CVE-2025-57804
h2 allows HTTP Request Smuggling due to illegal characters in headers
5.3CVSSMEDIUM
Description
h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an 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. This issue has been patched in version 4.3.0.
CVSS Vector Breakdown
Exploitability
AV:NAccess VectorNetwork
AC:LAccess ComplexityLow
Impact
C:NConfidentialityNone
I:LIntegrityL
A:NAvailabilityNone
Weaknesses
Affected Products
Exploitability
Official Patch Available
References
https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h
github.com
https://github.com/python-hyper/h2/commit/035e9899f95e3709af098f578bfc3cd302298e3a
github.com
https://lists.debian.org/debian-lts-announce/2025/09/msg00004.html
lists.debian.org
and 9 more references View all →
Timeline
Published
Aug 25, 2025
Last Updated
Nov 3, 2025
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