# CVE-2026-120756: Heap Buffer Over-read via Malformed Message Size in HDF5's H5O__chunk_deserialize

Status: published. Published: 2026-11-03. Updated: 2026-11-03.

HDF5 through 2.2.0 contains a heap buffer over-read vulnerability in H5O__chunk_deserialize() in src/H5Ocache.c. When decoding object header messages, the declared message size is checked against the end of the chunk image before the remainder of the message header (flags, and for version 2 object headers, the optional creation index) has been decoded. Because the message body begins after these fields, a message can start up to four bytes further into the chunk than the size check accounts for. A crafted HDF5 file can declare a message size that passes the check but extends past the end of the chunk image, causing the message’s decode callback, for example H5O__mtime_new_decode() in src/H5Omtime.c, to read beyond the heap allocation. The flaw is reachable during H5Fopen() on a crafted file. An attacker who can get a user or application to open a malicious file can cause a denial of service or limited disclosure of process memory.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- The HDF Group / HDF5: < 2.3.0 (affected). Source: CNA: HDFG

## Severity and prioritization

- CVSS 4.0: 1.9 (low); CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:L/VI:N/VA:L/SC:N/SI:N/SA:N/E:P; source: CNA: HDFG

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-125

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/HDFGroup/hdf5/pull/6589 (CNA: HDFG)

- https://github.com/HDFGroup/hdf5/issues/6401 (CNA: HDFG)

## Provenance

- CNA: HDFG: https://www.cve.org/CVERecord?id=CVE-2026-120756; retrieved 2026-11-03; source updated 2026-11-03

Revision: a98cea3526d1766d7677917e4861d965152be0020265836ad361e5613f7342be

Canonical: https://y2.dev/cve/CVE-2026-120756

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