h2 allows HTTP Request Smuggling due to illegal characters in headers
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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | h2 | < 4.3.0 | 4.3.0 |
References
- https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h
- https://nvd.nist.gov/vuln/detail/CVE-2025-57804
- https://github.com/python-hyper/h2/commit/035e9899f95e3709af098f578bfc3cd302298e3a
- https://github.com/python-hyper/h2
- https://lists.debian.org/debian-lts-announce/2025/09/msg00004.html
- https://pypi.org/project/h2
- https://github.com/advisories/GHSA-847f-9342-265h
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N- Weakness (CWE)
CWE-93- Platform
- Python (PyPI)
- Package
h2- Disclosed
- 25.08.2025
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-1435 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-847f-9342-265h - CC BY 4.0
Vulnerability history: h2
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 2
- First → latest
- 08.2025 08.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Medium 2
Disclosures per year
- 2025: 1
- 2026: 1
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