{"id":"CVE-2025-43859","aliases":["GHSA-vqfr-h8mv-ghfj","PYSEC-2026-348"],"url":"https://o3.security/vulnerability/CVE-2025-43859","summary":"h11 accepts some malformed Chunked-Encoding bodies","details":"h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.","published":"2025-04-24T18:15:53.328Z","modified":"2026-07-15T01:49:01.992742611Z","cvss":{"score":9.1,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"h11","fixedVersion":"0.16.0"}],"fix":{"url":"https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed","label":"python-hyper/h11@114803a"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/43xxx/CVE-2025-43859.json"},{"type":"ADVISORY","url":"https://github.com/python-hyper/h11/security/advisories/GHSA-vqfr-h8mv-ghfj"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-43859"},{"type":"FIX","url":"https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-07-15T01:49:01.992742611Z"}}