{"id":"CVE-2026-55165","state":"PUBLISHED","title":"Lemur : JWT verifier trusts attacker-supplied alg from token header — defense-in-depth gap; chain-dependent ATO with secret disclosure","description":"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.","publishedAt":"2026-08-18T18:49:59.538Z","updatedAt":"2026-08-18T19:35:55.129Z","severity":{"score":4.8,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","source":"GitHub_M"},"assigner":"GitHub_M","products":["Netflix lemur"],"affected":[{"vendor":"Netflix","product":"lemur","versions":[{"version":"< 1.9.2","status":"affected"}]}],"references":["https://github.com/Netflix/lemur/security/advisories/GHSA-r9gp-7f88-9r54","https://github.com/Netflix/lemur/commit/89898e665beed044cf7bd5927dfaa484e55bf242","https://github.com/Netflix/lemur/releases/tag/v1.9.2"],"weaknesses":["CWE-347"],"ingestedAt":"2026-10-29T20:12:24.861Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/55xxx/CVE-2026-55165.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-55165","state":"PUBLISHED","datePublished":"2026-08-18T18:49:59.538Z","dateUpdated":"2026-08-18T19:35:55.129Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-06-16T15:13:28.165Z"},"containers":{"cna":{"title":"Lemur : JWT verifier trusts attacker-supplied alg from token header — defense-in-depth gap; chain-dependent ATO with secret disclosure","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"Netflix","product":"lemur","versions":[{"version":"< 1.9.2","status":"affected"}]}],"references":[{"url":"https://github.com/Netflix/lemur/security/advisories/GHSA-r9gp-7f88-9r54","name":"https://github.com/Netflix/lemur/security/advisories/GHSA-r9gp-7f88-9r54","tags":["x_refsource_CONFIRM"]},{"url":"https://github.com/Netflix/lemur/commit/89898e665beed044cf7bd5927dfaa484e55bf242","name":"https://github.com/Netflix/lemur/commit/89898e665beed044cf7bd5927dfaa484e55bf242","tags":["x_refsource_MISC"]},{"url":"https://github.com/Netflix/lemur/releases/tag/v1.9.2","name":"https://github.com/Netflix/lemur/releases/tag/v1.9.2","tags":["x_refsource_MISC"]}],"metrics":[{"cvssV3_1":{"attackComplexity":"HIGH","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":4.8,"baseSeverity":"MEDIUM","confidentialityImpact":"LOW","integrityImpact":"LOW","privilegesRequired":"NONE","scope":"UNCHANGED","userInteraction":"NONE","vectorString":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N","version":"3.1"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-347","lang":"en","description":"CWE-347: Improper Verification of Cryptographic Signature","type":"CWE"}]}],"providerMetadata":{"shortName":"GitHub_M","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateUpdated":"2026-08-18T18:49:59.538Z"},"source":{"advisory":"GHSA-r9gp-7f88-9r54","discovery":"UNKNOWN"}},"adp":[{"title":"CISA ADP Vulnrichment","references":[{"url":"https://github.com/Netflix/lemur/security/advisories/GHSA-r9gp-7f88-9r54","tags":["exploit"]}],"metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-08-18T19:35:36.998493Z","id":"CVE-2026-55165","options":[{"Exploitation":"poc"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-08-18T19:35:55.129Z"}}]}}}