CVE-2026-34520

CRITICAL 9.1

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

CVSS Score

9.1

EPSS Score

0.0%

EPSS Percentile

14th

AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to version 3.13.4, the C parser (the default for most installs) accepted null bytes and control characters in response headers. This issue has been patched in version 3.13.4.

CWE	CWE-113
Vendor	aio-libs
Product	aiohttp
Published	Apr 1, 2026
Last Updated	Apr 4, 2026

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Affected Versions

aio-libs / aiohttp

< 3.13.4

References

NVD ↗ CVE.org ↗ EPSS Data ↗

github.com: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-63hf-3vf5-4wqf github.com: https://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4 github.com: https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4