CVE-2025-43859CRITICAL 9.1EPSS p40.1%

CVE-2025-43859CVE-2025-43859

Description

h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

Scoring

CVSS 3.19.1 (CRITICAL)
VectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
EPSS0.52% probability of exploitation · percentile 40.1% · 2026-06-19T12:03:05Z
Published2025-04-24
Last modified2026-04-15

Underlying weaknesses· 1

CWE-444

References

  1. https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed
  2. https://github.com/python-hyper/h11/security/advisories/GHSA-vqfr-h8mv-ghfj

1

TypeTargetConfidenceTier
WeaknessInconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')cwe-4440%live

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Sourced from NVD + FIRST.org EPSS. Curated for EU compliance use cases by Adam Lundqvist, Founder at SQUR.