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Advisory record · GHSA-vqfr-h8mv-ghfj

h11 accepts some malformed Chunked-Encoding bodies

CVSS 9.1 CriticalCriticalEPSS 0.0059 · 48th percentile

Critical, because:

  • CVSS score 9.1: 9.0 or more.

How we decide what’s critical

Published by the GitHub Advisory Database at 24 Apr 2025, 16:07 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

PackageEcosystemAffectedFixed in
h11PyPI<0.16.00.16.0
Identifiers
GHSA-vqfr-h8mv-ghfj · CVE-2025-43859 · PYSEC-2026-348
Weakness (CWE)
CWE-444
Tags
Published
24 Apr 2025
Updated
27 Oct 2026

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

h11 versions before 0.16.0 accept chunked-encoding bodies with malformed trailing line terminators instead of rejecting them. Exploitation needs a reverse proxy with a different chunked-encoding parsing bug, which can let an attacker smuggle requests past it. Upgrade to h11 0.16.0; the source text names 0.15.0 as the fixed version, so check the release notes.

Written 31 Oct 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

ScoreRatingCVSSVectorSource
9.1 Critical 3.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N CNA (GitHub_M)
  • CISA KEV Not listed.
  • EPSS 0.0059, the 48th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

  1. 31 Oct 2026, 14:07 UTC First seen by agentprime
  2. 31 Oct 2026, 14:07 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.