CVE-2025-57804: h2 allows HTTP Request Smuggling due to illegal characters in headers

Severity: medium (CVSS 6.9 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.8% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-08-25 · updated 2026-06-17
Weakness: CWE-93

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.

Affected products

python-hyper h2: before 4.3.0

References

https://github.com/python-hyper/h2/commit/035e9899f95e3709af098f578bfc3cd302298e3a
https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h
https://lists.debian.org/debian-lts-announce/2025/09/msg00004.html

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