = AttributeMessage::parse(&msg.data, length_size)?;
-            attrs.push(attr);
+            attrs.push(check_in_header(attr, header)?);
         }
     }
     Ok(attrs)
@@ -465,6 +477,7 @@ fn extract_attributes_with(
             } else {
                 AttributeMessage::parse_in_file(&msg.data, file_data, offset_size, length_size)
             };
+            let attr = attr.and_then(|a| check_in_header(a, header));
             match attr {
                 Ok(attr) => attrs.push(attr),
                 Err(e) => on_error(e)?,
@@ -573,7 +586,8 @@ mod tests {
 
     /// Build an f64 LE datatype message.
     fn build_f64_dt() -> Vec<u8> {
-        let mut buf = build_dt_header(1, 1, [0x00, 0x00, 0x02], 8);
+        // Sign bit 63 (bits 8-15 of the class bits).
+        let mut buf = build_dt_header(1, 1, [0x20, 63, 0x00], 8);
         let mut props = [0u8; 12];
         props[2..4].copy_from_slice(&64u16.to_le_bytes()); // bit_precision
         props[4] = 52; // exp_location
diff --git a/crates/clawhdf5-format/src/chunked_read.rs b/crates/clawhdf5-format/src/chunked_read.rs
index 3a3dc05..afff80d 100644
--- a/crates/clawhdf5-format/src/chunked_read.rs
+++ b/crates/clawhdf5-format/src/chunked_read.rs
@@ -148,6 +148,96 @@ pub(crate) fn checked_byte_len(elements: u64, elem_size: usize) -> Result<usize,
         })
 }
 
+/// The spatial chunk dimensions of a chunked layout (`chunk_dimensions` is
+/// the layout message's list: one per dataspace dimension, then the element
+/// size), after the checks libhdf5 makes when it opens a chunked dataset
+/// (`H5D__chunk_init` / `H5D__chunk_set_sizes`): the chunk rank must match
+/// the dataspace's, no chunk dimension may be 0, and a chunk indexed by a
+/// version-1 B-tree (`layout_version` below 4) may not be 4 GiB or more (the
+/// B-tree records chunk sizes in 32 bits; libhdf5: "chunk size must be < 4GB
+/// with v1 b-tree index"). The other chunk indexes allow larger chunks:
+/// HDF5 2.0 writes them with layout version 5. A zero chunk dimension used
+/// to read as all fill values, and a huge one to hang the reader.
+pub(crate) fn chunk_geometry(
+    chunk_dimensions: &[u32],
+    layout_version: u8,
+    dataspace: &Dataspace,
+    elem_size: usize,
+) -> Result<(usize, Vec<usize>), FormatError> {
+    let rank = chunk_dimensions.len().checked_sub(1).ok_or_else(|| {
+        FormatError::InvalidChunkDimensions("chunked layout has no dimensions".into())
+    })?;
+    if dataspace.dimensions.len() != rank {
+        return Err(FormatError::InvalidChunkDimensions(format!(
+            "dimensionality of chunks doesn't match the dataspace (chunk rank {rank}, \
+             dataspace rank {})",
+            dataspace.dimensions.len()
+        )));
+    }
+    let spatial = &chunk_dimensions[..rank];
+    if let Some(d) = spatial.iter().position(|&c| c == 0) {
+        return Err(FormatError::InvalidChunkDimensions(format!(
+            "chunk size must be > 0, dim = {d}"
+        )));
+    }
+    let bytes = spatial
+        .iter()
+        .fold(elem_size as u128, |acc, &c| acc * u128::from(c));
+    if layout_version < 4 && bytes > u128::from(u32::MAX) {
+        return Err(FormatError::InvalidChunkDimensions(format!(
+            "chunk size must be < 4GB with v1 b-tree index (chunk {spatial:?} of {elem_size}-byte elements)"
+        )));
+    }
+    Ok((rank, spatial.iter().map(|&c| c as usize).collect()))
+}
+
+/// The size of one element of `dt` as stored in the file: a
+/// variable-length element is its length (4), a global heap address
+/// (`offset_size`) and an index (4), not the 16 of [`Datatype::type_size`].
+fn stored_element_size(dt: &Datatype, offset_size: u8) -> u64 {
+    match dt {
+        Datatype::VariableLength { .. } => 8 + u64::from(offset_size),
+        Datatype::Array {
+            base_type,
+            dimensions,
+        } => dimensions
+            .iter()
+            .fold(stored_element_size(base_type, offset_size), |acc, &d| {
+                acc.saturating_mul(u64::from(d))
+            }),
+        _ => u64::from(dt.type_size()),
+    }
+}
+
+/// A chunked layout records the element size as its last dimension, and
+/// libhdf5 refuses a dataset whose datatype has another size
+/// (`H5D__chunk_set_sizes`: "stored datatype size in chunk layout does not
+/// match datatype description"). Reading it anyway laid the chunks out with
+/// the wrong element size.
+pub(crate) fn check_chunk_element_size(
+    layout: &DataLayout,
+    datatype: &Datatype,
+    offset_size: u8,
+) -> Result<(), FormatError> {
+    let DataLayout::Chunked {
+        chunk_dimensions, ..
+    } = layout
+    else {
+        return Ok(());
+    };
+    let Some(&stored) = chunk_dimensions.last() else {
+        return Ok(());
+    };
+    let expected = stored_element_size(datatype, offset_size);
+    if u64::from(stored) != expected {
+        return Err(FormatError::InvalidChunkDimensions(format!(
+            "stored datatype size in chunk layout does not match datatype description \
+             (layout {stored} bytes, datatype {expected})"
+        )));
+    }
+    Ok(())
+}
+
 /// Product of chunk dimensions times the element size, overflow-checked.
 pub(crate) fn checked_chunk_byte_len(
     chunk_dims: &[usize],
@@ -222,7 +312,76 @@ pub fn collect_chunk_info(
     offset_size: u8,
     length_size: u8,
 ) -> Result<Vec<ChunkInfo>, FormatError> {
-    collect_chunk_info_inner(file_data, btree_address, ndims, offset_size, length_size, 0)
+    collect_chunk_info_inner(
+        file_data,
+        btree_address,
+        ndims,
+        None,
+        offset_size,
+        length_size,
+        0,
+    )
+}
+
+/// [`collect_chunk_info`] for a layout with these `chunk_dimensions` (the
+/// layout message's list, element size last), checking every key of the
+/// B-tree as libhdf5 does (`H5D__btree_decode_key`): each coordinate offset
+/// must be a multiple of its chunk dimension. That includes the keys that
+/// only bound a node (internal-node keys and each node's final key), which
+/// is where a corrupt chunk dimension shows when the chunks themselves all
+/// start at offset 0 in that dimension (`cve-2018-11205`). A key that fails
+/// ("bad coordinate offset") means a corrupt index or chunk dimension; the
+/// chunks were read at the wrong place, or the dataset read as fill values.
+pub fn collect_chunk_info_checked(
+    file_data: &[u8],
+    btree_address: u64,
+    chunk_dimensions: &[u32],
+    offset_size: u8,
+    length_size: u8,
+) -> Result<Vec<ChunkInfo>, FormatError> {
+    collect_chunk_info_inner(
+        file_data,
+        btree_address,
+        chunk_dimensions.len(),
+        Some(chunk_dimensions),
+        offset_size,
+        length_size,
+        0,
+    )
+}
+
+/// Check one v1 B-tree chunk key's offsets (see
+/// [`collect_chunk_info_checked`]).
+fn check_key_offsets(offsets: &[u64], chunk_dimensions: &[u32]) -> Result<(), FormatError> {
+    for (&offset, &dim) in offsets.iter().zip(chunk_dimensions) {
+        if dim == 0 || offset % u64::from(dim) != 0 {
+            return Err(FormatError::ChunkedReadError(format!(
+                "bad coordinate offset {offsets:?} for chunk dimensions {chunk_dimensions:?}"
+            )));
+        }
+    }
+    Ok(())
+}
+
+/// Read the `ndims` 8-byte offsets of the chunk key at `pos` (after its
+/// chunk size and filter mask) and check them when `chunk_dimensions` is
+/// given.
+fn read_key_offsets(
+    file_data: &[u8],
+    pos: usize,
+    ndims: usize,
+    chunk_dimensions: Option<&[u32]>,
+) -> Result<Vec<u64>, FormatError> {
+    let mut offsets = Vec::with_capacity(ndims);
+    let mut kp = pos + 8;
+    for _ in 0..ndims {
+        offsets.push(read_offset(file_data, kp, CHUNK_KEY_OFFSET_SIZE)?);
+        kp += CHUNK_KEY_OFFSET_SIZE as usize;
+    }
+    if let Some(dims) = chunk_dimensions {
+        check_key_offsets(&offsets, dims)?;
+    }
+    Ok(offsets)
 }
 
 /// Width of each chunk offset in a v1 chunk B-tree key, independent of the
@@ -237,6 +396,7 @@ fn collect_chunk_info_inner(
     file_data: &[u8],
     btree_address: u64,
     ndims: usize,
+    chunk_dimensions: Option<&[u32]>,
     offset_size: u8,
     _length_size: u8,
     depth: usize,
@@ -296,12 +456,7 @@ fn collect_chunk_info_inner(
                 file_data[pos + 6],
                 file_data[pos + 7],
             ]);
-            let mut offsets = Vec::with_capacity(ndims);
-            let mut kp = pos + 8;
-            for _ in 0..ndims {
-                offsets.push(read_offset(file_data, kp, CHUNK_KEY_OFFSET_SIZE)?);
-                kp += CHUNK_KEY_OFFSET_SIZE as usize;
-            }
+            let offsets = read_key_offsets(file_data, pos, ndims, chunk_dimensions)?;
             pos += key_size;
 
             // Parse child address
@@ -315,7 +470,8 @@ fn collect_chunk_info_inner(
                 address,
             });
         }
-        // Skip final key
+        // The final key only bounds the node; libhdf5 still checks it.
+        read_key_offsets(file_data, pos, ndims, chunk_dimensions)?;
         Ok(chunks)
     } else {
         // Internal node: recurse into children
@@ -324,11 +480,13 @@ fn collect_chunk_info_inner(
 
         let mut child_addrs = Vec::with_capacity(entries_used);
         for _ in 0..entries_used {
-            pos += key_size; // skip key
+            read_key_offsets(file_data, pos, ndims, chunk_dimensions)?;
+            pos += key_size;
             let child_addr = read_offset(file_data, pos, offset_size)?;
             child_addrs.push(child_addr);
             pos += os;
         }
+        read_key_offsets(file_data, pos, ndims, chunk_dimensions)?;
 
         let mut all_chunks = Vec::new();
         for child_addr in child_addrs {
@@ -336,6 +494,7 @@ fn collect_chunk_info_inner(
                 file_data,
                 child_addr,
                 ndims,
+                chunk_dimensions,
                 offset_size,
                 _length_size,
                 depth + 1,
@@ -549,30 +708,13 @@ pub fn list_chunks(
         .ok_or_else(|| FormatError::ChunkedReadError("no address for chunked layout".into()))?;
 
     // Both v3 and v4 include element size as last dim (rank+1)
-    let ndims = chunk_dimensions.len();
-    let rank = ndims
-        .checked_sub(1)
-        .ok_or_else(|| FormatError::ChunkedReadError("chunked layout has no dimensions".into()))?;
-    let chunk_dims: Vec<usize> = chunk_dimensions[..rank]
-        .iter()
-        .map(|&d| d as usize)
-        .collect();
-
+    let (rank, chunk_dims) = chunk_geometry(chunk_dimensions, version, dataspace, elem_size)?;
     let ds_dims: Vec<usize> = dataspace.dimensions.iter().map(|&d| d as usize).collect();
-    if ds_dims.len() != rank {
-        return Err(FormatError::ChunkedReadError(format!(
-            "rank mismatch: dataspace has {} dims, layout has {} chunk dims (rank={})",
-            ds_dims.len(),
-            chunk_dimensions.len(),
-            rank
-        )));
-    }
 
     // Collect chunks based on version and index type
     let mut chunks = match (version, chunk_index_type) {
         (3, _) => {
-            let ndims = chunk_dimensions.len(); // rank+1
-            collect_chunk_info(file_data, addr, ndims, offset_size, length_size)?
+            collect_chunk_info_checked(file_data, addr, chunk_dimensions, offset_size, length_size)?
         }
         (4, Some(1)) => {
             // Single chunk — one chunk covering the entire dataset
@@ -679,6 +821,7 @@ pub fn read_chunked_data(
     offset_size: u8,
     length_size: u8,
 ) -> Result<Vec<u8>, FormatError> {
+    check_chunk_element_size(layout, datatype, offset_size)?;
     let elem_size = datatype.type_size() as usize;
     let (chunks, chunk_dims) = list_chunks(
         file_data,
@@ -802,12 +945,13 @@ pub fn read_chunked_data_cached(
     length_size: u8,
     cache: &ChunkCache,
 ) -> Result<Vec<u8>, FormatError> {
-    let (chunk_dimensions, addr_opt) = match layout {
+    let (chunk_dimensions, version, addr_opt) = match layout {
         DataLayout::Chunked {
             chunk_dimensions,
+            version,
             btree_address,
             ..
-        } => (chunk_dimensions, *btree_address),
+        } => (chunk_dimensions, *version, *btree_address),
         _ => {
             return Err(FormatError::ChunkedReadError(
                 "expected chunked layout".into(),
@@ -818,25 +962,10 @@ pub fn read_chunked_data_cached(
     let addr = addr_opt
         .ok_or_else(|| FormatError::ChunkedReadError("no address for chunked layout".into()))?;
 
+    check_chunk_element_size(layout, datatype, offset_size)?;
     let elem_size = datatype.type_size() as usize;
-    let ndims = chunk_dimensions.len();
-    let rank = ndims
-        .checked_sub(1)
-        .ok_or_else(|| FormatError::ChunkedReadError("chunked layout has no dimensions".into()))?;
-    let chunk_dims: Vec<usize> = chunk_dimensions[..rank]
-        .iter()
-        .map(|&d| d as usize)
-        .collect();
-
+    let (rank, chunk_dims) = chunk_geometry(chunk_dimensions, version, dataspace, elem_size)?;
     let ds_dims: Vec<usize> = dataspace.dimensions.iter().map(|&d| d as usize).collect();
-    if ds_dims.len() != rank {
-        return Err(FormatError::ChunkedReadError(format!(
-            "rank mismatch: dataspace has {} dims, layout has {} chunk dims (rank={})",
-            ds_dims.len(),
-            chunk_dimensions.len(),
-            rank
-        )));
-    }
 
     // The per-file cache is shared across datasets (and threads); every
     // lookup is keyed by this dataset's chunk-index address, so another
@@ -1123,12 +1252,13 @@ pub fn read_chunked_data_sweep(
     cache: &ChunkCache,
     sweep: &mut SweepContext,
 ) -> Result<Vec<u8>, FormatError> {
-    let (chunk_dimensions, addr_opt) = match layout {
+    let (chunk_dimensions, version, addr_opt) = match layout {
         DataLayout::Chunked {
             chunk_dimensions,
+            version,
             btree_address,
             ..
-        } => (chunk_dimensions, *btree_address),
+        } => (chunk_dimensions, *version, *btree_address),
         _ => {
             return Err(FormatError::ChunkedReadError(
                 "expected chunked layout".into(),
@@ -1139,25 +1269,10 @@ pub fn read_chunked_data_sweep(
     let addr = addr_opt
         .ok_or_else(|| FormatError::ChunkedReadError("no address for chunked layout".into()))?;
 
+    check_chunk_element_size(layout, datatype, offset_size)?;
     let elem_size = datatype.type_size() as usize;
-    let ndims = chunk_dimensions.len();
-    let rank = ndims
-        .checked_sub(1)
-        .ok_or_else(|| FormatError::ChunkedReadError("chunked layout has no dimensions".into()))?;
-    let chunk_dims: Vec<usize> = chunk_dimensions[..rank]
-        .iter()
-        .map(|&d| d as usize)
-        .collect();
-
+    let (rank, chunk_dims) = chunk_geometry(chunk_dimensions, version, dataspace, elem_size)?;
     let ds_dims: Vec<usize> = dataspace.dimensions.iter().map(|&d| d as usize).collect();
-    if ds_dims.len() != rank {
-        return Err(FormatError::ChunkedReadError(format!(
-            "rank mismatch: dataspace has {} dims, layout has {} chunk dims (rank={})",
-            ds_dims.len(),
-            chunk_dimensions.len(),
-            rank
-        )));
-    }
 
     // The per-file cache is shared across datasets (and threads); every
     // lookup is keyed by this dataset's chunk-index address, so another
@@ -1275,12 +1390,13 @@ pub fn read_chunked_data_indexed(
     length_size: u8,
     cache: &ChunkCache,
 ) -> Result<Vec<u8>, FormatError> {
-    let (chunk_dimensions, addr_opt) = match layout {
+    let (chunk_dimensions, version, addr_opt) = match layout {
         DataLayout::Chunked {
             chunk_dimensions,
+            version,
             btree_address,
             ..
-        } => (chunk_dimensions, *btree_address),
+        } => (chunk_dimensions, *version, *btree_address),
         _ => {
             return Err(FormatError::ChunkedReadError(
                 "expected chunked layout".into(),
@@ -1291,25 +1407,10 @@ pub fn read_chunked_data_indexed(
     let addr = addr_opt
         .ok_or_else(|| FormatError::ChunkedReadError("no address for chunked layout".into()))?;
 
+    check_chunk_element_size(layout, datatype, offset_size)?;
     let elem_size = datatype.type_size() as usize;
-    let ndims = chunk_dimensions.len();
-    let rank = ndims
-        .checked_sub(1)
-        .ok_or_else(|| FormatError::ChunkedReadError("chunked layout has no dimensions".into()))?;
-    let chunk_dims: Vec<usize> = chunk_dimensions[..rank]
-        .iter()
-        .map(|&d| d as usize)
-        .collect();
-
+    let (rank, chunk_dims) = chunk_geometry(chunk_dimensions, version, dataspace, elem_size)?;
     let ds_dims: Vec<usize> = dataspace.dimensions.iter().map(|&d| d as usize).collect();
-    if ds_dims.len() != rank {
-        return Err(FormatError::ChunkedReadError(format!(
-            "rank mismatch: dataspace has {} dims, layout has {} chunk dims (rank={})",
-            ds_dims.len(),
-            chunk_dimensions.len(),
-            rank
-        )));
-    }
 
     // Chunk index and assembly plan for this dataset, built on first access
     // and kept per dataset (keyed by chunk-index address) in the shared cache.
@@ -1620,11 +1721,12 @@ mod tests {
             write_offset(&mut buf, chunk.address, offset_size);
         }
 
-        // Final key (dummy)
+        // Final key (its offsets must be on the chunk grid, as libhdf5
+        // checks; 0 always is)
         buf.extend_from_slice(&0u32.to_le_bytes()); // chunk_size
         buf.extend_from_slice(&0u32.to_le_bytes()); // filter_mask
         for _ in 0..ndims {
-            write_offset(&mut buf, u64::MAX, 8);
+            write_offset(&mut buf, 0, 8);
         }
 
         buf
@@ -1632,6 +1734,48 @@ mod tests {
 
     // --- ChunkInfo collection tests ---
 
+    #[test]
+    fn checked_collection_refuses_keys_off_the_chunk_grid() {
+        let chunk = |offsets: Vec<u64>, address| ChunkInfo {
+            chunk_size: 80,
+            filter_mask: 0,
+            offsets,
+            address,
+        };
+        let good =
+            build_chunk_btree_leaf(&[chunk(vec![0, 0], 0x100), chunk(vec![10, 0], 0x200)], 2, 8);
+        assert_eq!(
+            collect_chunk_info_checked(&good, 0, &[10, 8], 8, 8)
+                .unwrap()
+                .len(),
+            2
+        );
+        // A chunk key off the grid.
+        let bad =
+            build_chunk_btree_leaf(&[chunk(vec![0, 0], 0x100), chunk(vec![7, 0], 0x200)], 2, 8);
+        assert!(collect_chunk_info(&bad, 0, 2, 8, 8).is_ok());
+        assert!(matches!(
+            collect_chunk_info_checked(&bad, 0, &[10, 8], 8, 8),
+            Err(FormatError::ChunkedReadError(m)) if m.starts_with("bad coordinate offset")
+        ));
+        // cve-2018-11205: the chunks all start at 0 in dimension 1, and only
+        // the node's final key shows the chunk dimension is wrong.
+        let mut two_d = build_chunk_btree_leaf(
+            &[chunk(vec![0, 0, 0], 0x100), chunk(vec![10, 0, 0], 0x200)],
+            3,
+            8,
+        );
+        // Final key: (20, 20, 0), the end of a 20 x 20 dataset.
+        let final_key = two_d.len() - 24;
+        two_d[final_key..final_key + 8].copy_from_slice(&20u64.to_le_bytes());
+        two_d[final_key + 8..final_key + 16].copy_from_slice(&20u64.to_le_bytes());
+        assert!(collect_chunk_info_checked(&two_d, 0, &[10, 20, 4], 8, 8).is_ok());
+        assert!(matches!(
+            collect_chunk_info_checked(&two_d, 0, &[10, 32788, 4], 8, 8),
+            Err(FormatError::ChunkedReadError(m)) if m.starts_with("bad coordinate offset [20, 20, 0]")
+        ));
+    }
+
     #[test]
     fn collect_two_chunks_from_leaf() {
         let ndims = 2; // rank+1 for 1D dataset
@@ -1750,6 +1894,51 @@ mod tests {
     use crate::dataspace::{Dataspace, DataspaceType};
     use crate::datatype::{Datatype, DatatypeByteOrder};
 
+    #[test]
+    fn chunk_geometry_matches_libhdf5_open_checks() {
+        let space = |dims: &[u64]| Dataspace {
+            space_type: DataspaceType::Simple,
+            rank: dims.len() as u8,
+            dimensions: dims.to_vec(),
+            max_dimensions: None,
+        };
+        for v in [3, 4] {
+            assert_eq!(
+                chunk_geometry(&[4, 5, 8], v, &space(&[10, 10]), 8).unwrap(),
+                (2, vec![4, 5])
+            );
+            // Rank mismatch.
+            assert!(matches!(
+                chunk_geometry(&[4, 8], v, &space(&[10, 10]), 8),
+                Err(FormatError::InvalidChunkDimensions(m)) if m.contains("doesn't match")
+            ));
+            // Zero dimension (a layout built in memory, bypassing the parser).
+            assert!(matches!(
+                chunk_geometry(&[4, 0, 8], v, &space(&[10, 10]), 8),
+                Err(FormatError::InvalidChunkDimensions(m)) if m.contains("must be > 0")
+            ));
+            assert!(chunk_geometry(&[0xFFFF_FFFF, 1], v, &space(&[10]), 1).is_ok());
+        }
+        // With a v1 B-tree index (layout version 3) the largest chunk is
+        // 4 GiB - 1 bytes: 0x80000000 x 4-byte elements (8 GiB) is refused.
+        // These dims used to hang the reader.
+        assert!(matches!(
+            chunk_geometry(&[0x8000_0000, 4], 3, &space(&[10]), 4),
+            Err(FormatError::InvalidChunkDimensions(m)) if m.contains("4GB with v1 b-tree")
+        ));
+        assert!(matches!(
+            chunk_geometry(&[0xFFFF_FFFF, 0xFFFF_FFFF, 1], 3, &space(&[10, 10]), 1),
+            Err(FormatError::InvalidChunkDimensions(m)) if m.contains("4GB with v1 b-tree")
+        ));
+        // The other chunk indexes (layout version 4, and 5, which is read as
+        // 4) allow chunks of 4 GiB and more; HDF5 2.0 writes them.
+        assert_eq!(
+            chunk_geometry(&[0x2000_0001, 8], 4, &space(&[10]), 8).unwrap(),
+            (1, vec![0x2000_0001])
+        );
+        assert!(chunk_geometry(&[0xFFFF_FFFF, 0xFFFF_FFFF, 1], 4, &space(&[10, 10]), 1).is_ok());
+    }
+
     fn make_f64_type() -> Datatype {
         Datatype::FloatingPoint {
             size: 8,
@@ -1863,8 +2052,8 @@ mod tests {
         let file_data = vec![0u8; 64];
         let result = read_chunked_data(&file_data, &layout, &dataspace, &datatype, None, 8, 8);
         assert!(
-            matches!(result, Err(FormatError::ChunkedReadError(_))),
-            "expected a clean ChunkedReadError, got {result:?}"
+            matches!(result, Err(FormatError::InvalidChunkDimensions(_))),
+            "expected a clean InvalidChunkDimensions, got {result:?}"
         );
     }
 
diff --git a/crates/clawhdf5-format/src/chunked_write.rs b/crates/clawhdf5-format/src/chunked_write.rs
index ff5081e..dcd0e43 100644
--- a/crates/clawhdf5-format/src/chunked_write.rs
+++ b/crates/clawhdf5-format/src/chunked_write.rs
@@ -383,39 +383,7 @@ fn serialize_v4_single_chunk(
     let ndims = chunk_dims.len() as u8 + 1;
     buf.push(ndims);
 
-    // dim_size_encoded_length: how many bytes per dimension
-    // We need to figure out the minimum encoding width
-    let max_dim = chunk_dims
-        .iter()
-        .map(|&d| d as u64)
-        .chain(core::iter::once(element_size as u64))
-        .max()
-        .unwrap_or(1);
-    let dim_encoded_len: u8 = if max_dim <= 0xFF {
-        1
-    } else if max_dim <= 0xFFFF {
-        2
-    } else {
-        4
-    };
-    buf.push(dim_encoded_len);
-
-    // dimension sizes (chunk dims + element size)
-    for &d in chunk_dims {
-        match dim_encoded_len {
-            1 => buf.push(d as u8),
-            2 => buf.extend_from_slice(&(d as u16).to_le_bytes()),
-            4 => buf.extend_from_slice(&d.to_le_bytes()),
-            _ => {}
-        }
-    }
-    // Element size dimension
-    match dim_encoded_len {
-        1 => buf.push(element_size as u8),
-        2 => buf.extend_from_slice(&(element_size as u16).to_le_bytes()),
-        4 => buf.extend_from_slice(&element_size.to_le_bytes()),
-        _ => {}
-    }
+    push_v4_chunk_dims(&mut buf, chunk_dims, element_size);
 
     // chunk index type = 1 (single chunk)
     buf.push(1);
@@ -465,6 +433,25 @@ fn serialize_v4_fixed_array(
 
 /// The part of a v4 chunked layout message before the chunk index type:
 /// version, class, flags and the chunk dimensions (plus the element size).
+/// Append a v4 layout's dimension width and its dimensions (the chunk
+/// dimensions, then the element size). Each takes the fewest bytes that hold
+/// the largest, as libhdf5 computes it (`H5D__chunk_set_sizes`:
+/// `(log2(dim) + 8) / 8`); HDF5 2.0.0 refuses any other width.
+pub(crate) fn push_v4_chunk_dims(buf: &mut Vec<u8>, chunk_dims: &[u32], element_size: u32) {
+    let max_dim = chunk_dims
+        .iter()
+        .copied()
+        .chain(core::iter::once(element_size))
+        .max()
+        .unwrap_or(1)
+        .max(1);
+    let width = (32 - max_dim.leading_zeros()).div_ceil(8) as usize;
+    buf.push(width as u8);
+    for &d in chunk_dims.iter().chain(core::iter::once(&element_size)) {
+        buf.extend_from_slice(&d.to_le_bytes()[..width]);
+    }
+}
+
 fn layout_v4_chunked_prefix(chunk_dims: &[u32], element_size: u32) -> Vec<u8> {
     let mut buf = Vec::new();
     buf.push(4); // version
@@ -476,35 +463,7 @@ fn layout_v4_chunked_prefix(chunk_dims: &[u32], element_size: u32) -> Vec<u8> {
     let ndims = chunk_dims.len() as u8 + 1;
     buf.push(ndims);
 
-    let max_dim = chunk_dims
-        .iter()
-        .map(|&d| d as u64)
-        .chain(core::iter::once(element_size as u64))
-        .max()
-        .unwrap_or(1);
-    let dim_encoded_len: u8 = if max_dim <= 0xFF {
-        1
-    } else if max_dim <= 0xFFFF {
-        2
-    } else {
-        4
-    };
-    buf.push(dim_encoded_len);
-
-    for &d in chunk_dims {
-        match dim_encoded_len {
-            1 => buf.push(d as u8),
-            2 => buf.extend_from_slice(&(d as u16).to_le_bytes()),
-            4 => buf.extend_from_slice(&d.to_le_bytes()),
-            _ => {}
-        }
-    }
-    match dim_encoded_len {
-        1 => buf.push(element_size as u8),
-        2 => buf.extend_from_slice(&(element_size as u16).to_le_bytes()),
-        4 => buf.extend_from_slice(&element_size.to_le_bytes()),
-        _ => {}
-    }
+    push_v4_chunk_dims(&mut buf, chunk_dims, element_size);
     buf
 }
 
diff --git a/crates/clawhdf5-format/src/data_layout.rs b/crates/clawhdf5-format/src/data_layout.rs
index 60a8243..59a9065 100644
--- a/crates/clawhdf5-format/src/data_layout.rs
+++ b/crates/clawhdf5-format/src/data_layout.rs
@@ -24,6 +24,34 @@ pub struct VdsMapping {
     pub virtual_selection: Vec<u8>,
 }
 
+/// Most dimensions a layout message can list (libhdf5 `H5O_LAYOUT_NDIMS`):
+/// 32 dataspace dimensions plus the element size.
+const MAX_LAYOUT_NDIMS: usize = 33;
+
+/// libhdf5's checks on a chunked layout message's dimensions
+/// (`H5O__layout_decode`): at most [`MAX_LAYOUT_NDIMS`], no dimension 0, and
+/// before version 4 at least one dataspace dimension plus the element size.
+/// A zero chunk dimension used to read the dataset as all fill values.
+fn check_chunk_dims(dims: Vec<u32>, layout_version: u8) -> Result<Vec<u32>, FormatError> {
+    if dims.len() > MAX_LAYOUT_NDIMS {
+        return Err(FormatError::InvalidChunkDimensions(
+            "dimensionality is too large".into(),
+        ));
+    }
+    if layout_version < 4 && dims.len() < 2 {
+        return Err(FormatError::InvalidChunkDimensions(
+            "bad dimensions for chunked storage".into(),
+        ));
+    }
+    if let Some(u) = dims.iter().position(|&d| d == 0) {
+        return Err(FormatError::InvalidChunkDimensions(format!(
+            "bad chunk dimension value when parsing layout message - chunk dimension must be \
+             positive: mesg->u.chunk.dim[{u}] = 0"
+        )));
+    }
+    Ok(dims)
+}
+
 /// Parsed HDF5 data layout message.
 #[derive(Debug, Clone, PartialEq)]
 pub enum DataLayout {
@@ -394,7 +422,7 @@ impl DataLayout {
                 Ok(DataLayout::Contiguous { address, size })
             }
             _ => Ok(DataLayout::Chunked {
-                chunk_dimensions: dims,
+                chunk_dimensions: check_chunk_dims(dims, 2)?,
                 btree_address: address,
                 version: 3,
                 chunk_index_type: None,
@@ -457,7 +485,7 @@ impl DataLayout {
                     p += 4;
                 }
                 Ok(DataLayout::Chunked {
-                    chunk_dimensions,
+                    chunk_dimensions: check_chunk_dims(chunk_dimensions, 3)?,
                     btree_address,
                     version: 3,
                     chunk_index_type: None,
@@ -506,47 +534,40 @@ impl DataLayout {
                 let dimensionality = data[pos + 1] as usize;
                 let dim_size_encoded_length = data[pos + 2] as usize;
                 let mut p = pos + 3;
+                if dimensionality > MAX_LAYOUT_NDIMS {
+                    return Err(FormatError::InvalidChunkDimensions(
+                        "dimensionality is too large".into(),
+                    ));
+                }
 
-                // dimension sizes
+                // Each dimension takes 1 to 8 bytes (libhdf5 writes the
+                // fewest that hold the largest one, so 3, 5, 6 and 7 occur:
+                // a chunk dimension of 70 000 takes 3). libhdf5 refuses 0
+                // and more than 8.
+                if dim_size_encoded_length == 0 || dim_size_encoded_length > 8 {
+                    return Err(FormatError::InvalidChunkDimensions(
+                        "encoded chunk dimension size is too large".into(),
+                    ));
+                }
                 ensure_len(data, p, dimensionality * dim_size_encoded_length)?;
                 let mut chunk_dimensions = Vec::with_capacity(dimensionality);
                 for _ in 0..dimensionality {
-                    let val = match dim_size_encoded_length {
-                        1 => data[p] as u32,
-                        2 => u16::from_le_bytes([data[p], data[p + 1]]) as u32,
-                        4 => u32::from_le_bytes([data[p], data[p + 1], data[p + 2], data[p + 3]]),
-                        8 => {
-                            // V4 chunked encodes dimension sizes as 8 bytes, but
-                            // our ChunkedStorageV4 stores them as u32. We read only
-                            // the low 4 bytes (little-endian). This silently
-                            // truncates dimensions > 4 GiB, which are not expected
-                            // in practice (HDF5 chunk dimensions are always small).
-                            // If the high bytes are non-zero, the file is malformed
-                            // or uses dimensions we cannot represent.
-                            let high = u32::from_le_bytes([
-                                data[p + 4],
-                                data[p + 5],
-                                data[p + 6],
-                                data[p + 7],
-                            ]);
-                            if high != 0 {
-                                return Err(FormatError::UnexpectedEof {
-                                    expected: p + 8,
-                                    available: data.len(),
-                                });
-                            }
-                            u32::from_le_bytes([data[p], data[p + 1], data[p + 2], data[p + 3]])
-                        }
-                        _ => {
-                            return Err(FormatError::UnexpectedEof {
-                                expected: p + dim_size_encoded_length,
-                                available: data.len(),
-                            });
-                        }
-                    };
+                    let val = data[p..p + dim_size_encoded_length]
+                        .iter()
+                        .rev()
+                        .fold(0u64, |acc, &b| (acc << 8) | u64::from(b));
+                    // Chunk dimensions are held as u32; HDF5 2.0 can write
+                    // larger ones (layout version 5), which are refused
+                    // rather than truncated.
+                    let val = u32::try_from(val).map_err(|_| {
+                        FormatError::InvalidChunkDimensions(format!(
+                            "chunk dimension {val} is larger than 2^32 - 1, which is not supported"
+                        ))
+                    })?;
                     chunk_dimensions.push(val);
                     p += dim_size_encoded_length;
                 }
+                let chunk_dimensions = check_chunk_dims(chunk_dimensions, 4)?;
 
                 // chunk index type
                 ensure_len(data, p, 1)?;
@@ -755,6 +776,100 @@ mod tests {
         );
     }
 
+    /// A v3 chunked layout message with these dims (element size last).
+    fn v3_chunked_msg(dims: &[u32]) -> Vec<u8> {
+        let mut buf = vec![3u8, 2, dims.len() as u8];
+        buf.extend_from_slice(&0x1000u64.to_le_bytes());
+        for d in dims {
+            buf.extend_from_slice(&d.to_le_bytes());
+        }
+        buf
+    }
+
+    #[test]
+    fn chunk_dimensions_are_checked_when_the_layout_is_parsed() {
+        assert!(DataLayout::parse(&v3_chunked_msg(&[4, 4, 8]), 8, 8).is_ok());
+        // A zero chunk dimension used to read as all fill values.
+        let err = DataLayout::parse(&v3_chunked_msg(&[4, 0, 8]), 8, 8).unwrap_err();
+        assert!(
+            matches!(&err, FormatError::InvalidChunkDimensions(m) if m.contains("dim[1] = 0")),
+            "{err:?}"
+        );
+        // Only the element-size dimension: libhdf5 "bad dimensions".
+        assert_eq!(
+            DataLayout::parse(&v3_chunked_msg(&[8]), 8, 8).unwrap_err(),
+            FormatError::InvalidChunkDimensions("bad dimensions for chunked storage".into())
+        );
+        assert_eq!(
+            DataLayout::parse(&v3_chunked_msg(&[1; 34]), 8, 8).unwrap_err(),
+            FormatError::InvalidChunkDimensions("dimensionality is too large".into())
+        );
+        // v1/v2 and v4 messages get the zero check too.
+        let mut v1 = v1v2_header(1, 2, 2);
+        v1.extend_from_slice(&0x1000u64.to_le_bytes());
+        v1.extend_from_slice(&0u32.to_le_bytes());
+        v1.extend_from_slice(&8u32.to_le_bytes());
+        assert!(matches!(
+            DataLayout::parse(&v1, 8, 8),
+            Err(FormatError::InvalidChunkDimensions(_))
+        ));
+        let mut v4 = vec![4u8, 2, 0, 2, 4];
+        v4.extend_from_slice(&0u32.to_le_bytes());
+        v4.extend_from_slice(&8u32.to_le_bytes());
+        v4.push(3); // fixed array index
+        v4.push(0); // page bits
+        v4.extend_from_slice(&0x1000u64.to_le_bytes());
+        assert!(matches!(
+            DataLayout::parse(&v4, 8, 8),
+            Err(FormatError::InvalidChunkDimensions(_))
+        ));
+    }
+
+    /// A v4 chunked layout (fixed array index) whose `dims` are each
+    /// encoded in `width` bytes.
+    fn v4_chunked_msg(width: u8, dims: &[u64]) -> Vec<u8> {
+        let mut m = vec![4u8, 2, 0, dims.len() as u8, width];
+        for &d in dims {
+            m.extend_from_slice(&d.to_le_bytes()[..width.min(8) as usize]);
+        }
+        m.push(3); // fixed array index
+        m.push(0); // page bits
+        m.extend_from_slice(&0x1000u64.to_le_bytes());
+        m
+    }
+
+    #[test]
+    fn v4_chunk_dimensions_take_1_to_8_bytes() {
+        // libhdf5 encodes each dimension in the fewest bytes that hold the
+        // largest: a chunk dimension of 70 000 takes 3, and 3, 5, 6 and 7
+        // were refused ("UnexpectedEof").
+        for width in 1..=8u8 {
+            let dims = [if width >= 3 { 70_000 } else { 200 }, 8];
+            let layout = DataLayout::parse(&v4_chunked_msg(width, &dims), 8, 8)
+                .unwrap_or_else(|e| panic!("width {width}: {e:?}"));
+            assert!(
+                matches!(&layout, DataLayout::Chunked { chunk_dimensions, .. }
+                    if chunk_dimensions.iter().map(|&d| u64::from(d)).eq(dims)),
+                "width {width}: {layout:?}"
+            );
+        }
+        // libhdf5 refuses 0 and more than 8 bytes.
+        for width in [0u8, 9] {
+            assert_eq!(
+                DataLayout::parse(&v4_chunked_msg(width, &[4, 8]), 8, 8).unwrap_err(),
+                FormatError::InvalidChunkDimensions(
+                    "encoded chunk dimension size is too large".into()
+                )
+            );
+        }
+        // A dimension past u32 cannot be represented and is refused, not
+        // truncated.
+        assert!(matches!(
+            DataLayout::parse(&v4_chunked_msg(5, &[1 << 32, 8]), 8, 8),
+            Err(FormatError::InvalidChunkDimensions(m)) if m.contains("2^32")
+        ));
+    }
+
     #[test]
     fn v1v2_rejects_bad_class_dimensionality_and_truncation() {
         assert_eq!(
diff --git a/crates/clawhdf5-format/src/data_read.rs b/crates/clawhdf5-format/src/data_read.rs
index 76c2b0e..b8f2ffe 100644
--- a/crates/clawhdf5-format/src/data_read.rs
+++ b/crates/clawhdf5-format/src/data_read.rs
@@ -303,6 +303,7 @@ pub fn read_raw_data_selection(
     use crate::selection::Selection;
 
     crate::partial_read::validate(selection, &dataspace.dimensions)?;
+    crate::chunked_read::check_chunk_element_size(layout, datatype, offset_size)?;
 
     // Read only what the selection's bounding box touches when that is
     // possible; everything below is the decode-everything-then-pick path,
@@ -360,6 +361,7 @@ pub fn read_raw_data_selection(
             chunk_index_type,
             ..
         } => {
+            crate::chunked_read::chunk_geometry(chunk_dimensions, *version, dataspace, elem_size)?;
             // For chunked data, only read chunks that intersect the selection
             let chunk_dims: Vec<u64> = chunk_dimensions.iter().map(|&d| d as u64).collect();
             let rank = dims.len();
@@ -400,10 +402,10 @@ pub fn read_raw_data_selection(
             } else {
                 // v3: B-tree v1
                 if let Some(addr) = btree_address {
-                    crate::chunked_read::collect_chunk_info(
+                    crate::chunked_read::collect_chunk_info_checked(
                         file_data,
                         *addr,
-                        rank + 1,
+                        chunk_dimensions,
                         offset_size,
                         length_size,
                     )?
diff --git a/crates/clawhdf5-format/src/datatype.rs b/crates/clawhdf5-format/src/datatype.rs
index ba85afa..12e427f 100644
--- a/crates/clawhdf5-format/src/datatype.rs
+++ b/crates/clawhdf5-format/src/datatype.rs
@@ -208,6 +208,31 @@ fn offset_bytes_for_size(compound_size: u32) -> usize {
 }
 
 /// Read an unsigned integer of 1, 2, 4, or 8 bytes (LE).
+/// The size field of the datatype message at `pos`, as stored (a
+/// variable-length type's stored size is not modelled in [`Datatype`]).
+fn stored_type_size(data: &[u8], pos: usize) -> Result<u32, FormatError> {
+    ensure_len(data, pos, 8)?;
+    Ok(LittleEndian::read_u32(&data[pos + 4..pos + 8]))
+}
+
+/// libhdf5 refuses an array type of more than `H5S_MAX_RANK` (32)
+/// dimensions.
+fn check_array_rank(ndims: usize) -> Result<(), FormatError> {
+    if ndims > 32 {
+        return Err(invalid("too many dimensions for array datatype"));
+    }
+    Ok(())
+}
+
+/// A zero-sized array dimension makes a zero-sized type, which libhdf5
+/// cannot open ("unable to retrieve size of datatype").
+fn check_array_dims(dims: &[u32]) -> Result<(), FormatError> {
+    if dims.contains(&0) {
+        return Err(invalid("zero-sized dimension specified"));
+    }
+    Ok(())
+}
+
 fn read_uint(data: &[u8], offset: usize, nbytes: usize) -> Result<u64, FormatError> {
     ensure_len(data, offset, nbytes)?;
     let slice = &data[offset..offset + nbytes];
@@ -232,10 +257,104 @@ fn read_uint(data: &[u8], offset: usize, nbytes: usize) -> Result<u64, FormatErr
 /// available stack is a few KB.
 const MAX_DATATYPE_DEPTH: u16 = 64;
 
+fn invalid(why: impl Into<String>) -> FormatError {
+    FormatError::InvalidDatatype(why.into())
+}
+
+/// libhdf5's bounds checks on an integer type's bit offset and precision
+/// (`H5O__dtype_decode_helper`): both must lie inside the type. (Newer
+/// libhdf5 checks bit fields the same way; HDF5 2.0, which h5py 3.16 ships,
+/// does not, and opens such a type.)
+fn check_integer_bits(size: u32, bit_offset: u16, bit_precision: u16) -> Result<(), FormatError> {
+    let bits = u64::from(size) * 8;
+    if u64::from(bit_offset) >= bits {
+        return Err(invalid("integer offset out of bounds"));
+    }
+    if bit_precision == 0 {
+        return Err(invalid("precision is zero"));
+    }
+    if u64::from(bit_offset) + u64::from(bit_precision) > bits {
+        return Err(invalid("integer offset+precision out of bounds"));
+    }
+    Ok(())
+}
+
+/// Whether the closed bit ranges `[a0, a1]` and `[b0, b1]` share a bit.
+fn ranges_overlap(a0: u64, a1: u64, b0: u64, b1: u64) -> bool {
+    a0 <= b1 && b0 <= a1
+}
+
+/// libhdf5's checks on a floating-point type's fields: exponent and mantissa
+/// must lie inside the type, be non-empty, and not overlap each other or the
+/// sign bit. (libhdf5 does not check a float's bit offset and precision.)
+///
+/// One libhdf5 check is left out on purpose: a sign bit position outside the
+/// type ("sign bit position out of bounds"). clawhdf5 up to v2.7.0 wrote 63
+/// there for every float, so every `f32` it wrote (every agent store's
+/// embeddings) would stop opening. The position is not used to decode an
+/// IEEE float, so reading such a type returns the right values.
+fn check_float_fields(
+    size: u32,
+    sign: u8,
+    epos: u8,
+    esize: u8,
+    mpos: u8,
+    msize: u8,
+) -> Result<(), FormatError> {
+    let bits = u64::from(size) * 8;
+    let (sign, epos, esize, mpos, msize) = (
+        u64::from(sign),
+        u64::from(epos),
+        u64::from(esize),
+        u64::from(mpos),
+        u64::from(msize),
+    );
+    if esize == 0 {
+        return Err(invalid("exponent size can't be zero"));
+    }
+    if epos >= bits {
+        return Err(invalid("exponent starting position out of bounds"));
+    }
+    if epos + esize > bits {
+        return Err(invalid("exponent range out of bounds"));
+    }
+    if msize == 0 {
+        return Err(invalid("mantissa size can't be zero"));
+    }
+    if mpos >= bits {
+        return Err(invalid("mantissa starting position out of bounds"));
+    }
+    if mpos + msize > bits {
+        return Err(invalid("mantissa range out of bounds"));
+    }
+    let (e_end, m_end) = (epos + esize - 1, mpos + msize - 1);
+    if ranges_overlap(sign, sign, epos, e_end) {
+        return Err(invalid("exponent and sign positions overlap"));
+    }
+    if ranges_overlap(sign, sign, mpos, m_end) {
+        return Err(invalid("mantissa and sign positions overlap"));
+    }
+    if ranges_overlap(epos, e_end, mpos, m_end) {
+        return Err(invalid("mantissa and exponent positions overlap"));
+    }
+    Ok(())
+}
+
 impl Datatype {
     /// Parse a datatype message from raw bytes.
     ///
     /// Returns `(Datatype, bytes_consumed)` for recursive parsing.
+    ///
+    /// A type libhdf5 refuses to decode is refused here too, with
+    /// [`FormatError::InvalidDatatype`] carrying libhdf5's reason: size 0,
+    /// integer/bit-field/float bit fields outside the type or overlapping,
+    /// a compound with no members, a member outside its compound, a
+    /// duplicate or overlapping member, an enum whose size differs from its
+    /// base type's or with an empty name, an array of more than 32
+    /// dimensions or a zero-sized one, an unaligned opaque tag length.
+    /// Reading such a type used to return data from a corrupt file. Checks
+    /// newer libhdf5 releases add but HDF5 2.0 (h5py 3.16) lacks are left
+    /// out, so a file h5py opens still opens here.
     pub fn parse(data: &[u8]) -> Result<(Datatype, usize), FormatError> {
         Self::parse_with_depth(data, 0)
     }
@@ -259,6 +378,14 @@ impl Datatype {
 
         let size = LittleEndian::read_u32(&data[4..8]);
         let mut pos = 8;
+        // libhdf5 refuses size 0 for every class. A fixed-length string is
+        // exempt: clawhdf5 up to v2.7.0 wrote an empty-string attribute
+        // with a size-0 string type, and refusing it would fail every
+        // attribute of such objects, while reading it (an empty string) is
+        // harmless.
+        if size == 0 && class_id != 3 {
+            return Err(invalid("invalid datatype size"));
+        }
 
         match class_id {
             0 => {
@@ -272,6 +399,7 @@ impl Datatype {
                 let signed = (bf0 >> 3) & 0x01 == 1;
                 let bit_offset = LittleEndian::read_u16(&data[pos..pos + 2]);
                 let bit_precision = LittleEndian::read_u16(&data[pos + 2..pos + 4]);
+                check_integer_bits(size, bit_offset, bit_precision)?;
                 pos += 4;
                 Ok((
                     Datatype::FixedPoint {
@@ -289,13 +417,23 @@ impl Datatype {
                 ensure_len(data, pos, 12)?;
                 let bo_low = bf0 & 0x01;
                 let bo_high = (bf0 >> 6) & 0x01;
+                // Bit 6 (with bit 0) is VAX order, defined by version 3; libhdf5
+                // ignores bit 6 in older versions, which this read as VAX,
+                // byte-swapping a little-endian float.
+                let bo_high = if version >= 3 { bo_high } else { 0 };
                 let byte_order = match (bo_high, bo_low) {
                     (0, 0) => DatatypeByteOrder::LittleEndian,
                     (0, 1) => DatatypeByteOrder::BigEndian,
-                    (1, 0) => DatatypeByteOrder::Vax,
+                    (1, 0) => {
+                        return Err(invalid("bad byte order for datatype message"));
+                    }
                     (1, 1) => DatatypeByteOrder::Vax,
                     _ => unreachable!(),
                 };
+                // Bits 4-5: mantissa normalization; 3 is undefined.
+                if (bf0 >> 4) & 0x03 == 3 {
+                    return Err(invalid("unknown floating-point normalization"));
+                }
                 let bit_offset = LittleEndian::read_u16(&data[pos..pos + 2]);
                 let bit_precision = LittleEndian::read_u16(&data[pos + 2..pos + 4]);
                 let exponent_location = data[pos + 4];
@@ -303,6 +441,14 @@ impl Datatype {
                 let mantissa_location = data[pos + 6];
                 let mantissa_size = data[pos + 7];
                 let exponent_bias = LittleEndian::read_u32(&data[pos + 8..pos + 12]);
+                check_float_fields(
+                    size,
+                    bf1,
+                    exponent_location,
+                    exponent_size,
+                    mantissa_location,
+                    mantissa_size,
+                )?;
                 pos += 12;
                 Ok((
                     Datatype::FloatingPoint {
@@ -371,6 +517,10 @@ impl Datatype {
             5 => {
                 // Opaque
                 let tag_len = bf0 as usize;
+                // libhdf5 writes the NUL-padded length, a multiple of 8.
+                if !tag_len.is_multiple_of(8) {
+                    return Err(invalid("opaque flag field must be aligned"));
+                }
                 ensure_len(data, pos, tag_len)?;
                 // The stored tag is NUL-padded to a multiple of 8 bytes; the
                 // tag itself ends at the first NUL (libhdf5 reads it with
@@ -384,7 +534,45 @@ impl Datatype {
             6 => {
                 // Compound
                 let num_members = (bf0 as u16) | ((bf1 as u16) << 8);
-                let mut members = Vec::with_capacity(num_members as usize);
+                if num_members == 0 {
+                    return Err(invalid("invalid number of members: 0"));
+                }
+                let mut members: Vec<CompoundMember> = Vec::with_capacity(num_members as usize);
+                // Each member's size in the compound as libhdf5 decodes it:
+                // its stored size, times a v1 member's array dimensions. A
+                // variable-length member takes 4 + offset size + 4 bytes on
+                // disk, not the 16 of `Datatype::type_size`.
+                let mut member_sizes: Vec<u64> = Vec::with_capacity(num_members as usize);
+                // libhdf5 checks each member as it is decoded: it must fit in
+                // the compound (by its own stored size, before a v1 member's
+                // array dimensions are applied), and must not repeat a name
+                // or overlap an earlier member (by its final size).
+                let check_member = |members: &[CompoundMember],
+                                    member_sizes: &[u64],
+                                    name: &str,
+                                    byte_offset: u64,
+                                    stored_size: u32,
+                                    final_size: u64|
+                 -> Result<(), FormatError> {
+                    if byte_offset + u64::from(stored_size) > u64::from(size) {
+                        return Err(invalid(
+                            "member type extends outside its parent compound type",
+                        ));
+                    }
+                    if let Some(j) = members.iter().position(|m| m.name == name) {
+                        return Err(invalid(format!(
+                            "duplicated compound field name '{name}', for fields {j} and {}",
+                            members.len()
+                        )));
+                    }
+                    let end = byte_offset + final_size;
+                    if members.iter().zip(member_sizes).any(|(m, &m_size)| {
+                        byte_offset < m.byte_offset + m_size && m.byte_offset < end
+                    }) {
+                        return Err(invalid("member overlaps with previous member"));
+                    }
+                    Ok(())
+                };
 
                 if (3..=5).contains(&version) {
                     // v3, v4 and v5 share the compact member encoding (name,
@@ -396,9 +584,20 @@ impl Datatype {
                         pos += name_len;
                         let byte_offset = read_uint(data, pos, ob)?;
                         pos += ob;
+                        let stored_size = stored_type_size(data, pos)?;
                         let (member_dt, consumed) =
                             Self::parse_with_depth(&data[pos..], depth + 1)?;
                         pos += consumed;
+                        let final_size = u64::from(stored_size);
+                        check_member(
+                            &members,
+                            &member_sizes,
+                            &name,
+                            byte_offset,
+                            stored_size,
+                            final_size,
+                        )?;
+                        member_sizes.push(final_size);
                         members.push(CompoundMember {
                             name,
                             byte_offset,
@@ -438,11 +637,11 @@ impl Datatype {
                                 let at = pos + 12 + 4 * j;
                                 LittleEndian::read_u32(&data[at..at + 4]) == 0
                             });
-                            if ndims > 4 || zero_dim {
-                                return Err(FormatError::InvalidDatatypeVersion {
-                                    class: class_id,
-                                    version,
-                                });
+                            if ndims > 4 {
+                                return Err(invalid("invalid number of dimensions for array"));
+                            }
+                            if zero_dim {
+                                return Err(invalid("zero-sized dimension specified"));
                             }
                             array_dims = (0..ndims)
                                 .map(|j| {
@@ -452,15 +651,28 @@ impl Datatype {
                                 .collect();
                             pos += 28;
                         }
+                        let stored_size = stored_type_size(data, pos)?;
                         let (mut member_dt, consumed) =
                             Self::parse_with_depth(&data[pos..], depth + 1)?;
                         pos += consumed;
+                        let final_size = array_dims.iter().fold(u64::from(stored_size), |a, &d| {
+                            a.saturating_mul(u64::from(d))
+                        });
                         if !array_dims.is_empty() {
                             member_dt = Datatype::Array {
                                 base_type: Box::new(member_dt),
                                 dimensions: array_dims,
                             };
                         }
+                        check_member(
+                            &members,
+                            &member_sizes,
+                            &name,
+                            byte_offset,
+                            stored_size,
+                            final_size,
+                        )?;
+                        member_sizes.push(final_size);
                         members.push(CompoundMember {
                             name,
                             byte_offset,
@@ -499,6 +711,9 @@ impl Datatype {
                 let (base_type, base_consumed) = Self::parse_with_depth(&data[pos..], depth + 1)?;
                 pos += base_consumed;
                 let base_size = base_type.type_size();
+                if base_size != size {
+                    return Err(invalid("ENUM datatype size does not match parent"));
+                }
                 let mut members = Vec::with_capacity(num_members as usize);
                 // Enum layout: base_type, then all names (null-terminated), then all values
                 // v1/v2: names are padded to 8-byte boundaries
@@ -506,6 +721,9 @@ impl Datatype {
                 let mut member_names = Vec::with_capacity(num_members as usize);
                 for _ in 0..num_members {
                     let (name, name_len) = read_null_terminated_string(data, pos)?;
+                    if name.is_empty() {
+                        return Err(invalid("0 length enum name"));
+                    }
                     if version < 3 {
                         let padded = (name_len + 7) & !7;
                         pos += padded;
@@ -566,6 +784,7 @@ impl Datatype {
                 if version == 2 {
                     ensure_len(data, pos, 4)?;
                     let ndims = data[pos] as usize;
+                    check_array_rank(ndims)?;
                     pos += 4; // ndims(1) + reserved(3)
                     ensure_len(data, pos, ndims * 4 + ndims * 4)?;
                     let mut dimensions = Vec::with_capacity(ndims);
@@ -573,6 +792,7 @@ impl Datatype {
                         dimensions.push(LittleEndian::read_u32(&data[pos..pos + 4]));
                         pos += 4;
                     }
+                    check_array_dims(&dimensions)?;
                     // skip permutation indices
                     pos += ndims * 4;
                     let (base_type, consumed) = Self::parse_with_depth(&data[pos..], depth + 1)?;
@@ -589,6 +809,7 @@ impl Datatype {
                     // type); HDF5 1.14+/2.0 with `libver=latest` emits v5.
                     ensure_len(data, pos, 1)?;
                     let ndims = data[pos] as usize;
+                    check_array_rank(ndims)?;
                     pos += 1;
                     ensure_len(data, pos, ndims * 4)?;
                     let mut dimensions = Vec::with_capacity(ndims);
@@ -596,6 +817,7 @@ impl Datatype {
                         dimensions.push(LittleEndian::read_u32(&data[pos..pos + 4]));
                         pos += 4;
                     }
+                    check_array_dims(&dimensions)?;
                     let (base_type, consumed) = Self::parse_with_depth(&data[pos..], depth + 1)?;
                     pos += consumed;
                     Ok((
@@ -652,6 +874,70 @@ impl Datatype {
         }
     }
 
+    /// [`Self::parse`] for the datatype message of an object whose header
+    /// has version `header_version`: a version-1 header, which has no
+    /// checksum, additionally gets [`Self::check_unused_bits`], as libhdf5
+    /// does. Use this wherever the header is at hand.
+    pub fn parse_in_header(
+        data: &[u8],
+        header_version: u8,
+    ) -> Result<(Datatype, usize), FormatError> {
+        let parsed = Self::parse(data)?;
+        if header_version == 1 {
+            parsed.0.check_unused_bits()?;
+        }
+        Ok(parsed)
+    }
+
+    /// libhdf5's guard against a corrupt numeric type in a header without
+    /// a checksum (`H5T_is_numeric_with_unusual_unused_bits`, HDF5 1.14.4+):
+    /// an integer, float or bit field wider than a byte whose precision and
+    /// offset leave more than half its bits unused is taken for corruption
+    /// (e.g. a 3-bit integer in 4 bytes, `cve-2024-29162`, or a 32-bit float
+    /// in 65525 bytes, `cve-2024-32614`), anywhere in the type. libhdf5
+    /// skips the check for checksummed (version-2) headers and when the
+    /// file is opened with `H5Pset_relax_file_integrity_checks`; so does
+    /// [`Self::parse_in_header`], which has no such option.
+    pub fn check_unused_bits(&self) -> Result<(), FormatError> {
+        match self {
+            Datatype::FixedPoint {
+                size,
+                bit_offset,
+                bit_precision,
+                ..
+            }
+            | Datatype::FloatingPoint {
+                size,
+                bit_offset,
+                bit_precision,
+                ..
+            }
+            | Datatype::BitField {
+                size,
+                bit_offset,
+                bit_precision,
+                ..
+            } => {
+                let bits = u64::from(*size) * 8;
+                let prec = u64::from(*bit_precision);
+                if *size > 1 && prec < bits && bits > 2 * (prec + u64::from(*bit_offset)) {
+                    return Err(invalid(format!(
+                        "datatype has unusually large # of unused bits (prec = {prec} bits, \
+                         size = {size} bytes), possibly corrupted file"
+                    )));
+                }
+                Ok(())
+            }
+            Datatype::Compound { members, .. } => members
+                .iter()
+                .try_for_each(|m| m.datatype.check_unused_bits()),
+            Datatype::Enumeration { base_type, .. }
+            | Datatype::VariableLength { base_type, .. }
+            | Datatype::Array { base_type, .. } => base_type.check_unused_bits(),
+            _ => Ok(()),
+        }
+    }
+
     /// Serialize datatype to HDF5 message bytes.
     pub fn serialize(&self) -> Vec<u8> {
         match self {
@@ -863,9 +1149,23 @@ impl Datatype {
     }
 
     /// Check that this datatype can be written: every part of it has an
-    /// on-disk encoding. [`Self::serialize`] cannot report errors, so the
-    /// writer calls this first.
+    /// on-disk encoding, and the encoding is one the reader (and libhdf5)
+    /// accepts. [`Self::serialize`] cannot report errors, so the writer calls
+    /// this first. A compound with no fields or a repeated field name, or an
+    /// enum member with an empty name, is refused here: libhdf5 and h5py
+    /// refuse such types, and so does [`Self::parse`], so writing one made a
+    /// file that could not be read back.
     pub fn check_encodable(&self) -> Result<(), FormatError> {
+        self.check_encodable_parts()?;
+        Self::parse(&self.serialize()).map_err(|e| {
+            FormatError::SerializationError(format!(
+                "datatype cannot be written: HDF5 readers refuse it ({e})"
+            ))
+        })?;
+        Ok(())
+    }
+
+    fn check_encodable_parts(&self) -> Result<(), FormatError> {
         match self {
             Datatype::Opaque { tag, .. } if opaque_tag_text(tag).len() > MAX_OPAQUE_TAG_LEN => {
                 Err(FormatError::SerializationError(format!(
@@ -878,10 +1178,10 @@ impl Datatype {
             )),
             Datatype::Compound { members, .. } => members
                 .iter()
-                .try_for_each(|m| m.datatype.check_encodable()),
+                .try_for_each(|m| m.datatype.check_encodable_parts()),
             Datatype::Enumeration { base_type, .. }
             | Datatype::VariableLength { base_type, .. }
-            | Datatype::Array { base_type, .. } => base_type.check_encodable(),
+            | Datatype::Array { base_type, .. } => base_type.check_encodable_parts(),
             _ => Ok(()),
         }
     }
@@ -985,7 +1285,8 @@ mod tests {
     ) -> Vec<u8> {
         // LE byte order: bo_low=0, bo_high=0
         let bf0 = 0x00u8;
-        let bf1 = 0x00u8;
+        // Sign bit: the top bit.
+        let bf1 = (size * 8 - 1) as u8;
         // mantissa norm = 2 (MSB not stored) in bits 24-31... wait, that's bf2
         let bf2 = 0x02u8; // norm = 2
         let mut buf = build_dt_header(1, 1, [bf0, bf1, bf2], size);
@@ -1012,7 +1313,7 @@ mod tests {
         let levels = MAX_DATATYPE_DEPTH as usize + 10;
         let mut data = Vec::new();
         for _ in 0..levels {
-            data.extend_from_slice(&build_dt_header(9, 3, [0, 0, 0], 0));
+            data.extend_from_slice(&build_dt_header(9, 3, [0, 0, 0], 16));
         }
         data.extend_from_slice(&build_fixed_point(4, false, false, 0, 32));
 
@@ -1026,7 +1327,7 @@ mod tests {
         let levels = MAX_DATATYPE_DEPTH as usize - 1;
         let mut data = Vec::new();
         for _ in 0..levels {
-            data.extend_from_slice(&build_dt_header(9, 3, [0, 0, 0], 0));
+            data.extend_from_slice(&build_dt_header(9, 3, [0, 0, 0], 16));
         }
         data.extend_from_slice(&build_fixed_point(4, false, false, 0, 32));
 
@@ -1176,8 +1477,8 @@ mod tests {
 
     #[test]
     fn test_opaque() {
-        // tag_len = 4, tag = "BLOB"
-        let mut buf = build_dt_header(5, 1, [4, 0, 0], 64);
+        // tag = "BLOB"; the stored length is the NUL-padded length, 8
+        let mut buf = build_dt_header(5, 1, [8, 0, 0], 64);
         buf.extend_from_slice(b"BLOB");
         // Pad to 8 bytes
         buf.extend_from_slice(&[0, 0, 0, 0]);
@@ -2049,4 +2350,273 @@ mod tests {
         };
         assert_eq!(dt.type_size(), 48);
     }
+    /// Every check here mirrors one in libhdf5's `H5O__dtype_decode_helper`;
+    /// the error text is libhdf5's.
+    fn invalid_reason(data: &[u8]) -> String {
+        match Datatype::parse(data) {
+            Err(FormatError::InvalidDatatype(why)) => why,
+            other => panic!("expected InvalidDatatype, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn size_zero_is_refused() {
+        // cve-2017-17508: a variable-length string member of stored size 0.
+        let mut data = build_dt_header(9, 1, [1, 0, 0], 0);
+        data.extend_from_slice(&build_fixed_point(1, false, false, 0, 8));
+        assert_eq!(invalid_reason(&data), "invalid datatype size");
+        // Except a fixed-length string, which clawhdf5 <= v2.7.0 wrote for an
+        // empty-string attribute.
+        assert!(Datatype::parse(&build_dt_header(3, 1, [0, 0, 0], 0)).is_ok());
+        assert_eq!(
+            invalid_reason(&build_fixed_point(0, false, false, 0, 0)),
+            "invalid datatype size"
+        );
+    }
+
+    #[test]
+    fn integer_bits_must_lie_inside_the_type() {
+        assert_eq!(
+            invalid_reason(&build_fixed_point(4, false, false, 32, 1)),
+            "integer offset out of bounds"
+        );
+        assert_eq!(
+            invalid_reason(&build_fixed_point(4, false, false, 0, 0)),
+            "precision is zero"
+        );
+        assert_eq!(
+            invalid_reason(&build_fixed_point(4, false, false, 8, 25)),
+            "integer offset+precision out of bounds"
+        );
+        // A partial-precision integer inside its bytes is fine.
+        assert!(Datatype::parse(&build_fixed_point(4, false, false, 12, 8)).is_ok());
+    }
+
+    #[test]
+    fn float_fields_must_lie_inside_the_type_and_not_overlap() {
+        // (sign, epos, esize, mpos, msize) on an f32
+        let f32_with = |sign: u8, epos: u8, esize: u8, mpos: u8, msize: u8| {
+            let mut data = build_dt_header(1, 1, [0x20, sign, 0], 4);
+            data.extend_from_slice(&0u16.to_le_bytes());
+            data.extend_from_slice(&32u16.to_le_bytes());
+            data.extend_from_slice(&[epos, esize, mpos, msize]);
+            data.extend_from_slice(&127u32.to_le_bytes());
+            data
+        };
+        assert!(Datatype::parse(&f32_with(31, 23, 8, 0, 23)).is_ok());
+        for (fields, why) in [
+            ((31, 23, 0, 0, 23), "exponent size can't be zero"),
+            (
+                (31, 32, 8, 0, 23),
+                "exponent starting position out of bounds",
+            ),
+            ((31, 30, 8, 0, 23), "exponent range out of bounds"),
+            ((31, 23, 8, 0, 0), "mantissa size can't be zero"),
+            (
+                (31, 23, 8, 40, 1),
+                "mantissa starting position out of bounds",
+            ),
+            // cve-2024-29163: a 128-bit mantissa in a 4-byte float.
+            ((31, 23, 8, 0, 128), "mantissa range out of bounds"),
+            ((23, 23, 8, 0, 23), "exponent and sign positions overlap"),
+            ((0, 23, 8, 0, 23), "mantissa and sign positions overlap"),
+            // cve-2026-34734.
+            (
+                (31, 20, 8, 0, 23),
+                "mantissa and exponent positions overlap",
+            ),
+        ] {
+            let (sign, epos, esize, mpos, msize) = fields;
+            assert_eq!(
+                invalid_reason(&f32_with(sign, epos, esize, mpos, msize)),
+                why,
+                "{fields:?}"
+            );
+        }
+        // Normalization 3 is undefined; bit 6 (VAX) needs bit 0 from v3.
+        let mut data = f32_with(31, 23, 8, 0, 23);
+        data[1] = 0x30;
+        assert_eq!(
+            invalid_reason(&data),
+            "unknown floating-point normalization"
+        );
+        let mut data = f32_with(31, 23, 8, 0, 23);
+        data[0] = 0x31; // version 3
+        data[1] = 0x60;
+        assert_eq!(invalid_reason(&data), "bad byte order for datatype message");
+    }
+
+    #[test]
+    fn unusual_unused_bits_are_refused_in_version_1_headers_only() {
+        // cve-2024-29162: a 3-bit integer in 4 bytes.
+        let data = build_fixed_point(4, false, true, 0, 3);
+        assert!(Datatype::parse_in_header(&data, 2).is_ok());
+        assert_eq!(
+            match Datatype::parse_in_header(&data, 1) {
+                Err(FormatError::InvalidDatatype(why)) => why,
+                other => panic!("{other:?}"),
+            },
+            "datatype has unusually large # of unused bits (prec = 3 bits, size = 4 bytes), \
+             possibly corrupted file"
+        );
+        // Half the bits used (with the offset) is not unusual; nor is a
+        // 1-byte type; nor a full-precision one.
+        for (size, offset, prec) in [(4u32, 0u16, 16u16), (4, 8, 8), (1, 0, 1), (8, 0, 64)] {
+            let data = build_fixed_point(size, false, true, offset, prec);
+            assert!(
+                Datatype::parse_in_header(&data, 1).is_ok(),
+                "{size} {offset} {prec}"
+            );
+        }
+        // Nested: a compound member's type is checked too.
+        let member = build_fixed_point(4, false, true, 0, 15);
+        let data = compound_v3(4, &[("a", 0, member)]);
+        assert!(Datatype::parse_in_header(&data, 2).is_ok());
+        assert!(Datatype::parse_in_header(&data, 1).is_err());
+    }
+
+    #[test]
+    fn f32_written_by_clawhdf5_up_to_2_7_0_still_parses() {
+        // Those versions put the sign bit at 63 whatever the float's size;
+        // libhdf5 refuses it ("sign bit position out of bounds").
+        let mut data = build_dt_header(1, 1, [0x20, 63, 0], 4);
+        data.extend_from_slice(&0u16.to_le_bytes());
+        data.extend_from_slice(&32u16.to_le_bytes());
+        data.extend_from_slice(&[23, 8, 0, 23]);
+        data.extend_from_slice(&127u32.to_le_bytes());
+        assert!(Datatype::parse(&data).is_ok());
+    }
+
+    #[test]
+    fn float_bit_6_is_vax_order_only_from_version_3() {
+        // h5py opens a v1 float with bit 6 set as an ordinary little-endian
+        // float; it used to be read as VAX order.
+        let mut data = build_float(4, 23, 8, 0, 23, 127);
+        data[1] |= 0x40;
+        match Datatype::parse(&data).unwrap().0 {
+            Datatype::FloatingPoint { byte_order, .. } => {
+                assert_eq!(byte_order, DatatypeByteOrder::LittleEndian)
+            }
+            other => panic!("{other:?}"),
+        }
+        data[0] = 0x31;
+        data[1] |= 0x01;
+        match Datatype::parse(&data).unwrap().0 {
+            Datatype::FloatingPoint { byte_order, .. } => {
+                assert_eq!(byte_order, DatatypeByteOrder::Vax)
+            }
+            other => panic!("{other:?}"),
+        }
+    }
+
+    #[test]
+    fn opaque_tag_length_must_be_padded() {
+        let mut data = build_dt_header(5, 1, [4, 0, 0], 4);
+        data.extend_from_slice(b"BLOB");
+        assert_eq!(invalid_reason(&data), "opaque flag field must be aligned");
+    }
+
+    /// A v3 compound of `size` bytes with `(name, offset, member)` members.
+    fn compound_v3(size: u32, members: &[(&str, u8, Vec<u8>)]) -> Vec<u8> {
+        let n = members.len() as u8;
+        let mut data = build_dt_header(6, 3, [n, 0, 0], size);
+        for (name, off, dt) in members {
+            data.extend_from_slice(name.as_bytes());
+            data.push(0);
+            data.push(*off);
+            data.extend_from_slice(dt);
+        }
+        data
+    }
+
+    #[test]
+    fn compound_members_are_checked() {
+        let i4 = build_fixed_point(4, false, true, 0, 32);
+        // cve-2016-4332: no members.
+        assert_eq!(
+            invalid_reason(&compound_v3(8, &[])),
+            "invalid number of members: 0"
+        );
+        assert_eq!(
+            invalid_reason(&compound_v3(
+                8,
+                &[("a", 0, i4.clone()), ("b", 6, i4.clone())]
+            )),
+            "member type extends outside its parent compound type"
+        );
+        assert_eq!(
+            invalid_reason(&compound_v3(
+                8,
+                &[("a", 0, i4.clone()), ("a", 4, i4.clone())]
+            )),
+            "duplicated compound field name 'a', for fields 0 and 1"
+        );
+        assert_eq!(
+            invalid_reason(&compound_v3(
+                8,
+                &[("a", 0, i4.clone()), ("b", 2, i4.clone())]
+            )),
+            "member overlaps with previous member"
+        );
+        assert_eq!(
+            invalid_reason(&compound_v3(
+                8,
+                &[("b", 4, i4.clone()), ("a", 2, i4.clone())]
+            )),
+            "member overlaps with previous member"
+        );
+        // Members out of offset order, and gaps, are fine.
+        assert!(Datatype::parse(&compound_v3(12, &[("b", 8, i4.clone()), ("a", 0, i4)])).is_ok());
+    }
+
+    #[test]
+    fn enum_is_checked() {
+        let base = build_fixed_point(4, false, true, 0, 32);
+        let enum_of = |size: u32, names: &[&str]| {
+            let mut data = build_dt_header(8, 3, [names.len() as u8, 0, 0], size);
+            data.extend_from_slice(&base);
+            for n in names {
+                data.extend_from_slice(n.as_bytes());
+                data.push(0);
+            }
+            for i in 0..names.len() as u32 {
+                data.extend_from_slice(&i.to_le_bytes());
+            }
+            data
+        };
+        assert!(Datatype::parse(&enum_of(4, &["RED", "GREEN"])).is_ok());
+        // cve-2024-32618.
+        assert_eq!(
+            invalid_reason(&enum_of(4, &["", "GREEN"])),
+            "0 length enum name"
+        );
+        assert_eq!(
+            invalid_reason(&enum_of(2, &["RED"])),
+            "ENUM datatype size does not match parent"
+        );
+    }
+
+    #[test]
+    fn array_dimensions_are_checked() {
+        let base = build_fixed_point(4, false, true, 0, 32);
+        let array_v3 = |dims: &[u32]| {
+            let n = dims.iter().product::<u32>().max(1);
+            let mut data = build_dt_header(10, 3, [0, 0, 0], 4 * n);
+            data.push(dims.len() as u8);
+            for d in dims {
+                data.extend_from_slice(&d.to_le_bytes());
+            }
+            data.extend_from_slice(&base);
+            data
+        };
+        assert!(Datatype::parse(&array_v3(&[2, 3])).is_ok());
+        assert_eq!(
+            invalid_reason(&array_v3(&[2, 0])),
+            "zero-sized dimension specified"
+        );
+        assert_eq!(
+            invalid_reason(&array_v3(&[1; 33])),
+            "too many dimensions for array datatype"
+        );
+    }
 }
diff --git a/crates/clawhdf5-format/src/ea_writer.rs b/crates/clawhdf5-format/src/ea_writer.rs
index e0f0375..f669534 100644
--- a/crates/clawhdf5-format/src/ea_writer.rs
+++ b/crates/clawhdf5-format/src/ea_writer.rs
@@ -7,7 +7,9 @@ extern crate alloc;
 use alloc::{vec, vec::Vec};
 
 use crate::checksum::jenkins_lookup3;
-use crate::chunked_write::{WrittenChunk, filtered_chunk_size_len, push_addr, push_index_element};
+use crate::chunked_write::{
+    WrittenChunk, filtered_chunk_size_len, push_addr, push_index_element, push_v4_chunk_dims,
+};
 
 /// Serialize a v4 Extensible Array layout message.
 pub(crate) fn serialize_v4_extensible_array(
@@ -24,35 +26,7 @@ pub(crate) fn serialize_v4_extensible_array(
     let ndims = chunk_dims.len() as u8 + 1;
     buf.push(ndims);
 
-    let max_dim = chunk_dims
-        .iter()
-        .map(|&d| d as u64)
-        .chain(core::iter::once(element_size as u64))
-        .max()
-        .unwrap_or(1);
-    let dim_encoded_len: u8 = if max_dim <= 0xFF {
-        1
-    } else if max_dim <= 0xFFFF {
-        2
-    } else {
-        4
-    };
-    buf.push(dim_encoded_len);
-
-    for &d in chunk_dims {
-        match dim_encoded_len {
-            1 => buf.push(d as u8),
-            2 => buf.extend_from_slice(&(d as u16).to_le_bytes()),
-            4 => buf.extend_from_slice(&d.to_le_bytes()),
-            _ => unreachable!("unexpected dim_encoded_len: {dim_encoded_len}"),
-        }
-    }
-    match dim_encoded_len {
-        1 => buf.push(element_size as u8),
-        2 => buf.extend_from_slice(&(element_size as u16).to_le_bytes()),
-        4 => buf.extend_from_slice(&element_size.to_le_bytes()),
-        _ => unreachable!("unexpected dim_encoded_len: {dim_encoded_len}"),
-    }
+    push_v4_chunk_dims(&mut buf, chunk_dims, element_size);
 
     // chunk index type = 4 (Extensible Array)
     buf.push(4);
diff --git a/crates/clawhdf5-format/src/error.rs b/crates/clawhdf5-format/src/error.rs
index bb81939..a9a0c2b 100644
--- a/crates/clawhdf5-format/src/error.rs
+++ b/crates/clawhdf5-format/src/error.rs
@@ -201,6 +201,28 @@ pub enum FormatError {
     DuplicateDatasetName(String),
     /// Integer overflow in size computation (malformed data protection).
     Overflow(String),
+    /// An object header that libhdf5 refuses to load (the reason is
+    /// libhdf5's own error text): a misaligned or overrunning message, a
+    /// wrong message count, contradictory message flags, a message of a
+    /// class that cannot be shared flagged shareable, …
+    InvalidObjectHeader(&'static str),
+    /// A datatype message libhdf5 refuses to decode (the reason is
+    /// libhdf5's own error text): size 0, bit fields outside the type,
+    /// an empty enum name, a compound member outside its compound, …
+    InvalidDatatype(String),
+    /// A chunked layout whose chunk dimensions libhdf5 refuses: a zero
+    /// dimension, a rank that does not match the dataspace, an element size
+    /// that is not the datatype's, or a chunk of 4 GiB or more indexed by a
+    /// version-1 B-tree.
+    InvalidChunkDimensions(String),
+    /// The superblock's end-of-file address lies past the end of the file:
+    /// the file was truncated (libhdf5 refuses to open it).
+    TruncatedFile {
+        /// End of file recorded in the superblock (relative to byte 0).
+        stored_eof: u64,
+        /// The file's actual length in bytes.
+        actual_len: u64,
+    },
 }
 
 impl fmt::Display for FormatError {
@@ -445,6 +467,25 @@ impl fmt::Display for FormatError {
             FormatError::Overflow(msg) => {
                 write!(f, "integer overflow: {msg}")
             }
+            FormatError::InvalidObjectHeader(why) => {
+                write!(f, "corrupt object header: {why}")
+            }
+            FormatError::InvalidDatatype(why) => {
+                write!(f, "invalid datatype: {why}")
+            }
+            FormatError::InvalidChunkDimensions(why) => {
+                write!(f, "invalid chunk dimensions: {why}")
+            }
+            FormatError::TruncatedFile {
+                stored_eof,
+                actual_len,
+            } => {
+                write!(
+                    f,
+                    "truncated file: the superblock records end of file {stored_eof}, \
+                     but the file is {actual_len} bytes"
+                )
+            }
         }
     }
 }
diff --git a/crates/clawhdf5-format/src/object_header.rs b/crates/clawhdf5-format/src/object_header.rs
index 4b8067f..2c3c819 100644
--- a/crates/clawhdf5-format/src/object_header.rs
+++ b/crates/clawhdf5-format/src/object_header.rs
@@ -108,10 +108,20 @@ impl ObjectHeader {
             return Err(FormatError::InvalidObjectHeaderVersion(version));
         }
 
-        let num_messages = LittleEndian::read_u16(&data[offset + 2..offset + 4]);
+        let num_messages = LittleEndian::read_u16(&data[offset + 2..offset + 4]) as usize;
         let reference_count = LittleEndian::read_u32(&data[offset + 4..offset + 8]);
         let header_data_size = LittleEndian::read_u32(&data[offset + 8..offset + 12]) as usize;
 
+        // libhdf5 (H5O__prefix_deserialize): a header with messages needs room
+        // for at least one message header, and one without has an empty chunk.
+        if (num_messages > 0 && header_data_size < V1_MSG_HEADER_SIZE)
+            || (num_messages == 0 && header_data_size > 0)
+        {
+            return Err(FormatError::InvalidObjectHeader(
+                "bad object header chunk size",
+            ));
+        }
+
         // Pad to 8-byte alignment: header prefix is 12 bytes, pad to 16
         let padding = 4; // pad 12-byte prefix to 16-byte alignment
         let msg_start = offset
@@ -124,64 +134,23 @@ impl ObjectHeader {
         ensure_len(data, msg_start, header_data_size)?;
 
         let mut messages = Vec::new();
-        let mut pos = msg_start;
-        let msg_end =
-            msg_start
-                .checked_add(header_data_size)
-                .ok_or(FormatError::UnexpectedEof {
-                    expected: usize::MAX,
-                    available: data.len(),
-                })?;
-
-        for _ in 0..num_messages {
-            if pos + 8 > msg_end {
-                break;
-            }
-            let msg_type_raw = LittleEndian::read_u16(&data[pos..pos + 2]);
-            let msg_data_size = LittleEndian::read_u16(&data[pos + 2..pos + 4]) as usize;
-            let msg_flags = data[pos + 4];
-            // reserved(3) at pos+5..pos+8
-            pos += 8;
-
-            ensure_len(data, pos, msg_data_size)?;
-            let msg_type = MessageType::from_u16(msg_type_raw);
-
-            check_unknown_message(msg_type, msg_flags)?;
-
-            if msg_type != MessageType::Nil {
-                messages.push(HeaderMessage {
-                    msg_type,
-                    size: msg_data_size,
-                    flags: msg_flags,
-                    creation_order: None,
-                    data: data[pos..pos + msg_data_size].to_vec(),
-                });
-            }
-
-            pos += msg_data_size;
-
-            // Follow continuations
-            if msg_type == MessageType::ObjectHeaderContinuation {
-                let cont_msg_data = &messages
-                    .last()
-                    .ok_or(FormatError::InvalidObjectHeaderSignature)?
-                    .data;
-                if cont_msg_data.len() >= (offset_size as usize + length_size as usize) {
-                    let cont_offset = read_offset(cont_msg_data, 0, offset_size)? as usize;
-                    let cont_length =
-                        read_offset(cont_msg_data, offset_size as usize, length_size)? as usize;
-                    // Parse continuation block (v1: just raw messages, no signature)
-                    let cont_msgs = Self::parse_v1_continuation(
-                        data,
-                        cont_offset,
-                        cont_length,
-                        offset_size,
-                        length_size,
-                        32, // max continuation depth
-                    )?;
-                    messages.extend(cont_msgs);
-                }
-            }
+        let chunk0_count = Self::parse_v1_chunk(
+            data,
+            msg_start,
+            header_data_size,
+            offset_size,
+            length_size,
+            MAX_V1_CONTINUATION_DEPTH,
+            &mut messages,
+        )?;
+        // libhdf5 reads every message in the first chunk and refuses a header
+        // whose prefix claims fewer than that (continuation chunks are read
+        // later and not held to the count). Stopping after the claimed number
+        // silently dropped the rest.
+        if chunk0_count > num_messages {
+            return Err(FormatError::InvalidObjectHeader(
+                "bad object header message count",
+            ));
         }
 
         Ok(ObjectHeader {
@@ -196,72 +165,87 @@ impl ObjectHeader {
         })
     }
 
-    fn parse_v1_continuation(
+    /// Parse the messages of one version-1 chunk (`length` bytes at
+    /// `offset`, no signature), following continuation messages as they are
+    /// met. Returns how many messages (NIL ones included) this chunk itself
+    /// holds.
+    ///
+    /// A version-1 chunk is filled with messages whose sizes are multiples of
+    /// 8; libhdf5 refuses a message that is not aligned, that runs past the
+    /// end of the chunk, or leftover bytes too few for a message header (a
+    /// "gap", which only version 2 allows).
+    #[allow(clippy::too_many_arguments)]
+    fn parse_v1_chunk(
         data: &[u8],
         offset: usize,
         length: usize,
         offset_size: u8,
         length_size: u8,
         depth_remaining: u16,
-    ) -> Result<Vec<HeaderMessage>, FormatError> {
+        messages: &mut Vec<HeaderMessage>,
+    ) -> Result<usize, FormatError> {
         if depth_remaining == 0 {
             return Err(FormatError::NestingDepthExceeded);
         }
         ensure_len(data, offset, length)?;
-        let mut messages = Vec::new();
+        let end = offset + length;
         let mut pos = offset;
-        let end = offset.saturating_add(length);
+        let mut count = 0usize;
 
-        while pos + 8 <= end {
+        while pos < end {
+            if end - pos < V1_MSG_HEADER_SIZE {
+                return Err(FormatError::InvalidObjectHeader(
+                    "gap found in early version of file format",
+                ));
+            }
             let msg_type_raw = LittleEndian::read_u16(&data[pos..pos + 2]);
             let msg_data_size = LittleEndian::read_u16(&data[pos + 2..pos + 4]) as usize;
             let msg_flags = data[pos + 4];
-            pos += 8;
+            // reserved(3) at pos+5..pos+8
+            pos += V1_MSG_HEADER_SIZE;
 
-            if pos + msg_data_size > end {
-                break;
+            if !msg_data_size.is_multiple_of(8) {
+                return Err(FormatError::InvalidObjectHeader("message not aligned"));
             }
+            if msg_data_size > end - pos {
+                return Err(FormatError::InvalidObjectHeader(
+                    "message size exceeds buffer end",
+                ));
+            }
+            let body = &data[pos..pos + msg_data_size];
+            check_message(1, msg_type_raw, msg_flags, body, offset_size, length_size)?;
+            count += 1;
 
             let msg_type = MessageType::from_u16(msg_type_raw);
-
-            check_unknown_message(msg_type, msg_flags)?;
-
             if msg_type != MessageType::Nil {
                 messages.push(HeaderMessage {
                     msg_type,
                     size: msg_data_size,
                     flags: msg_flags,
                     creation_order: None,
-                    data: data[pos..pos + msg_data_size].to_vec(),
+                    data: body.to_vec(),
                 });
             }
-
             pos += msg_data_size;
 
-            // Recursive continuations
+            // Follow continuations (v1 continuation chunks are just raw
+            // messages, no signature); check_message has checked the body.
             if msg_type == MessageType::ObjectHeaderContinuation {
-                let cont_msg_data = &messages
-                    .last()
-                    .ok_or(FormatError::InvalidObjectHeaderSignature)?
-                    .data;
-                if cont_msg_data.len() >= (offset_size as usize + length_size as usize) {
-                    let cont_offset = read_offset(cont_msg_data, 0, offset_size)? as usize;
-                    let cont_length =
-                        read_offset(cont_msg_data, offset_size as usize, length_size)? as usize;
-                    let cont_msgs = Self::parse_v1_continuation(
-                        data,
-                        cont_offset,
-                        cont_length,
-                        offset_size,
-                        length_size,
-                        depth_remaining - 1,
-                    )?;
-                    messages.extend(cont_msgs);
-                }
+                let cont_offset = read_offset(body, 0, offset_size)? as usize;
+                let cont_length = read_offset(body, offset_size as usize, length_size)? as usize;
+                Self::parse_v1_chunk(
+                    data,
+                    cont_offset,
+                    cont_length,
+                    offset_size,
+                    length_size,
+                    depth_remaining - 1,
+                    messages,
+                )?;
             }
         }
 
-        Ok(messages)
+        Ok(count)
     }
 
     fn parse_v2(
@@ -278,6 +262,11 @@ impl ObjectHeader {
             return Err(FormatError::InvalidObjectHeaderVersion(version));
         }
         let flags = data[offset + 5];
+        if flags & !V2_HDR_ALL_FLAGS != 0 {
+            return Err(FormatError::InvalidObjectHeader(
+                "unknown object header status flag(s)",
+            ));
+        }
 
         let mut pos = offset + 6;
 
@@ -297,7 +286,14 @@ impl ObjectHeader {
         // Optional attribute storage thresholds (flags bit 4)
         if flags & 0x10 != 0 {
             ensure_len(data, pos, 4)?;
-            // max_compact_attrs(2) + min_dense_attrs(2) — read but don't store for now
+            // max_compact_attrs(2) + min_dense_attrs(2) — checked, not stored
+            let max_compact = LittleEndian::read_u16(&data[pos..pos + 2]);
+            let min_dense = LittleEndian::read_u16(&data[pos + 2..pos + 4]);
+            if max_compact < min_dense {
+                return Err(FormatError::InvalidObjectHeader(
+                    "bad object header attribute phase change values",
+                ));
+            }
             pos += 4;
         }
 
@@ -312,6 +308,14 @@ impl ObjectHeader {
         ensure_len(data, pos, chunk_size_width as usize)?;
         let chunk0_size = read_offset(data, pos, chunk_size_width)? as usize;
         pos += chunk_size_width as usize;
+        // Bit 2: attribute creation order tracked → messages include creation order field
+        let has_creation_order = flags & 0x04 != 0;
+        let msg_header_size = if has_creation_order { 6 } else { 4 };
+        if chunk0_size > 0 && chunk0_size < msg_header_size {
+            return Err(FormatError::InvalidObjectHeader(
+                "bad object header chunk size",
+            ));
+        }
 
         let chunk0_msg_start = pos;
         let chunk0_msg_end = pos
@@ -335,9 +339,6 @@ impl ObjectHeader {
             }
         }
 
-        // Bit 2: attribute creation order tracked → messages include creation order field
-        let has_creation_order = flags & 0x04 != 0;
-
         // Parse messages from chunk0
         let mut messages = Vec::new();
         let mut continuations = Vec::new();
@@ -396,8 +397,20 @@ impl ObjectHeader {
     ) -> Result<(), FormatError> {
         let msg_header_size = if has_creation_order { 6 } else { 4 };
         let mut pos = start;
+        let mut null_count = 0usize;
 
-        while pos + msg_header_size <= end {
+        while pos < end {
+            // Leftover bytes too few for a message header are a gap, which
+            // libhdf5 allows only in a chunk without NIL messages (a writer
+            // that leaves a gap had no NIL message to put the space in).
+            if end - pos < msg_header_size {
+                if null_count != 0 {
+                    return Err(FormatError::InvalidObjectHeader(
+                        "gap in chunk with no null messages",
+                    ));
+                }
+                break;
+            }
             let msg_type_raw = data[pos] as u16;
             let msg_data_size = LittleEndian::read_u16(&data[pos + 1..pos + 3]) as usize;
             let msg_flags = data[pos + 3];
@@ -408,32 +421,36 @@ impl ObjectHeader {
             };
             pos += msg_header_size;
 
-            if pos + msg_data_size > end {
-                // Could be padding at end of chunk
-                break;
+            // `end` is where the messages stop and the checksum starts.
+            // libhdf5 bounds a message by the chunk including its checksum,
+            // but a message that runs into the checksum still fails there:
+            // its loop stops at the checksum, and reading the checksum from
+            // past its start overruns the chunk ("ran off end of input
+            // buffer while decoding"). Both refuse it; only the text
+            // differs.
+            if msg_data_size > end - pos {
+                return Err(FormatError::InvalidObjectHeader(
+                    "message size exceeds buffer end",
+                ));
             }
+            let body = &data[pos..pos + msg_data_size];
+            check_message(2, msg_type_raw, msg_flags, body, offset_size, length_size)?;
 
             let msg_type = MessageType::from_u16(msg_type_raw);
-
-            check_unknown_message(msg_type, msg_flags)?;
-
-            let msg_data = data[pos..pos + msg_data_size].to_vec();
-
             if msg_type == MessageType::ObjectHeaderContinuation {
-                // Parse continuation offset/length from message data
-                if msg_data.len() >= (offset_size as usize + length_size as usize) {
-                    let cont_off = read_offset(&msg_data, 0, offset_size)? as usize;
-                    let cont_len =
-                        read_offset(&msg_data, offset_size as usize, length_size)? as usize;
-                    continuations.push((cont_off, cont_len));
-                }
-            } else if msg_type != MessageType::Nil {
+                // check_message has checked the body holds both fields.
+                let cont_off = read_offset(body, 0, offset_size)? as usize;
+                let cont_len = read_offset(body, offset_size as usize, length_size)? as usize;
+                continuations.push((cont_off, cont_len));
+            } else if msg_type == MessageType::Nil {
+                null_count += 1;
+            } else {
                 messages.push(HeaderMessage {
                     msg_type,
                     size: msg_data_size,
                     flags: msg_flags,
                     creation_order,
-                    data: msg_data,
+                    data: body.to_vec(),
                 });
             }
 
@@ -496,22 +513,149 @@ impl ObjectHeader {
     }
 }
 
-/// Header message flag bit 7: fail if the message is unknown, always.
+/// Size of a version-1 message header: type(2) + size(2) + flags(1) + reserved(3).
+const V1_MSG_HEADER_SIZE: usize = 8;
+
+/// How deep version-1 continuation chunks may chain (malformed-data guard).
+const MAX_V1_CONTINUATION_DEPTH: u16 = 32;
+
+/// Every defined version-2 object header status flag (libhdf5
+/// `H5O_HDR_ALL_FLAGS`): chunk-0 size width (bits 0-1), attribute creation
+/// order tracked/indexed, attribute phase-change values, times stored.
+const V2_HDR_ALL_FLAGS: u8 = 0x3F;
+
+// Header message flag bits (libhdf5 `H5O_MSG_FLAG_*`). Bit 0 (constant) needs
+// no check. Bit 3 (fail if unknown and the file is opened for writing) never
+// fails a read: the parser only ever reads, as libhdf5 ignores it for a
+// read-only open.
+const MSG_FLAG_SHARED: u8 = 0x02;
+const MSG_FLAG_DONTSHARE: u8 = 0x04;
+const MSG_FLAG_FAIL_IF_UNKNOWN_AND_OPEN_FOR_WRITE: u8 = 0x08;
+const MSG_FLAG_MARK_IF_UNKNOWN: u8 = 0x10;
+const MSG_FLAG_WAS_UNKNOWN: u8 = 0x20;
+const MSG_FLAG_SHAREABLE: u8 = 0x40;
+/// Fail if the message is unknown, whatever the access mode.
 const MSG_FLAG_FAIL_IF_UNKNOWN_ALWAYS: u8 = 0x80;
 
-/// Refuse an unknown message the file says no reader may skip.
+/// Message type ids libhdf5 has a class for (`H5O_msg_class_g`): 0x00-0x18
+/// except 0x09 (a test-only "bogus" message). Anything else is an unknown
+/// message.
+fn is_known_message(id: u16) -> bool {
+    id <= 0x18 && id != 0x09
+}
+
+/// Message classes that may be shared (`H5O_SHARE_IS_SHARABLE`): dataspace,
+/// datatype, the two fill-value messages, filter pipeline and attribute.
+fn is_shareable_message(id: u16) -> bool {
+    matches!(id, 0x01 | 0x03 | 0x04 | 0x05 | 0x0B | 0x0C)
+}
+
+/// Check one header message the way libhdf5 does while it loads an object
+/// header (`H5O__chunk_deserialize`), so an object libhdf5 refuses to open is
+/// refused here too instead of being read from a corrupt header:
 ///
-/// The parser only ever reads, so bit 3 (fail only when opened for writing)
-/// is ignored, as libhdf5 ignores it for a read-only open; bit 7 fails
-/// regardless of access mode. This had the two the wrong way round, failing
-/// objects libhdf5 reads and reading ones it refuses (`tbogus.h5`).
-fn check_unknown_message(msg_type: MessageType, msg_flags: u8) -> Result<(), FormatError> {
-    match msg_type {
-        MessageType::Unknown(id) if msg_flags & MSG_FLAG_FAIL_IF_UNKNOWN_ALWAYS != 0 => {
-            Err(FormatError::UnsupportedMessage(id))
-        }
-        _ => Ok(()),
+/// - contradictory flag combinations;
+/// - an unknown message the file says no reader may skip (bit 7). This had
+///   bits 3 and 7 the wrong way round once, failing objects libhdf5 reads
+///   and reading ones it refuses (`tbogus.h5`);
+/// - a known message whose class cannot be shared, flagged shared or
+///   shareable (`cve-2016-4332`);
+/// - the messages libhdf5 decodes while loading the header, whose decode
+///   errors fail the load: continuation, reference count (which a version-1
+///   header cannot hold), and both modification-time messages.
+fn check_message(
+    header_version: u8,
+    id: u16,
+    flags: u8,
+    body: &[u8],
+    offset_size: u8,
+    length_size: u8,
+) -> Result<(), FormatError> {
+    let bad_flags = FormatError::InvalidObjectHeader("bad flag combination for message");
+    if flags & MSG_FLAG_SHARED != 0 && flags & MSG_FLAG_DONTSHARE != 0 {
+        return Err(bad_flags);
     }
+    if flags & MSG_FLAG_WAS_UNKNOWN != 0
+        && (flags & MSG_FLAG_FAIL_IF_UNKNOWN_AND_OPEN_FOR_WRITE != 0
+            || flags & MSG_FLAG_MARK_IF_UNKNOWN == 0)
+    {
+        return Err(bad_flags);
+    }
+
+    if !is_known_message(id) {
+        if flags & MSG_FLAG_FAIL_IF_UNKNOWN_ALWAYS != 0 {
+            return Err(FormatError::UnsupportedMessage(id));
+        }
+        return Ok(());
+    }
+    if flags & (MSG_FLAG_SHARED | MSG_FLAG_SHAREABLE) != 0 && !is_shareable_message(id) {
+        return Err(FormatError::InvalidObjectHeader(
+            "message of unshareable class flagged as shareable",
+        ));
+    }
+
+    let overrun = FormatError::InvalidObjectHeader("ran off end of input buffer while decoding");
+    match id {
+        // Continuation: address + length, and the chunk cannot be empty.
+        0x10 => {
+            if body.len() < offset_size as usize + length_size as usize {
+                return Err(overrun);
+            }
+            if read_offset(body, offset_size as usize, length_size)? == 0 {
+                return Err(FormatError::InvalidObjectHeader(
+                    "invalid continuation chunk size (0)",
+                ));
+            }
+        }
+        // Reference count: version-2 headers only; version 0 then a u32.
+        0x16 => {
+            if header_version == 1 {
+                return Err(FormatError::InvalidObjectHeader(
+                    "object header version does not support reference count message",
+                ));
+            }
+            match body.first() {
+                None => return Err(overrun),
+                Some(0) => {}
+                Some(_) => {
+                    return Err(FormatError::InvalidObjectHeader(
+                        "bad version number for reference count message",
+                    ));
+                }
+            }
+            if body.len() < 5 {
+                return Err(overrun);
+            }
+        }
+        // Old modification time: "YYYYMMDDhhmmss" and 2 reserved bytes.
+        0x0E => {
+            if body.len() < 16 {
+                return Err(overrun);
+            }
+            if !body[..14].iter().all(u8::is_ascii_digit) {
+                return Err(FormatError::InvalidObjectHeader(
+                    "badly formatted modification time message",
+                ));
+            }
+        }
+        // New modification time: version 1, 3 reserved bytes, u32 seconds.
+        0x12 => {
+            match body.first() {
+                None => return Err(overrun),
+                Some(1) => {}
+                Some(_) => {
+                    return Err(FormatError::InvalidObjectHeader(
+                        "bad version number for mtime message",
+                    ));
+                }
+            }
+            if body.len() < 8 {
+                return Err(overrun);
+            }
+        }
+        _ => {}
+    }
+    Ok(())
 }
 
 #[cfg(test)]
@@ -528,11 +672,14 @@ mod tests {
         // Calculate total header message data size
         let mut msg_bytes = Vec::new();
         for (mtype, mdata, mflags) in messages {
+            // v1 message sizes are multiples of 8 (the data is zero-padded).
+            let padded = mdata.len().div_ceil(8) * 8;
             msg_bytes.extend_from_slice(&mtype.to_le_bytes()); // type(2)
-            msg_bytes.extend_from_slice(&(mdata.len() as u16).to_le_bytes()); // size(2)
+            msg_bytes.extend_from_slice(&(padded as u16).to_le_bytes()); // size(2)
             msg_bytes.push(*mflags); // flags(1)
             msg_bytes.extend_from_slice(&[0u8; 3]); // reserved(3)
             msg_bytes.extend_from_slice(mdata); // data
+            msg_bytes.resize(msg_bytes.len() + padded - mdata.len(), 0);
         }
 
         let mut buf = Vec::new();
@@ -622,9 +769,10 @@ mod tests {
         let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
         assert_eq!(hdr.messages.len(), 2);
         assert_eq!(hdr.messages[0].msg_type, MessageType::Dataspace);
-        assert_eq!(hdr.messages[0].data, vec![1, 2, 3, 4]);
+        // v1 message data is padded to a multiple of 8 bytes.
+        assert_eq!(hdr.messages[0].data, vec![1, 2, 3, 4, 0, 0, 0, 0]);
         assert_eq!(hdr.messages[1].msg_type, MessageType::DataLayout);
-        assert_eq!(hdr.messages[1].data, vec![5, 6]);
+        assert_eq!(hdr.messages[1].data[..2], [5, 6]);
     }
 
     #[test]
@@ -650,7 +798,7 @@ mod tests {
         // Bit 3 = fail if unknown *and the file is opened for writing*. This
         // parser only reads, so libhdf5 (read-only) opens such an object and
         // so must we. Bits 4/5 (mark if unknown / was unknown) never fail.
-        for flags in [0x08u8, 0x10, 0x20, 0x38] {
+        for flags in [0x08u8, 0x10, 0x30] {
             let messages = [(0x00FFu16, &[0xAA][..], flags)];
             let data = build_v1_header(&messages, 8, 8);
             let hdr = ObjectHeader::parse(&data, 0, 8, 8).unwrap();
@@ -658,6 +806,231 @@ mod tests {
         }
     }
 
+    #[test]
+    fn contradictory_message_flags_are_refused() {
+        // libhdf5: "bad flag combination for message" for shared + don't
+        // share, was-unknown without mark-if-unknown, and was-unknown with
+        // fail-if-unknown-on-write.
+        for flags in [0x06u8, 0x20, 0x38] {
+            let data = build_v1_header(&[(0x00FFu16, &[0xAA][..], flags)], 8, 8);
+            assert_eq!(
+                ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+                FormatError::InvalidObjectHeader("bad flag combination for message"),
+                "flags {flags:#x}"
+            );
+            let data = build_v2_header(0x00, &[(0xF0, &[1, 2], flags)], None);
+            assert_eq!(
+                ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+                FormatError::InvalidObjectHeader("bad flag combination for message"),
+                "flags {flags:#x}"
+            );
+        }
+    }
+
+    #[test]
+    fn unshareable_message_flagged_shareable_is_refused() {
+        // A layout (0x08) or modification time (0x12) message cannot be
+        // shared; bit 1 (shared) or bit 6 (shareable) on one is corruption
+        // (cve-2016-4332). A datatype (0x03) may be shareable.
+        let mtime = [1u8, 0, 0, 0, 0x10, 0x20, 0x30, 0x40];
+        for (id, flags) in [(0x08u16, 0x40u8), (0x08, 0x02), (0x12, 0x40)] {
+            let data = build_v1_header(&[(id, &mtime[..], flags)], 8, 8);
+            assert_eq!(
+                ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+                FormatError::InvalidObjectHeader(
+                    "message of unshareable class flagged as shareable"
+                ),
+                "id {id:#x} flags {flags:#x}"
+            );
+        }
+        let data = build_v1_header(&[(0x03, &[0u8; 8][..], 0x40)], 8, 8);
+        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
+        // An unknown message is never checked for shareability.
+        let data = build_v1_header(&[(0x00FF, &[0u8; 8][..], 0x40)], 8, 8);
+        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
+    }
+
+    #[test]
+    fn v1_message_must_be_aligned() {
+        // cve-2018-13873: a v1 message whose size is not a multiple of 8.
+        let mut data = build_v1_header(&[(0x01, &[0u8; 8][..], 0)], 8, 8);
+        data[16 + 2] = 7; // size field of the only message
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("message not aligned")
+        );
+    }
+
+    #[test]
+    fn message_overrunning_its_chunk_is_refused() {
+        // It used to end the chunk quietly, dropping this message and any
+        // after it.
+        let mut data = build_v1_header(&[(0x01, &[0u8; 8][..], 0)], 8, 8);
+        data[16 + 2] = 16;
+        data.resize(data.len() + 64, 0);
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("message size exceeds buffer end")
+        );
+        let mut data = build_v2_header(0x00, &[(0x01, &[1, 2], 0)], None);
+        data[7 + 1] = 9; // size of the only message (after OHDR, ver, flags, chunk size)
+        let chk = crate::checksum::jenkins_lookup3(&data[..data.len() - 4]);
+        let n = data.len();
+        data[n - 4..].copy_from_slice(&chk.to_le_bytes());
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("message size exceeds buffer end")
+        );
+    }
+
+    #[test]
+    fn v1_gap_after_last_message_is_refused() {
+        // Fewer than 8 bytes left over: a gap, which only version 2 allows.
+        let mut data = build_v1_header(&[(0x01, &[0u8; 8][..], 0)], 8, 8);
+        data[8] += 4; // header_data_size
+        data.extend_from_slice(&[0u8; 4]);
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("gap found in early version of file format")
+        );
+    }
+
+    #[test]
+    fn v1_chunk_holding_more_messages_than_the_prefix_says_is_refused() {
+        // cve-2024-32619: the prefix says 1 message, the chunk holds 2. The
+        // second used to be dropped silently.
+        let mut data = build_v1_header(&[(0x01, &[0u8; 8][..], 0), (0x03, &[0u8; 8][..], 0)], 8, 8);
+        data[2] = 1;
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("bad object header message count")
+        );
+        // Fewer in the chunk than the prefix says is fine (the rest may be in
+        // continuation chunks; libhdf5 only enforces that with strict checks).
+        data[2] = 3;
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap().messages.len(),
+            2
+        );
+    }
+
+    #[test]
+    fn v1_prefix_chunk_size_must_fit_the_message_count() {
+        let mut data = build_v1_header(&[], 8, 8);
+        data[8] = 8; // no messages but a non-empty chunk
+        data.extend_from_slice(&[0u8; 8]);
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("bad object header chunk size")
+        );
+    }
+
+    #[test]
+    fn v1_header_cannot_hold_a_reference_count_message() {
+        // cve-2018-11204.
+        let data = build_v1_header(&[(0x16, &[0, 2, 0, 0, 0][..], 0)], 8, 8);
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader(
+                "object header version does not support reference count message"
+            )
+        );
+        let data = build_v2_header(0x00, &[(0x16, &[0, 2, 0, 0, 0], 0)], None);
+        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
+    }
+
+    #[test]
+    fn modification_time_messages_are_decoded_with_the_header() {
+        // cve-2024-33873 (version 0) and cve-2024-33874 (empty message).
+        for (body, why) in [
+            (
+                &[0u8, 0, 0, 0, 1, 2, 3, 4][..],
+                "bad version number for mtime message",
+            ),
+            (&[][..], "ran off end of input buffer while decoding"),
+        ] {
+            let data = build_v2_header(0x00, &[(0x12, body, 0)], None);
+            assert_eq!(
+                ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+                FormatError::InvalidObjectHeader(why)
+            );
+        }
+        let data = build_v2_header(0x00, &[(0x12, &[1, 0, 0, 0, 1, 2, 3, 4], 0)], None);
+        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
+        // The old (0x0E) message is 14 ASCII digits and 2 reserved bytes.
+        let data = build_v1_header(&[(0x0E, &b"20110414214255\0\0"[..], 0)], 8, 8);
+        assert!(ObjectHeader::parse(&data, 0, 8, 8).is_ok());
+        let data = build_v1_header(&[(0x0E, &b"2011041421425x\0\0"[..], 0)], 8, 8);
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("badly formatted modification time message")
+        );
+    }
+
+    #[test]
+    fn continuation_message_must_hold_a_nonempty_chunk() {
+        let mut cont = [0u8; 16];
+        cont[..8].copy_from_slice(&64u64.to_le_bytes());
+        let data = build_v2_header(0x00, &[(0x10, &cont, 0)], None);
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("invalid continuation chunk size (0)")
+        );
+        let data = build_v2_header(0x00, &[(0x10, &cont[..8], 0)], None);
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("ran off end of input buffer while decoding")
+        );
+    }
+
+    #[test]
+    fn v2_prefix_is_checked() {
+        let data = build_v2_header(0x40, &[(0x01, &[1], 0)], None);
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("unknown object header status flag(s)")
+        );
+        // build_v2_header writes max_compact 8, min_dense 6; swap them.
+        let mut data = build_v2_header(0x10, &[(0x01, &[1], 0)], None);
+        data[6] = 6;
+        data[8] = 8;
+        let chk = crate::checksum::jenkins_lookup3(&data[..data.len() - 4]);
+        let n = data.len();
+        data[n - 4..].copy_from_slice(&chk.to_le_bytes());
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("bad object header attribute phase change values")
+        );
+    }
+
+    #[test]
+    fn v2_gap_is_allowed_only_without_nil_messages() {
+        // Three bytes after the last message: a gap (a message header is 4).
+        let mut data = build_v2_header(0x00, &[(0x01, &[1, 2, 3], 0), (0x03, &[], 0)], None);
+        // Turn the empty datatype message (4 header bytes) into a 3-byte gap
+        // by shrinking the chunk.
+        let n = data.len();
+        data.truncate(n - 5);
+        data[6] -= 1;
+        let chk = crate::checksum::jenkins_lookup3(&data);
+        data.extend_from_slice(&chk.to_le_bytes());
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap().messages.len(),
+            1
+        );
+
+        let mut data = build_v2_header(0x00, &[(0x00, &[1, 2, 3], 0), (0x03, &[], 0)], None);
+        let n = data.len();
+        data.truncate(n - 5);
+        data[6] -= 1;
+        let chk = crate::checksum::jenkins_lookup3(&data);
+        data.extend_from_slice(&chk.to_le_bytes());
+        assert_eq!(
+            ObjectHeader::parse(&data, 0, 8, 8).unwrap_err(),
+            FormatError::InvalidObjectHeader("gap in chunk with no null messages")
+        );
+    }
+
     #[test]
     fn parse_v2_unknown_message_flags() {
         let data = build_v2_header(0x00, &[(0xF0, &[1, 2], 0x08)], None);
diff --git a/crates/clawhdf5-format/src/superblock.rs b/crates/clawhdf5-format/src/superblock.rs
index d2ec4d6..a565321 100644
--- a/crates/clawhdf5-format/src/superblock.rs
+++ b/crates/clawhdf5-format/src/superblock.rs
@@ -100,6 +100,42 @@ pub mod swmr_flags {
 }
 
 impl Superblock {
+    /// Where the HDF5 data ends, relative to the superblock, for a file of
+    /// `file_len` bytes whose superblock is at `user_block` (both counted
+    /// from the start of the file), with libhdf5's truncation check
+    /// (`H5F__super_read`).
+    ///
+    /// The superblock records the end of the file's data as an absolute
+    /// address. A file shorter than that was truncated, and libhdf5 refuses
+    /// to open it ("truncated file"); so does this, with
+    /// [`FormatError::TruncatedFile`]. Bytes past that address are not part
+    /// of the file: libhdf5 fails any read of them ("addr overflow" /
+    /// "address plus size exceeds file eoa"), so a reader should parse only
+    /// the data up to the returned end. As libhdf5 does for a SWMR reader,
+    /// the check is skipped for a version-3 superblock whose writer is still
+    /// writing it in SWMR mode (it extends the file as it goes); the data
+    /// then ends at the end of the file.
+    ///
+    /// When the superblock's recorded base address differs from where the
+    /// superblock actually is (a user block added or removed after the file
+    /// was written), libhdf5 moves the recorded end of file by the same
+    /// amount, and so does this.
+    pub fn data_end(&self, user_block: u64, file_len: u64) -> Result<u64, FormatError> {
+        let eof =
+            i128::from(self.eof_address) - i128::from(self.base_address) + i128::from(user_block);
+        if eof < 0 || eof > i128::from(file_len) {
+            if self.version >= 3 && self.is_swmr_write() {
+                return Ok(file_len.saturating_sub(user_block));
+            }
+            return Err(FormatError::TruncatedFile {
+                stored_eof: u64::try_from(eof).unwrap_or(self.eof_address),
+                actual_len: file_len,
+            });
+        }
+        // 0 <= eof <= file_len, so it fits a u64.
+        Ok((eof as u64).saturating_sub(user_block))
+    }
+
     /// Whether the file was opened with write access when the superblock was written.
     pub fn is_write_access(&self) -> bool {
         self.consistency_flags & swmr_flags::WRITE_ACCESS != 0
@@ -537,6 +573,30 @@ mod tests {
         buf
     }
 
+    #[test]
+    fn data_end_refuses_truncated_files_like_libhdf5() {
+        // build_v2_bytes records base 0, end of file 2048.
+        let sb = Superblock::parse(&build_v2_bytes(8, 2), 0).unwrap();
+        assert_eq!(sb.data_end(0, 2048), Ok(2048));
+        // Bytes past the recorded end are not part of the file.
+        assert_eq!(sb.data_end(0, 4096), Ok(2048));
+        assert_eq!(
+            sb.data_end(0, 2047),
+            Err(FormatError::TruncatedFile {
+                stored_eof: 2048,
+                actual_len: 2047
+            })
+        );
+        // A user block added in front after the file was written (the
+        // recorded base address is still 0): the end moves with it.
+        assert_eq!(sb.data_end(512, 2560), Ok(2048));
+        assert!(sb.data_end(512, 2559).is_err());
+        // A v3 superblock still being written in SWMR mode is not checked.
+        let mut swmr = Superblock::parse(&build_v2_bytes(8, 3), 0).unwrap();
+        swmr.consistency_flags = swmr_flags::WRITE_ACCESS | swmr_flags::SWMR_WRITE;
+        assert_eq!(swmr.data_end(0, 1000), Ok(1000));
+    }
+
     #[test]
     fn parse_v0_8byte_offsets() {
         let data = build_v0_bytes(8);
diff --git a/crates/clawhdf5-io/src/async_read.rs b/crates/clawhdf5-io/src/async_read.rs
index d9e2ffe..30969fa 100644
--- a/crates/clawhdf5-io/src/async_read.rs
+++ b/crates/clawhdf5-io/src/async_read.rs
@@ -281,8 +281,13 @@ impl AsyncHDF5File {
         // HDF5 addresses are relative to the superblock: drop any user block
         // so they index `data` directly.
         let user_block = find_signature(&data)?;
+        let whole_len = data.len() as u64;
         data.drain(..user_block);
         let superblock = Superblock::parse(&data, 0)?;
+        // Refuse a truncated file, and keep nothing past the end of file the
+        // superblock records, as libhdf5 does.
+        let end = superblock.data_end(user_block as u64, whole_len)?;
+        data.truncate(end as usize);
         Ok(Self { data, superblock })
     }
 
@@ -312,7 +317,7 @@ impl AsyncHDF5File {
 
         let dt_msg =
             find_msg(&header, MessageType::Datatype).ok_or(FormatError::DatasetMissingData)?;
-        let (datatype, _) = Datatype::parse(&dt_msg.data)?;
+        let (datatype, _) = Datatype::parse_in_header(&dt_msg.data, header.version)?;
 
         let ds_msg =
             find_msg(&header, MessageType::Dataspace).ok_or(FormatError::DatasetMissingShape)?;
@@ -605,6 +610,28 @@ mod tests {
         tokio::fs::remove_file(&path).await.ok();
     }
 
+    /// As libhdf5 does: a truncated file is refused, and bytes past the end
+    /// of file the superblock records are dropped.
+    #[tokio::test]
+    async fn async_refuses_truncated_files_and_drops_trailing_bytes() {
+        let bytes = make_test_hdf5_f64("v", &[1.0, 2.0]);
+        let truncated = bytes[..bytes.len() - 8].to_vec();
+        let err = AsyncHDF5File::from_bytes(truncated).err().unwrap();
+        assert!(
+            matches!(
+                err,
+                AsyncHDF5Error::Format(FormatError::TruncatedFile { .. })
+            ),
+            "{err}"
+        );
+
+        let mut appended = bytes.clone();
+        appended.extend_from_slice(&[0xAB; 64]);
+        let file = AsyncHDF5File::from_bytes(appended).unwrap();
+        assert_eq!(file.as_bytes().len(), bytes.len());
+        assert_eq!(file.read_f64("v").await.unwrap(), vec![1.0, 2.0]);
+    }
+
     #[tokio::test]
     async fn async_error_display() {
         let io_err = AsyncHDF5Error::Io(io::Error::new(io::ErrorKind::NotFound, "gone"));
diff --git a/crates/clawhdf5-io/src/mpi_vol.rs b/crates/clawhdf5-io/src/mpi_vol.rs
index 7dac39e..87a82ea 100644
--- a/crates/clawhdf5-io/src/mpi_vol.rs
+++ b/crates/clawhdf5-io/src/mpi_vol.rs
@@ -180,8 +180,7 @@ fn mpi_collective_read(vol: &MpiVol, location: &str, path: &str) -> Result<Vec<u
     use clawhdf5_format::{
         data_layout::DataLayout, data_read::read_raw_data_full, dataspace::Dataspace,
         datatype::Datatype, filter_pipeline::FilterPipeline, group_v2::resolve_path_any,
-        message_type::MessageType, object_header::ObjectHeader, signature::split_user_block,
-        superblock::Superblock,
+        message_type::MessageType, object_header::ObjectHeader,
     };
     use mpi::traits::*;
 
@@ -193,9 +192,9 @@ fn mpi_collective_read(vol: &MpiVol, location: &str, path: &str) -> Result<Vec<u
 
     if rank == 0 {
         let file = std::fs::read(location).map_err(VolError::Io)?;
-        // Addresses are relative to the superblock: skip any user block.
-        let (_, bytes) = split_user_block(&file).map_err(|e| VolError::DataError(e.to_string()))?;
-        let sb = Superblock::parse(bytes, 0).map_err(|e| VolError::DataError(e.to_string()))?;
+        // From the superblock to the recorded end of file; truncated files
+        // are refused.
+        let (bytes, sb) = crate::vol::hdf5_view(&file)?;
         let addr = resolve_path_any(&bytes, &sb, path)
             .map_err(|e| VolError::NotFound(format!("{path}: {e}")))?;
         let oh = ObjectHeader::parse(&bytes, addr as usize, sb.offset_size, sb.length_size)
@@ -205,8 +204,8 @@ fn mpi_collective_read(vol: &MpiVol, location: &str, path: &str) -> Result<Vec<u
             .iter()
             .find(|m| m.msg_type == MessageType::Datatype)
             .ok_or_else(|| VolError::DataError("no datatype".into()))?;
-        let (datatype, _) =
-            Datatype::parse(&dt.data).map_err(|e| VolError::DataError(e.to_string()))?;
+        let (datatype, _) = Datatype::parse_in_header(&dt.data, oh.version)
+            .map_err(|e| VolError::DataError(e.to_string()))?;
         let ds = oh
             .messages
             .iter()
diff --git a/crates/clawhdf5-io/src/vol.rs b/crates/clawhdf5-io/src/vol.rs
index 47e1388..2699aae 100644
--- a/crates/clawhdf5-io/src/vol.rs
+++ b/crates/clawhdf5-io/src/vol.rs
@@ -210,6 +210,24 @@ pub struct NativeVol {
     location: Option<String>,
 }
 
+/// The HDF5 bytes of a whole file and its superblock: from the superblock
+/// (addresses are relative to it, so any user block is skipped) to the end
+/// of file the superblock records. A file shorter than that is truncated
+/// and refused, and nothing past it is read, as in libhdf5.
+pub(crate) fn hdf5_view(
+    whole: &[u8],
+) -> Result<(&[u8], clawhdf5_format::superblock::Superblock), VolError> {
+    use clawhdf5_format::{signature::split_user_block, superblock::Superblock};
+    let err = |e: clawhdf5_format::error::FormatError| VolError::DataError(e.to_string());
+    let (user_block, data) = split_user_block(whole).map_err(err)?;
+    let sb = Superblock::parse(data, 0).map_err(err)?;
+    let end = sb
+        .data_end(user_block.len() as u64, whole.len() as u64)
+        .map_err(err)?;
+    // data_end is at most the file length less the user block.
+    Ok((&data[..end as usize], sb))
+}
+
 impl NativeVol {
     /// Create a new native VOL connector.
     pub fn new() -> Self {
@@ -264,6 +282,8 @@ impl VirtualObjectLayer for NativeVol {
 
     fn open(&mut self, location: &str) -> Result<(), VolError> {
         let data = std::fs::read(location)?;
+        // Refuse a truncated file at open, as libhdf5 does.
+        hdf5_view(&data)?;
         self.data = Some(data);
         self.location = Some(location.to_string());
         Ok(())
@@ -283,13 +303,10 @@ impl VirtualObjectLayer for NativeVol {
         use clawhdf5_format::{
             data_layout::DataLayout, data_read::read_raw_data_full, dataspace::Dataspace,
             datatype::Datatype, filter_pipeline::FilterPipeline, group_v2::resolve_path_any,
-            message_type::MessageType, object_header::ObjectHeader, signature::split_user_block,
-            superblock::Superblock,
+            message_type::MessageType, object_header::ObjectHeader,
         };
 
-        // Addresses are relative to the superblock: skip any user block.
-        let (_, data) = split_user_block(data).map_err(|e| VolError::DataError(e.to_string()))?;
-        let sb = Superblock::parse(data, 0).map_err(|e| VolError::DataError(e.to_string()))?;
+        let (data, sb) = hdf5_view(data)?;
         let addr = resolve_path_any(data, &sb, path)
             .map_err(|e| VolError::NotFound(format!("{path}: {e}")))?;
 
@@ -301,8 +318,8 @@ impl VirtualObjectLayer for NativeVol {
             .iter()
             .find(|m| m.msg_type == MessageType::Datatype)
             .ok_or_else(|| VolError::DataError("missing datatype".into()))?;
-        let (datatype, _) =
-            Datatype::parse(&dt_msg.data).map_err(|e| VolError::DataError(e.to_string()))?;
+        let (datatype, _) = Datatype::parse_in_header(&dt_msg.data, header.version)
+            .map_err(|e| VolError::DataError(e.to_string()))?;
 
         let ds_msg = header
             .messages
@@ -387,6 +404,36 @@ mod tests {
         assert!(vol.as_bytes().is_none());
     }
 
+    /// As libhdf5 does: a file shorter than the end of file its superblock
+    /// records is truncated and refused (at open, and when read from
+    /// memory), and bytes appended past that end are not part of the file.
+    #[test]
+    fn native_vol_refuses_truncated_files_and_ignores_trailing_bytes() {
+        use clawhdf5_format::file_writer::FileWriter as FmtWriter;
+
+        let mut fw = FmtWriter::new();
+        fw.create_dataset("x").with_f64_data(&[1.0, 2.0, 3.0]);
+        let bytes = fw.finish().unwrap();
+
+        let truncated = bytes[..bytes.len() - 8].to_vec();
+        let err = NativeVol::from_bytes(truncated.clone())
+            .read_dataset("x")
+            .unwrap_err();
+        assert!(err.to_string().contains("truncated"), "{err}");
+        let dir = std::env::temp_dir().join(format!("clawhdf5_vol_trunc_{}", std::process::id()));
+        std::fs::create_dir_all(&dir).unwrap();
+        let path = dir.join("truncated.h5");
+        std::fs::write(&path, &truncated).unwrap();
+        let err = NativeVol::open_path(path.to_str().unwrap()).err().unwrap();
+        assert!(err.to_string().contains("truncated"), "{err}");
+        std::fs::remove_dir_all(&dir).ok();
+
+        let mut appended = bytes.clone();
+        appended.extend_from_slice(&[0xAB; 64]);
+        let raw = NativeVol::from_bytes(appended).read_dataset("x").unwrap();
+        assert_eq!(raw.len(), 24);
+    }
+
     #[test]
     fn vol_error_display() {
         let err = VolError::Unsupported("read_dataset".into());
diff --git a/crates/clawhdf5/src/lazy.rs b/crates/clawhdf5/src/lazy.rs
index 75ff609..d893485 100644
--- a/crates/clawhdf5/src/lazy.rs
+++ b/crates/clawhdf5/src/lazy.rs
@@ -43,6 +43,8 @@ pub struct LazyFile<R: HDF5Read> {
     /// Offset of the superblock in the file (the user-block size); every
     /// HDF5 address is relative to it.
     base: usize,
+    /// End of the HDF5 data (`Superblock::data_end`, absolute).
+    end: usize,
     superblock: Superblock,
     root_header: ObjectHeader,
     /// Cache of parsed object headers, keyed by address.
@@ -74,9 +76,13 @@ impl<R: HDF5Read> LazyFile<R> {
     ///
     /// Parses only the superblock and root group object header.
     pub fn open(reader: R) -> Result<Self, Error> {
+        let whole_len = reader.as_bytes().len() as u64;
         let (user_block, data) = signature::split_user_block(reader.as_bytes())?;
         let base = user_block.len();
         let superblock = Superblock::parse(data, 0)?;
+        // Refuse a truncated file; read nothing past the recorded end of file.
+        let end = base + superblock.data_end(base as u64, whole_len)? as usize;
+        let data = &reader.as_bytes()[base..end];
         let root_header = ObjectHeader::parse(
             data,
             superblock.root_group_address as usize,
@@ -86,6 +92,7 @@ impl<R: HDF5Read> LazyFile<R> {
         Ok(Self {
             reader,
             base,
+            end,
             superblock,
             root_header,
             header_cache: RefCell::new(HashMap::new()),
@@ -104,7 +111,7 @@ impl<R: HDF5Read> LazyFile<R> {
     }
 
     fn hdf5_bytes(&self) -> &[u8] {
-        &self.reader.as_bytes()[self.base..]
+        &self.reader.as_bytes()[self.base..self.end]
     }
 
     /// Returns a reference to the parsed superblock.
@@ -479,7 +486,7 @@ impl<'f, R: HDF5Read> LazyDataset<'f, R> {
 
     fn datatype(&self) -> Result<Datatype, Error> {
         let data = self.required_payload(MessageType::Datatype)?;
-        let (dt, _) = Datatype::parse(&data)?;
+        let (dt, _) = Datatype::parse_in_header(&data, self.header.version)?;
         Ok(dt)
     }
 
diff --git a/crates/clawhdf5/src/mmap_file.rs b/crates/clawhdf5/src/mmap_file.rs
index eeb1efb..7f544ca 100644
--- a/crates/clawhdf5/src/mmap_file.rs
+++ b/crates/clawhdf5/src/mmap_file.rs
@@ -35,6 +35,8 @@ pub struct MmapFile {
     /// Offset of the superblock in the mapped file (the user-block size);
     /// every HDF5 address is relative to it.
     base: usize,
+    /// End of the HDF5 data (`Superblock::data_end`, absolute).
+    end: usize,
     superblock: Superblock,
 }
 
@@ -42,12 +44,16 @@ impl MmapFile {
     /// Open an HDF5 file using memory-mapped I/O.
     pub fn open<P: AsRef<std::path::Path>>(path: P) -> Result<Self, Error> {
         let reader = MmapReader::open(path).map_err(Error::Io)?;
+        let whole_len = reader.as_bytes().len() as u64;
         let (user_block, data) = signature::split_user_block(reader.as_bytes())?;
         let base = user_block.len();
         let superblock = Superblock::parse(data, 0)?;
+        // Refuse a truncated file; read nothing past the recorded end of file.
+        let end = base + superblock.data_end(base as u64, whole_len)? as usize;
         Ok(Self {
             reader,
             base,
+            end,
             superblock,
         })
     }
@@ -55,7 +61,7 @@ impl MmapFile {
     /// The file's bytes from the superblock on — the space HDF5 addresses
     /// index into.
     fn hdf5_bytes(&self) -> &[u8] {
-        &self.reader.as_bytes()[self.base..]
+        &self.reader.as_bytes()[self.base..self.end]
     }
 
     /// Size of the user block before the superblock (0 for most files).
@@ -426,7 +432,7 @@ impl<'f> MmapDataset<'f> {
 
     fn datatype(&self) -> Result<Datatype, Error> {
         let data = self.required_payload(MessageType::Datatype)?;
-        let (dt, _) = Datatype::parse(&data)?;
+        let (dt, _) = Datatype::parse_in_header(&data, self.header.version)?;
         Ok(dt)
     }
 
diff --git a/crates/clawhdf5/src/reader.rs b/crates/clawhdf5/src/reader.rs
index 65c0292..6e506b4 100644
--- a/crates/clawhdf5/src/reader.rs
+++ b/crates/clawhdf5/src/reader.rs
@@ -45,26 +45,34 @@ impl Backing {
     }
 }
 
-/// The file's bytes, viewed from the superblock on. A file may start with a
-/// user block (the superblock at 512, 1024, …); every HDF5 address is
-/// relative to the superblock, so all parsing goes through [`Self::as_bytes`].
+/// The file's bytes, viewed from the superblock on and up to the end of
+/// file the superblock records. A file may start with a user block (the
+/// superblock at 512, 1024, …); every HDF5 address is relative to the
+/// superblock, so all parsing goes through [`Self::as_bytes`].
 struct FileData {
     backing: Backing,
     /// Offset of the superblock in the file (the user-block size).
     base: usize,
+    /// End of the HDF5 data in the file (`Superblock::data_end`, absolute).
+    end: usize,
 }
 
 impl FileData {
-    /// Locate the superblock and parse it.
+    /// Locate the superblock and parse it. A truncated file is refused, and
+    /// bytes past the recorded end of file are not read, as in libhdf5.
     fn new(backing: Backing) -> Result<(Self, Superblock), Error> {
-        let (user_block, hdf5) = signature::split_user_block(backing.whole_file())?;
+        let whole = backing.whole_file();
+        let (user_block, hdf5) = signature::split_user_block(whole)?;
         let base = user_block.len();
         let superblock = Superblock::parse(hdf5, 0)?;
-        Ok((Self { backing, base }, superblock))
+        let end = superblock.data_end(base as u64, whole.len() as u64)?;
+        // data_end is at most the file length (less the user block).
+        let end = base + end as usize;
+        Ok((Self { backing, base, end }, superblock))
     }
 
     fn as_bytes(&self) -> &[u8] {
-        &self.backing.whole_file()[self.base..]
+        &self.backing.whole_file()[self.base..self.end]
     }
 
     fn len(&self) -> usize {
@@ -850,7 +858,7 @@ impl<'f> Dataset<'f> {
 
     fn datatype(&self) -> Result<Datatype, Error> {
         let data = self.required_payload(MessageType::Datatype)?;
-        let (dt, _) = Datatype::parse(&data)?;
+        let (dt, _) = Datatype::parse_in_header(&data, self.header.version)?;
         Ok(dt)
     }
 
diff --git a/crates/clawhdf5/tests/chunk_index_interop.rs b/crates/clawhdf5/tests/chunk_index_interop.rs
index b72b7e2..3ff0420 100644
--- a/crates/clawhdf5/tests/chunk_index_interop.rs
+++ b/crates/clawhdf5/tests/chunk_index_interop.rs
@@ -515,6 +515,31 @@ fn we_write_maxshape_larger_than_shape() {
     check_we_write(&cases);
 }
 
+/// A version-4 layout must encode every chunk dimension in the fewest bytes
+/// that hold the largest one, as libhdf5 does: HDF5 2.0.0 (h5py 3.16)
+/// refuses a wider encoding ("stored chunk dimension encoding length does
+/// not match value calculated from chunk dimensions"). We rounded 3 bytes
+/// up to 4, so h5py could not open any dataset we wrote with a chunk
+/// dimension from 65 536 to 16 777 215.
+#[test]
+fn we_write_chunk_dimensions_in_the_fewest_bytes() {
+    const U: u64 = u64::MAX;
+    let mut cases = vec![
+        // Single chunk, Fixed Array, Extensible Array, v2 B-tree.
+        wcase("single_70000", &[70_000], &[70_000], None),
+        wcase("fa_70000", &[140_000], &[70_000], None),
+        wcase("ea_70000", &[140_000], &[70_000], Some(&[U])),
+        wcase("bt2_70000", &[2, 70_000], &[1, 70_000], Some(&[U, U])),
+        // 2 bytes and 1 byte still, with the element size (4) the largest.
+        wcase("fa_300", &[600], &[300], None),
+        wcase("fa_3", &[6], &[3], None),
+    ];
+    let mut filtered = wcase("single_70000_deflate", &[70_000], &[70_000], None);
+    filtered.deflate = true;
+    cases.push(filtered);
+    check_we_write(&cases);
+}
+
 /// More than one unlimited dimension needs a version-2 B-tree chunk index,
 /// as the library uses; an Extensible Array for `(None, None)` made libhdf5
 /// refuse the whole file ("already found unlimited dimension").
diff --git a/crates/clawhdf5/tests/fixtures/written_by_v2_7_0.h5 b/crates/clawhdf5/tests/fixtures/written_by_v2_7_0.h5
new file mode 100644
index 0000000..ebb7cf1
Binary files /dev/null and b/crates/clawhdf5/tests/fixtures/written_by_v2_7_0.h5 differ
diff --git a/crates/clawhdf5/tests/fixtures/written_by_v2_7_0_paged.h5 b/crates/clawhdf5/tests/fixtures/written_by_v2_7_0_paged.h5
new file mode 100644
index 0000000..8f83714
Binary files /dev/null and b/crates/clawhdf5/tests/fixtures/written_by_v2_7_0_paged.h5 differ
diff --git a/crates/clawhdf5/tests/h5py_chunked_read_tests.rs b/crates/clawhdf5/tests/h5py_chunked_read_tests.rs
index 2b9e902..40bfbf5 100644
--- a/crates/clawhdf5/tests/h5py_chunked_read_tests.rs
+++ b/crates/clawhdf5/tests/h5py_chunked_read_tests.rs
@@ -343,3 +343,41 @@ print("OK")
     let want: Vec<u8> = line[990..].iter().flat_map(|v| v.to_le_bytes()).collect();
     assert_eq!(tail, want);
 }
+
+// ---------------------------------------------------------------------------
+// Chunk dimension widths in layout version 4
+// ---------------------------------------------------------------------------
+
+/// A version-4 layout stores every chunk dimension in the fewest bytes that
+/// hold the largest one (the element size included). A chunk dimension of
+/// 70 000 takes 3 bytes and 2^32 + 1 elements would take 5; widths other
+/// than 1, 2, 4 and 8 were refused, so these h5py files did not open.
+#[test]
+fn h5py_layout_v4_chunk_dimensions_of_3_bytes_read() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("enc3.h5");
+    let p = path.display().to_string();
+    run_python(&format!(
+        r#"
+import h5py, numpy as np
+with h5py.File("{p}", "w", libver="latest") as f:
+    f.create_dataset("single", data=np.arange(70000, dtype="<u4"), chunks=(70000,))
+    ds = f.create_dataset("ea", shape=(10,), maxshape=(None,), chunks=(70000,), dtype="<f8")
+    ds[:] = np.arange(10.0)
+    f.create_dataset("fa", data=np.arange(3 * 70000, dtype="<i4").reshape(3, 70000),
+                     chunks=(1, 70000), compression="gzip")
+with h5py.File("{p}", "r") as f:
+    raw = open("{p}", "rb").read()
+    # version 4, class 2 (chunked), flags, 2 or 3 dims, then 3 bytes each
+    assert raw.find(bytes([4, 2, 0, 2, 3])) > 0 or raw.find(bytes([4, 2, 1, 2, 3])) > 0
+"#
+    ));
+    let file = File::open(&path).unwrap();
+    let single = file.dataset("single").unwrap().read_u64().unwrap();
+    assert!(single.iter().copied().eq(0..70000), "single");
+    let ea = file.dataset("ea").unwrap().read_f64().unwrap();
+    assert_eq!(ea, (0..10).map(f64::from).collect::<Vec<_>>());
+    let fa = file.dataset("fa").unwrap().read_i32().unwrap();
+    assert!(fa.iter().copied().eq(0..3 * 70000), "fa");
+}
diff --git a/crates/clawhdf5/tests/header_validation_interop.rs b/crates/clawhdf5/tests/header_validation_interop.rs
new file mode 100644
index 0000000..2f52d27
--- /dev/null
+++ b/crates/clawhdf5/tests/header_validation_interop.rs
@@ -0,0 +1,510 @@
+//! Corrupt files that libhdf5 refuses must be refused here too, not read.
+//!
+//! h5py writes a valid file, the script corrupts a copy the way a damaged or
+//! malicious file would be, and records whether h5py (libhdf5) still opens
+//! and reads the object. clawhdf5 must agree: read the valid file, refuse
+//! each corrupt one. Skipped when python3 with h5py is unavailable, unless
+//! `CLAWHDF5_REQUIRE_INTEROP=1`.
+
+use std::path::Path;
+use std::process::Command;
+
+use clawhdf5::File;
+
+fn python() -> String {
+    std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
+}
+
+fn interop_required() -> bool {
+    std::env::var("CLAWHDF5_REQUIRE_INTEROP").is_ok_and(|v| v == "1")
+}
+
+fn python_available() -> bool {
+    Command::new(python())
+        .args(["-c", "import h5py, numpy"])
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false)
+}
+
+macro_rules! skip_if_no_python {
+    () => {
+        if !python_available() {
+            assert!(
+                !interop_required(),
+                "CLAWHDF5_REQUIRE_INTEROP=1 but python3 with h5py is not available"
+            );
+            eprintln!("SKIP: python3 with h5py not available");
+            return;
+        }
+    };
+}
+
+/// Runs `body` (Python, with `h5py`, `numpy as np`, `struct` imported and
+/// `d` the output directory) and then, for every `NAME.h5` it wrote,
+/// prints `NAME ok` when h5py opens and reads dataset `d` and `NAME ERROR`
+/// otherwise. Returns those lines, sorted.
+fn h5py_verdicts(dir: &Path, body: &str) -> Vec<String> {
+    let script = format!(
+        r#"
+import h5py, numpy as np, struct, os, glob
+d = "{dir}"
+{body}
+for path in sorted(glob.glob(os.path.join(d, "*.h5"))):
+    name = os.path.basename(path)[:-3]
+    try:
+        with h5py.File(path, "r") as f:
+            f["d"][()]
+        print(name, "ok")
+    except Exception:
+        print(name, "ERROR")
+"#,
+        dir = dir.display()
+    );
+    let out = Command::new(python())
+        .args(["-c", &script])
+        .output()
+        .expect("failed to run python");
+    assert!(
+        out.status.success(),
+        "python failed:\n{}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    let mut lines: Vec<String> = String::from_utf8_lossy(&out.stdout)
+        .lines()
+        .map(str::to_owned)
+        .collect();
+    lines.sort();
+    lines
+}
+
+/// Whether clawhdf5 opens and reads dataset `d` of `path` (as raw bytes of
+/// whatever type it has).
+fn clawhdf5_reads(path: &Path) -> Result<(), String> {
+    let file = File::open(path).map_err(|e| format!("open: {e}"))?;
+    let ds = file.dataset("d").map_err(|e| format!("dataset: {e}"))?;
+    ds.dtype().map_err(|e| format!("dtype: {e}"))?;
+    ds.shape().map_err(|e| format!("shape: {e}"))?;
+    file.read_multi(&["d"])
+        .map(|_| ())
+        .map_err(|e| format!("read: {e}"))
+}
+
+/// h5py's verdict for each file must be `expected`, and clawhdf5 must read
+/// exactly the files h5py reads.
+fn assert_agrees_with_h5py(dir: &Path, verdicts: &[String], expected: &[&str]) {
+    assert_eq!(verdicts, expected, "h5py's view changed");
+    for line in verdicts {
+        let (name, verdict) = line.split_once(' ').unwrap();
+        let ours = clawhdf5_reads(&dir.join(format!("{name}.h5")));
+        match verdict {
+            "ok" => assert!(ours.is_ok(), "{name}: h5py reads it, we fail: {ours:?}"),
+            _ => assert!(ours.is_err(), "{name}: h5py refuses it, we read it"),
+        }
+    }
+}
+
+#[test]
+fn chunk_dimensions_libhdf5_refuses_are_refused() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    // A chunked int32 dataset (chunk 37, element size 4 after it in the
+    // layout message), with libver earliest (layout v3) and latest (v4).
+    // The corrupt copies set the chunk dimension to 0 (which read as all
+    // fill values), to 0x80000000 (an 8 GiB chunk) and to 38, which the
+    // chunk index's offsets 37 and 74 are not multiples of (libhdf5: "bad
+    // coordinate offset"; the chunks were read at the wrong place). A v4
+    // layout indexes chunks by position, not offset, but both libraries
+    // refuse the changed chunk grid there too.
+    let verdicts = h5py_verdicts(
+        dir.path(),
+        r#"
+for libver in ("earliest", "latest"):
+    good = os.path.join(d, f"{libver}_good.h5")
+    with h5py.File(good, "w", libver=libver) as f:
+        f.create_dataset("d", data=np.arange(100, dtype="<i4"), chunks=(37,))
+    data = bytearray(open(good, "rb").read())
+    if libver == "earliest":
+        at = data.find(struct.pack("<II", 37, 4))
+        width = 4
+    else:
+        # v4: flags, ndims, bytes per dim, then the dims
+        at = data.find(bytes([4, 2, 0, 2]))
+        width = data[at + 4]
+        at += 5
+    assert at > 0
+    for name, value in (("zero", 0), ("huge", 0x80000000), ("offgrid", 38)):
+        bad = bytearray(data)
+        if value >= 1 << (8 * width):
+            continue
+        bad[at:at + width] = value.to_bytes(width, "little")
+        open(os.path.join(d, f"{libver}_{name}.h5"), "wb").write(bad)
+"#,
+    );
+    assert_agrees_with_h5py(
+        dir.path(),
+        &verdicts,
+        &[
+            "earliest_good ok",
+            "earliest_huge ERROR",
+            "earliest_offgrid ERROR",
+            "earliest_zero ERROR",
+            "latest_good ok",
+            "latest_offgrid ERROR",
+            "latest_zero ERROR",
+        ],
+    );
+}
+
+/// Python: `fix_ohdr(buf, off)` recomputes the Jenkins lookup3 checksum of
+/// the version-2 object header chunk 0 at `off`, so a field can be changed
+/// in a `libver="latest"` file without the checksum failing first.
+const FIX_OHDR_PY: &str = r#"
+def _rot(x, k):
+    return ((x << k) | (x >> (32 - k))) & 0xFFFFFFFF
+def lookup3(data):
+    M = 0xFFFFFFFF
+    n = len(data); a = b = c = (0xDEADBEEF + n) & M; i = 0
+    w = lambda j: int.from_bytes(data[j:j + 4], "little")
+    while n > 12:
+        a = (a + w(i)) & M; b = (b + w(i + 4)) & M; c = (c + w(i + 8)) & M
+        a = (a - c) & M; a ^= _rot(c, 4); c = (c + b) & M
+        b = (b - a) & M; b ^= _rot(a, 6); a = (a + c) & M
+        c = (c - b) & M; c ^= _rot(b, 8); b = (b + a) & M
+        a = (a - c) & M; a ^= _rot(c, 16); c = (c + b) & M
+        b = (b - a) & M; b ^= _rot(a, 19); a = (a + c) & M
+        c = (c - b) & M; c ^= _rot(b, 4); b = (b + a) & M
+        n -= 12; i += 12
+    if n == 0:
+        return c
+    t = bytes(data[i:]) + bytes(12)
+    w = lambda j: int.from_bytes(t[j:j + 4], "little")
+    a = (a + w(0)) & M; b = (b + w(4)) & M; c = (c + w(8)) & M
+    c ^= b; c = (c - _rot(b, 14)) & M
+    a ^= c; a = (a - _rot(c, 11)) & M
+    b ^= a; b = (b - _rot(a, 25)) & M
+    c ^= b; c = (c - _rot(b, 16)) & M
+    a ^= c; a = (a - _rot(c, 4)) & M
+    b ^= a; b = (b - _rot(a, 14)) & M
+    c ^= b; c = (c - _rot(b, 24)) & M
+    return c
+def fix_ohdr(buf, off):
+    assert buf[off:off + 4] == b"OHDR"
+    flags = buf[off + 5]; p = off + 6
+    if flags & 0x20: p += 16
+    if flags & 0x10: p += 4
+    width = 1 << (flags & 3)
+    end = p + width + int.from_bytes(buf[p:p + width], "little")
+    buf[end:end + 4] = lookup3(bytes(buf[off:end])).to_bytes(4, "little")
+"#;
+
+/// A chunked layout records the element size as its last dimension; libhdf5
+/// refuses a dataset whose datatype has another size ("stored datatype size
+/// in chunk layout does not match datatype description"). This read the
+/// chunks laid out with the wrong element size.
+#[test]
+fn chunk_layout_element_size_must_match_the_datatype() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let body = format!(
+        "{FIX_OHDR_PY}{}",
+        r#"
+for libver in ("earliest", "latest"):
+    good = os.path.join(d, f"{libver}_good.h5")
+    with h5py.File(good, "w", libver=libver) as f:
+        f.create_dataset("d", data=np.arange(100, dtype="<i4"), chunks=(37,))
+    data = bytearray(open(good, "rb").read())
+    if libver == "earliest":
+        at = data.find(struct.pack("<II", 37, 4)) + 4
+        width = 4
+    else:
+        # v4: version, class, flags, ndims, bytes per dim, then the dims
+        at = data.find(bytes([4, 2, 0, 2, 1, 37, 4])) + 6
+        width = 1
+    assert at > 6
+    for size in (2, 8):
+        bad = bytearray(data)
+        bad[at:at + width] = size.to_bytes(width, "little")
+        if libver == "latest":
+            fix_ohdr(bad, bad.rfind(b"OHDR", 0, at))
+        open(os.path.join(d, f"{libver}_size{size}.h5"), "wb").write(bad)
+"#
+    );
+    let verdicts = h5py_verdicts(dir.path(), &body);
+    assert_agrees_with_h5py(
+        dir.path(),
+        &verdicts,
+        &[
+            "earliest_good ok",
+            "earliest_size2 ERROR",
+            "earliest_size8 ERROR",
+            "latest_good ok",
+            "latest_size2 ERROR",
+            "latest_size8 ERROR",
+        ],
+    );
+    for name in ["earliest_size2", "latest_size8"] {
+        let err = clawhdf5_reads(&dir.path().join(format!("{name}.h5"))).unwrap_err();
+        assert!(
+            err.contains("stored datatype size in chunk layout"),
+            "{name}: {err}"
+        );
+    }
+}
+
+/// A v2 object header message whose size runs past the messages into the
+/// chunk's checksum is refused by libhdf5 whether it runs 1 byte or more
+/// into the checksum (its loop stops at the checksum, and then the checksum
+/// read and the size check fail), so it is refused here too.
+#[test]
+fn v2_header_message_running_into_the_checksum_is_refused() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    let body = format!(
+        "{FIX_OHDR_PY}{}",
+        r#"
+good = os.path.join(d, "good.h5")
+with h5py.File(good, "w", libver="latest") as f:
+    f.create_dataset("d", data=np.arange(4, dtype="<i4"))
+raw = bytearray(open(good, "rb").read())
+off = raw.rfind(b"OHDR")
+flags = raw[off + 5]; p = off + 6
+if flags & 0x20: p += 16
+if flags & 0x10: p += 4
+width = 1 << (flags & 3)
+end = p + width + int.from_bytes(raw[p:p + width], "little"); p += width
+hdr = 6 if flags & 0x04 else 4
+last = p
+while p + hdr <= end:
+    last = p
+    p += hdr + int.from_bytes(raw[p + 1:p + 3], "little")
+assert p == end
+size = int.from_bytes(raw[last + 1:last + 3], "little")
+for k in (1, 4, 5):
+    bad = bytearray(raw)
+    bad[last + 1:last + 3] = (size + k).to_bytes(2, "little")
+    fix_ohdr(bad, off)
+    open(os.path.join(d, f"into_checksum_{k}.h5"), "wb").write(bad)
+"#
+    );
+    let verdicts = h5py_verdicts(dir.path(), &body);
+    assert_agrees_with_h5py(
+        dir.path(),
+        &verdicts,
+        &[
+            "good ok",
+            "into_checksum_1 ERROR",
+            "into_checksum_4 ERROR",
+            "into_checksum_5 ERROR",
+        ],
+    );
+}
+
+#[test]
+fn truncated_files_are_refused_and_nothing_past_the_end_of_file_is_read() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    // The superblock records where the file's data ends. A copy missing its
+    // last bytes is truncated: libhdf5 refuses to open it (this read what
+    // was left). Bytes appended after the end are not part of the file, and
+    // a superblock moved by prepending a user block (its recorded base
+    // address now wrong) has its end of file moved with it; both still read.
+    let verdicts = h5py_verdicts(
+        dir.path(),
+        r#"
+for libver in ("earliest", "latest"):
+    good = os.path.join(d, f"{libver}_good.h5")
+    with h5py.File(good, "w", libver=libver) as f:
+        f.create_dataset("d", data=np.arange(100, dtype="<i4"))
+        f.attrs["note"] = "x" * 64
+    data = open(good, "rb").read()
+    open(os.path.join(d, f"{libver}_truncated.h5"), "wb").write(data[:-8])
+    open(os.path.join(d, f"{libver}_appended.h5"), "wb").write(data + b"\0" * 64)
+    open(os.path.join(d, f"{libver}_moved.h5"), "wb").write(b"\0" * 512 + data)
+"#,
+    );
+    assert_agrees_with_h5py(
+        dir.path(),
+        &verdicts,
+        &[
+            "earliest_appended ok",
+            "earliest_good ok",
+            "earliest_moved ok",
+            "earliest_truncated ERROR",
+            "latest_appended ok",
+            "latest_good ok",
+            "latest_moved ok",
+            "latest_truncated ERROR",
+        ],
+    );
+    for name in ["earliest_truncated", "latest_truncated"] {
+        let err = File::open(dir.path().join(format!("{name}.h5")))
+            .err()
+            .unwrap_or_else(|| panic!("{name} opened"));
+        assert!(err.to_string().contains("truncated file"), "{name}: {err}");
+    }
+}
+
+#[test]
+fn header_and_datatype_damage_libhdf5_refuses_is_refused() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    // Each case damages one field of a valid file h5py wrote (libver
+    // earliest, so version-1 object headers); all of these read before.
+    let verdicts = h5py_verdicts(
+        dir.path(),
+        r#"
+def write(name, make):
+    path = os.path.join(d, name + ".h5")
+    with h5py.File(path, "w", libver="earliest") as f:
+        make(f)
+    return bytearray(open(path, "rb").read())
+
+def save(name, data):
+    open(os.path.join(d, name + ".h5"), "wb").write(data)
+
+# A layout message (type 8, 24 bytes, v1 header) flagged shareable.
+data = write("layout_good", lambda f: f.create_dataset("d", data=np.arange(4, dtype="<i4")))
+at = data.find(bytes([8, 0, 24, 0]))
+assert at > 0
+bad = bytearray(data); bad[at + 4] |= 0x40
+save("layout_shareable", bad)
+# A message size that is not a multiple of 8 in a v1 header.
+bad = bytearray(data); bad[at + 2] = 23
+save("layout_unaligned", bad)
+
+# A compound whose second field repeats the first's name, or overlaps it.
+dt = np.dtype([("aa", "<i4"), ("bb", "<i4")])
+data = write("compound_good", lambda f: f.create_dataset("d", data=np.zeros(2, dt)))
+at = data.find(b"bb\0")
+bad = bytearray(data); bad[at:at + 2] = b"aa"
+save("compound_duplicate", bad)
+bad = bytearray(data); bad[at + 8:at + 12] = struct.pack("<I", 2)
+save("compound_overlap", bad)
+
+# An enum member with an empty name.
+et = h5py.enum_dtype({"RED": 0, "GREEN": 1}, basetype="i4")
+data = write("enum_good", lambda f: f.create_dataset("d", data=np.zeros(2, "<i4"), dtype=et))
+at = data.find(b"RED\0")
+bad = bytearray(data); bad[at:at + 3] = b"\0\0\0"
+save("enum_empty_name", bad)
+
+# A float whose exponent overlaps its mantissa (exponent at bit 20).
+data = write("float_good", lambda f: f.create_dataset("d", data=np.zeros(3, "<f4")))
+at = data.find(bytes([0, 0, 32, 0, 23, 8, 0, 23]))
+bad = bytearray(data); bad[at + 4] = 20
+save("float_overlap", bad)
+"#,
+    );
+    assert_agrees_with_h5py(
+        dir.path(),
+        &verdicts,
+        &[
+            "compound_duplicate ERROR",
+            "compound_good ok",
+            "compound_overlap ERROR",
+            "enum_empty_name ERROR",
+            "enum_good ok",
+            "float_good ok",
+            "float_overlap ERROR",
+            "layout_good ok",
+            "layout_shareable ERROR",
+            "layout_unaligned ERROR",
+        ],
+    );
+}
+
+/// Runs a Python script (with `h5py`, `numpy as np` and `struct` imported,
+/// `d` the output directory) and fails the test if it fails.
+fn run_python(dir: &Path, body: &str) {
+    let script = format!(
+        "import h5py, numpy as np, struct, os\nd = \"{}\"\n{body}",
+        dir.display()
+    );
+    let out = Command::new(python())
+        .args(["-c", &script])
+        .output()
+        .expect("failed to run python");
+    assert!(
+        out.status.success(),
+        "python failed:\n{}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+}
+
+/// libhdf5 limits a chunk to under 4 GiB only when a version-1 B-tree
+/// indexes it; HDF5 2.0 writes larger chunks with layout version 5 (libver
+/// v200), and h5py reads them. These were refused as "chunk size must be <
+/// 4GB". Ignored by default: h5py writes a 4 GiB chunk and both libraries
+/// hold it in memory (about 9 GiB in all).
+#[test]
+#[ignore = "writes and reads a 4 GiB chunk (about 9 GiB of memory)"]
+fn chunks_of_4_gib_and_more_read_with_layout_v5() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    run_python(
+        dir.path(),
+        r#"
+with h5py.File(os.path.join(d, "big.h5"), "w", libver=("v200", "v200")) as f:
+    ds = f.create_dataset("d", shape=(10,), maxshape=(None,), chunks=(2**29 + 1,),
+                          dtype="<f8", compression="gzip", compression_opts=1)
+    ds[:] = np.arange(10.0)
+with h5py.File(os.path.join(d, "big.h5"), "r") as f:
+    assert list(f["d"][:]) == list(np.arange(10.0))
+"#,
+    );
+    let file = File::open(dir.path().join("big.h5")).unwrap();
+    let values = file.dataset("d").unwrap().read_f64().unwrap();
+    assert_eq!(values, (0..10).map(f64::from).collect::<Vec<_>>());
+}
+
+/// A variable-length compound member takes 4 + offset size + 4 bytes, which
+/// is 12 in a file with 4-byte offsets, and libhdf5 checks for overlapping
+/// members with that stored size. A member right after one was refused as
+/// "member overlaps with previous member" (the check took the 16 bytes of
+/// an 8-byte-offset file), and with it every attribute of the object.
+#[test]
+fn variable_length_compound_members_in_files_with_4_byte_offsets() {
+    skip_if_no_python!();
+    let dir = tempfile::tempdir().unwrap();
+    run_python(
+        dir.path(),
+        r#"
+from h5py import h5f, h5p
+dt = np.dtype([("s", h5py.string_dtype()), ("i", "<i4")])
+a = np.array([("x", 1), ("yy", 2)], dtype=dt)
+for libver in ("earliest", "latest"):
+    fcpl = h5p.create(h5p.FILE_CREATE)
+    fcpl.set_sizes(4, 4)
+    fapl = h5p.create(h5p.FILE_ACCESS)
+    low = h5f.LIBVER_EARLIEST if libver == "earliest" else h5f.LIBVER_LATEST
+    fapl.set_libver_bounds(low, h5f.LIBVER_LATEST)
+    path = os.path.join(d, f"{libver}.h5")
+    with h5py.File(h5f.create(path.encode(), h5f.ACC_TRUNC, fcpl=fcpl, fapl=fapl)) as f:
+        f.create_dataset("c", data=a)
+        f.attrs["c"] = a
+        f.attrs["note"] = "kept"
+    with h5py.File(path, "r") as f:
+        assert f["c"][1]["i"] == 2
+        assert f.attrs["note"] == "kept"
+"#,
+    );
+    for libver in ["earliest", "latest"] {
+        let file = File::open(dir.path().join(format!("{libver}.h5"))).unwrap();
+        let dtype = file.dataset("c").unwrap().dtype();
+        assert!(
+            matches!(&dtype, Ok(clawhdf5::DType::Compound(fields))
+                if fields.iter().map(|f| f.0.as_str()).eq(["s", "i"])),
+            "{libver}: {dtype:?}"
+        );
+        // (Variable-length values in files with 4-byte offsets are not
+        // decoded yet; see docs/known-issues.md. The attributes must at
+        // least be listed without errors.)
+        let (attrs, errors) = file.root().attrs_with_errors().unwrap();
+        assert!(errors.is_empty(), "{libver}: {errors:?}");
+        assert!(
+            attrs.contains_key("c") && attrs.contains_key("note"),
+            "{libver}: {attrs:?}"
+        );
+    }
+}
diff --git a/crates/clawhdf5/tests/legacy_writer_files.rs b/crates/clawhdf5/tests/legacy_writer_files.rs
new file mode 100644
index 0000000..e826064
--- /dev/null
+++ b/crates/clawhdf5/tests/legacy_writer_files.rs
@@ -0,0 +1,141 @@
+//! Files written by older clawhdf5 releases must keep opening, even where
+//! libhdf5 refuses them: stricter validation of corrupt files must not lock
+//! users out of their own data.
+//!
+//! `fixtures/written_by_v2_7_0.h5` and `written_by_v2_7_0_paged.h5` were
+//! written by clawhdf5 v2.7.0 (`FileBuilder` / `FileWriter` with every
+//! datatype, layout and attribute kind it could write). v2.7.0 wrote the sign
+//! bit of every float at position 63 and a size-0 string type for an empty
+//! string attribute; libhdf5 refuses both, this reader must not.
+
+use std::path::PathBuf;
+
+use clawhdf5::{AttrValue, File};
+
+fn fixture(name: &str) -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .join("tests/fixtures")
+        .join(name)
+}
+
+#[test]
+fn every_object_of_a_v2_7_0_file_reads() {
+    let file = File::open(fixture("written_by_v2_7_0.h5")).unwrap();
+
+    let (attrs, errors) = file.root().attrs_with_errors().unwrap();
+    assert!(errors.is_empty(), "{errors:?}");
+    // (It reads as no strings, as it did before.)
+    assert!(
+        matches!(&attrs["empty"], AttrValue::StringArray(v) if v.iter().all(String::is_empty)),
+        "{:?}",
+        attrs["empty"]
+    );
+    assert!(matches!(&attrs["title"], AttrValue::String(s) if s == "old"));
+
+    let f32s = |name: &str| file.dataset(name).unwrap().read_f32().unwrap();
+    assert_eq!(f32s("f32"), [1.0, 2.0, 3.0]);
+    assert_eq!(
+        f32s("f32_2d"),
+        (0..60).map(|x| x as f32).collect::<Vec<_>>()
+    );
+    assert_eq!(
+        f32s("chunked"),
+        (0..1000).map(|x| x as f32).collect::<Vec<_>>()
+    );
+    assert!(f32s("empty").is_empty());
+    assert_eq!(
+        file.dataset("f64").unwrap().read_f64().unwrap(),
+        [1.0, 2.0, 3.0]
+    );
+    assert_eq!(file.dataset("i32").unwrap().read_i32().unwrap(), [1, -2, 3]);
+    assert_eq!(file.dataset("i64").unwrap().read_i64().unwrap(), [1, -2, 3]);
+    assert_eq!(file.dataset("u64").unwrap().read_u64().unwrap(), [1, 2, 3]);
+    assert_eq!(
+        file.dataset("chunked_2d").unwrap().read_i32().unwrap(),
+        (0..600).collect::<Vec<_>>()
+    );
+    for name in ["unlimited", "maxshape"] {
+        assert_eq!(
+            file.dataset(name).unwrap().read_f64().unwrap(),
+            (0..100).map(|x| x as f64).collect::<Vec<_>>(),
+            "{name}"
+        );
+    }
+    assert_eq!(
+        file.dataset("compact").unwrap().read_i32().unwrap(),
+        [7, 8, 9]
+    );
+    for name in ["u8", "compound", "enum", "enum8"] {
+        file.dataset(name)
+            .unwrap()
+            .dtype()
+            .unwrap_or_else(|e| panic!("{name}: {e}"));
+    }
+
+    let grp = file.group("grp").unwrap();
+    let (attrs, errors) = grp.attrs_with_errors().unwrap();
+    assert!(errors.is_empty(), "{errors:?}");
+    assert_eq!(attrs.len(), 21);
+    assert_eq!(grp.dataset("d").unwrap().read_f32().unwrap(), [4.0, 5.0]);
+
+    let paged = File::open(fixture("written_by_v2_7_0_paged.h5")).unwrap();
+    assert_eq!(paged.dataset("d").unwrap().read_f32().unwrap(), [1.0, 2.0]);
+}
+
+/// Datatypes libhdf5 (and this reader) refuse — a compound with a repeated
+/// field name or no fields, an enum member with an empty name — must not be
+/// written: they made files that did not read back. Valid neighbours of each
+/// still write and read back.
+#[test]
+fn datatypes_the_reader_refuses_are_not_written() {
+    use clawhdf5::{CompoundTypeBuilder, EnumTypeBuilder, FileBuilder};
+
+    let write_compound = |dt, raw: Vec<u8>| {
+        let mut b = FileBuilder::new();
+        b.create_dataset("d").with_compound_data(dt, raw, 1);
+        b.finish()
+    };
+    let write_enum = |dt| {
+        let mut b = FileBuilder::new();
+        b.create_dataset("d").with_enum_u8_data(dt, &[0, 1]);
+        b.finish()
+    };
+    let refused = |r: Result<Vec<u8>, clawhdf5::Error>, what: &str| {
+        let err = r.expect_err("written");
+        let msg = err.to_string();
+        assert!(msg.contains("datatype cannot be written"), "{msg}");
+        assert!(msg.contains(what), "{msg}");
+    };
+
+    let dup = CompoundTypeBuilder::new()
+        .f64_field("x")
+        .f64_field("x")
+        .build();
+    refused(
+        write_compound(dup, vec![0; 16]),
+        "duplicated compound field name 'x'",
+    );
+    refused(
+        write_compound(CompoundTypeBuilder::new().build(), vec![]),
+        "invalid",
+    );
+    let empty_name = EnumTypeBuilder::u8_based()
+        .u8_value("A", 0)
+        .u8_value("", 1)
+        .build();
+    refused(write_enum(empty_name), "0 length enum name");
+
+    let ok = CompoundTypeBuilder::new()
+        .f64_field("x")
+        .f64_field("y")
+        .build();
+    let bytes = write_compound(ok, vec![0; 16]).unwrap();
+    let file = File::from_bytes(bytes).unwrap();
+    file.dataset("d").unwrap().dtype().unwrap();
+    let ok = EnumTypeBuilder::u8_based()
+        .u8_value("A", 0)
+        .u8_value("B", 1)
+        .build();
+    let file = File::from_bytes(write_enum(ok).unwrap()).unwrap();
+    file.dataset("d").unwrap().dtype().unwrap();
+}
diff --git a/docs/known-issues.md b/docs/known-issues.md
index 4ea7518..617784a 100644
--- a/docs/known-issues.md
+++ b/docs/known-issues.md
@@ -129,13 +129,46 @@ fill-value item that did is fixed).
   failing the others.
 - **Other readers:**
   - VL-string datasets are not readable through `File`.
+  - Variable-length values inside a compound (and VL-string attributes) in
+    a file with 4-byte offsets (`sizeof_addr = 4`) fail with
+    `GlobalHeapObjectNotFound` or come back as `Raw`: these paths assume
+    the 16-byte element of an 8-byte-offset file. The datatype itself reads
+    (it was refused as "member overlaps with previous member" until
+    2026-09-26).
   - Metadata cache images are not supported.
   - x87 long double and binary128 are refused.
   - N-Bit on 64-bit scale-offset data and some N-Bit parameter layouts fail.
 - **Filters:** blosc, blosc2, bitshuffle, bzip2, LZF and zfp are not
   implemented.
-- **Header checks:** on 12 CVE datasets libhdf5 rejects a corrupt header and
-  we read data anyway. We need stricter header checks.
+- ~~**Header checks:** on 12 CVE datasets libhdf5 rejects a corrupt header and
+  we read data anyway. We need stricter header checks.~~ **Fixed
+  2026-09-26** (counted again: 18 objects on the CVE corpus that libhdf5
+  refuses; some read as wrong data, e.g. a zero chunk dimension read as all
+  fill values): object headers, datatypes, chunk dimensions and chunk-index
+  offsets are checked as libhdf5 checks them, and truncated files are
+  refused. 17 of the 18 now
+  fail as in libhdf5 (conformance on tank, `conformance/run.sh --no-fetch`,
+  2026-09-26: 571 of 697 ok). Still read where libhdf5 refuses:
+  - `cve-2024-32624.h5` `/Dset_OBJREF`: a dataspace whose storage size
+    overflows 64 bits. `File::dataset` and `shape()` succeed (libhdf5
+    refuses at open); reading the values fails.
+  - `cve-2020-10810.h5`, `cve-2020-10812.h5` (whole files libhdf5 cannot
+    open, not among the 18): libhdf5 decodes the superblock extension's File
+    Space Info and metadata-cache-image messages at open and refuses these
+    files; we do not decode those messages at open.
+  - Deliberately not refused, because clawhdf5 up to v2.7.0 wrote them: a
+    float sign bit position outside the type, and a size-0 string type.
+  - Not refused because current libhdf5 reads it though HDF5 2.0.0
+    (h5py 3.16) refuses it: a v4 chunked layout whose dimensions are
+    encoded in more bytes than they need (HDFGroup/hdf5@e124c36,
+    2026-06-05, relaxed that check; clawhdf5 wrote such layouts until
+    2026-09-26).
+  - Not refused because HDF5 2.0 (h5py 3.16) reads them though newer
+    libhdf5 refuses them: bit-field offset/precision outside the type, an
+    unknown variable-length kind, an array type whose stored size is not
+    its element count times its base size.
+  - (`cve-2024-32616` `/group1/dset3` and `cve-2025-2309`'s `Comp_OBJREF`
+    attribute are h5py/numpy type-mapping failures, not libhdf5 refusals.)
 - **Writer:**
   - Nested groups beyond one level: path-like names are now refused, not
     created.