{"id":"CVE-2026-34525","aliases":["GHSA-c427-h43c-vf67","PYSEC-2026-2103"],"title":"AIOHTTP accepts duplicate Host headers","summary":"AIOHTTP accepts duplicate Host headers","severity":"medium","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":"2026-09-10T03:51:01.505705751Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-c427-h43c-vf67","references":[{"url":"https://github.com/aio-libs/aiohttp/security/advisories/GHSA-c427-h43c-vf67"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-34525"},{"url":"https://github.com/aio-libs/aiohttp/commit/53e2e6fc58b89c6185be7820bd2c9f40216b3000"},{"url":"https://github.com/aio-libs/aiohttp/commit/e00ca3cca92c465c7913c4beb763a72da9ed8349"},{"url":"https://github.com/aio-libs/aiohttp"},{"url":"https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4"}],"tags":["osv","pip"],"epss":0.00288,"epssPercentile":0.21621,"ingestedAt":"2026-07-13T18:57:56.245Z","slug":"CVE-2026-34525","body":"## Overview\n\n### Summary\n\nMultiple Host headers were allowed in aiohttp.\n\n### Impact\n\nMostly this doesn't affect aiohttp security itself, but if a reverse proxy is applying security rules depending on the target Host, it is theoretically possible that the proxy and aiohttp could process different host names, possibly resulting in bypassing a security check on the proxy and getting a request processed by aiohttp in a privileged sub app when using `Application.add_domain()`.\n\n-----\n\nPatch: https://github.com/aio-libs/aiohttp/commit/e00ca3cca92c465c7913c4beb763a72da9ed8349\nPatch: https://github.com/aio-libs/aiohttp/commit/53e2e6fc58b89c6185be7820bd2c9f40216b3000\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":"sunlit","depthScore":28,"depthScoreParts":{"impact":27.5,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}