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PYSEC-2026-1435

h2 allows HTTP Request Smuggling due to illegal characters in headers

Quick fix

PYSEC-2026-1435 — h2: upgrade to the fixed version with the command below.

pip install --upgrade 'h2>=4.3.0'

Details

### Summary

HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls.

Are you affected?

Enter the version of the package you're using.

Affected packages

PyPI/h2

Introduced in: 0Fixed in: 4.3.0

Fixpip install --upgrade 'h2>=4.3.0'

References

https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h[WEB]
https://nvd.nist.gov/vuln/detail/CVE-2025-57804[ADVISORY]
https://github.com/python-hyper/h2/commit/035e9899f95e3709af098f578bfc3cd302298e3a[WEB]
https://github.com/python-hyper/h2[PACKAGE]
https://lists.debian.org/debian-lts-announce/2025/09/msg00004.html[WEB]
https://pypi.org/project/h2[PACKAGE]
https://github.com/advisories/GHSA-847f-9342-265h[ADVISORY]