CVE-2026-34520: AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass

April 1, 2026 (updated April 6, 2026)

The C parser (the default for most installs) accepted null bytes and control characters is response headers.

References

github.com/advisories/GHSA-63hf-3vf5-4wqf
github.com/aio-libs/aiohttp
github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4
github.com/aio-libs/aiohttp/releases/tag/v3.13.4
github.com/aio-libs/aiohttp/security/advisories/GHSA-63hf-3vf5-4wqf
nvd.nist.gov/vuln/detail/CVE-2026-34520

Code Behaviors & Features

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