{"cve":"CVE-2025-59420","nvd":{"published":"2025-09-22T18:15:46.910","modified":"2026-10-19T16:10:00.260","status":"Modified","description":"Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.4, Authlib’s JWS verification accepts tokens that declare unknown critical header parameters (crit), violating RFC 7515 “must‑understand” semantics. An attacker can craft a signed token with a critical header (for example, bork or cnf) that strict verifiers reject but Authlib accepts. In mixed‑language fleets, this enables split‑brain verification and can lead to policy bypass, replay, or privilege escalation. This issue has been patched in version 1.6.4.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N","score":7.5},"cwes":["CWE-345","CWE-863"],"references":["https://github.com/authlib/authlib/commit/6b1813e4392eb7c168c276099ff7783b176479df","https://github.com/authlib/authlib/security/advisories/GHSA-9ggr-2464-2j32","https://lists.debian.org/debian-lts-announce/2025/10/msg00032.html"],"products":["authlib:authlib"],"source":"nvd","synced_at":"2026-10-19T22:10:08.713834912+00:00"},"epss":{"cve":"CVE-2025-59420","score":0.00262,"percentile":0.17824,"date":"2026-10-25"},"kev":null,"osv":[{"id":"GHSA-9ggr-2464-2j32","url":"https://osv.dev/vulnerability/GHSA-9ggr-2464-2j32","summary":"Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)","modified":"2026-09-10T03:50:28.176043463Z","packages":[{"ecosystem":"PyPI","name":"authlib","purl":"pkg:pypi/authlib","ranges":["<1.6.4"],"fixed":["1.6.4"]}]},{"id":"PYSEC-2026-1200","url":"https://osv.dev/vulnerability/PYSEC-2026-1200","summary":"Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)","modified":"2026-07-07T17:46:32.324326134Z","packages":[{"ecosystem":"PyPI","name":"authlib","purl":"pkg:pypi/authlib","ranges":["<1.6.4"],"fixed":["1.6.4"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}