id: PYSEC-2026-1435
published: "2026-07-07T16:03:01.951354Z"
modified: "2026-07-07T17:24:17.277303Z"
aliases:
- CVE-2025-57804
- GHSA-847f-9342-265h
summary: h2 allows HTTP Request Smuggling due to illegal characters in headers
details: "### Summary\n\nHTTP/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:
- package:
    name: h2
    ecosystem: PyPI
    purl: pkg:pypi/h2
  ranges:
  - type: ECOSYSTEM
    events:
    - introduced: "0"
    - fixed: 4.3.0
  versions:
  - 0.1.0
  - 1.0.0
  - 1.1.0
  - 1.1.1
  - 2.0.0
  - 2.1.0
  - 2.1.1
  - 2.1.2
  - 2.1.3
  - 2.1.4
  - 2.1.5
  - 2.2.0
  - 2.2.1
  - 2.2.2
  - 2.2.3
  - 2.2.4
  - 2.2.5
  - 2.2.6
  - 2.2.7
  - 2.3.0
  - 2.3.1
  - 2.3.2
  - 2.3.3
  - 2.3.4
  - 2.4.0
  - 2.4.1
  - 2.4.2
  - 2.4.3
  - 2.4.4
  - 2.5.0
  - 2.5.1
  - 2.5.2
  - 2.5.3
  - 2.5.4
  - 2.6.0
  - 2.6.1
  - 2.6.2
  - 3.0.0
  - 3.0.1
  - 3.1.0
  - 3.1.1
  - 3.2.0
  - 4.0.0
  - 4.1.0
  - 4.2.0
references:
- type: WEB
  url: https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h
- type: ADVISORY
  url: https://nvd.nist.gov/vuln/detail/CVE-2025-57804
- type: WEB
  url: https://github.com/python-hyper/h2/commit/035e9899f95e3709af098f578bfc3cd302298e3a
- type: PACKAGE
  url: https://github.com/python-hyper/h2
- type: WEB
  url: https://lists.debian.org/debian-lts-announce/2025/09/msg00004.html
- type: PACKAGE
  url: https://pypi.org/project/h2
- type: ADVISORY
  url: https://github.com/advisories/GHSA-847f-9342-265h
severity:
- type: CVSS_V4
  score: 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