CVE-2026-34520
CRITICAL9.1EPSS 0.08%AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass
Published: 4/1/2026Modified: 4/6/2026
Description
### Summary The C parser (the default for most installs) accepted null bytes and control characters is response headers. ### Impact An attacker could send header values that are interpreted differently than expected due to the presence of control characters. For example, `request.url.origin()` may return a different value than the raw Host header, or what a reverse proxy interpreted it as., potentially resulting in some kind of security bypass. ----- Patch: https://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4
Affected packages (2)
- Debian/python-aiohttpfrom 0
- PyPI/aiohttpfrom 0, < 3.13.4
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 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 |
| osv | CVSS 3.1 | CRITICAL9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H |
References (6)
- ADVISORYhttps://nvd.nist.gov/vuln/detail/CVE-2026-34520
- ADVISORYhttps://security-tracker.debian.org/tracker/CVE-2026-34520
- PATCHhttps://github.com/aio-libs/aiohttp
- WEBhttps://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4
- WEBhttps://github.com/aio-libs/aiohttp/releases/tag/v3.13.4
- WEBhttps://github.com/aio-libs/aiohttp/security/advisories/GHSA-63hf-3vf5-4wqf