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Advisory record · GHSA-63hf-3vf5-4wqf

AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass

CVSS 2.7 LowEPSS 0.0048 · 41st percentile

Published by the GitHub Advisory Database at 1 Apr 2026, 21:49 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

PackageEcosystemAffectedFixed in
aiohttpPyPI<3.13.43.13.4
Identifiers
GHSA-63hf-3vf5-4wqf · CVE-2026-34520 · PYSEC-2026-2102
Weakness (CWE)
CWE-113
Tags
Published
1 Apr 2026
Updated
10 Sept 2026

Machine-written summary

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

The aiohttp package, versions before 3.13.4, is affected. Its C parser (llhttp) accepted null bytes and control characters in request header values, so an attacker who can send crafted HTTP requests may get headers interpreted differently than intended, which could enable a security bypass. The source gives few details on the exact bypass scenario. Upgrade aiohttp to version 3.13.4 or later.

Written 5 Nov 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
2.7 Low 4.0 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:U CNA (GitHub_M)
  • CISA KEV Not listed.
  • EPSS 0.0048, the 41st percentile, on 14 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

  1. 1 Apr 2026, 00:01 UTC Published by the maintainers as a GitHub repository advisory
  2. 31 Oct 2026, 14:34 UTC First seen by agentprime
  3. 31 Oct 2026, 14:34 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.