{"id":"CVE-2026-34520","aliases":["GHSA-63hf-3vf5-4wqf","PYSEC-2026-2102"],"title":"AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass","summary":"AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass","severity":"critical","cvss":9.1,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H","vendor":"aiohttp","product":"aiohttp","ecosystem":"pip","affected":["aiohttp < 3.13.4"],"patched":["aiohttp 3.13.4"],"published":"2026-04-01","updated":"2026-09-10","sourceUpdated":"2026-09-10T03:50:43.255807027Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-63hf-3vf5-4wqf","references":[{"url":"https://github.com/aio-libs/aiohttp/security/advisories/GHSA-63hf-3vf5-4wqf"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-34520"},{"url":"https://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4"},{"url":"https://github.com/aio-libs/aiohttp"},{"url":"https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/aiohttp/PYSEC-2026-2102.yaml"}],"tags":["osv","pip"],"epss":0.00461,"epssPercentile":0.3922,"ingestedAt":"2026-07-13T18:57:53.610Z","slug":"CVE-2026-34520","body":"## Overview\n\n### Summary\n\nThe C parser (the default for most installs) accepted null bytes and control characters in request headers.\n\n### Impact\n\nAn 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.\n\n-----\n\nPatch: https://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4\n\n## Affected packages\n\n- `aiohttp < 3.13.4`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `aiohttp 3.13.4`","depth":"midnight","depthScore":50,"depthScoreParts":{"impact":50.1,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}