CVE-2026-34520

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

CVSS 2.7 LOWEPSS 0.5%CWE-113

Vexday Risk Score

8Low

SSVC decision (CISA)

Track

No exploitation signal → monitor

CVSS 2.7EPSS 0.5%KEV nãoPoC —Nuclei —Metasploit —Patch —

Lifecycle

01 Apr 2026Published on NVD

Recommendation: Monitor — no exploitation signal at the moment.

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.

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

Affected products

aio-libs · aiohttp

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References

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