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High severityNVD Advisory· Published Sep 22, 2025· Updated Nov 3, 2025

Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)

CVE-2025-59420

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.

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Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
authlibPyPI
< 1.6.41.6.4

Affected products

20

Patches

Vulnerability mechanics

References

5

News mentions

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