{"id":"CVE-2026-55165","aliases":["GHSA-r9gp-7f88-9r54","PYSEC-2026-2589"],"url":"https://o3.security/vulnerability/CVE-2026-55165","summary":"Lemur : JWT verifier trusts attacker-supplied alg from token header — defense-in-depth gap; chain-dependent ATO with secret disclosure","details":"Lemur manages TLS certificate creation. Prior to 1.9.2, the JWT verifier in lemur/auth/service.py:130-137 used fetch_token_header to read header_data[\"alg\"] from an unverified token and passed that attacker-controlled value to decode_with_multiple_secrets. PyJWT 2.x rejects alg=none with the configured key, so the flaw is a defense-in-depth gap rather than a direct authentication bypass in the shipped configuration. The unpinned algorithm can become exploitable after an asymmetric-signing migration through algorithm confusion, and it weakens algorithm-based anomaly detection because the token chooses the recorded value. A separate disclosure of LEMUR_TOKEN_SECRET would also permit forged HS256 tokens, although that disclosure is an independent prerequisite. The fix introduces the server-controlled LEMUR_TOKEN_ALGORITHMS allowlist and defaults it to HS256. This issue is fixed in version 1.9.2.","published":"2026-08-18T18:49:59.538Z","modified":"2026-08-20T03:54:30.364998204Z","cvss":{"score":4.8,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N"},"epss":{"score":0.00126,"percentile":0.02566,"asOf":"2026-09-06"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"lemur","fixedVersion":"1.9.2"}],"fix":{"url":"https://github.com/Netflix/lemur/commit/89898e665beed044cf7bd5927dfaa484e55bf242","label":"Netflix/lemur@89898e6"},"references":[{"type":"WEB","url":"https://github.com/Netflix/lemur/releases/tag/v1.9.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55165.json"},{"type":"ADVISORY","url":"https://github.com/Netflix/lemur/security/advisories/GHSA-r9gp-7f88-9r54"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55165"},{"type":"FIX","url":"https://github.com/Netflix/lemur/commit/89898e665beed044cf7bd5927dfaa484e55bf242"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-20T03:54:30.364998204Z"}}