What’s Vulnerable › Vulnerabilities

CVE-2025-59420: Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2025-09-22 · updated 2026-10-19
• Weakness: CWE-345

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.

Affected products

• authlib: before 1.6.4 — fixed in 1.6.4

How to fix

Upgrade to a fixed version: authlib 1.6.4. Check the vendor advisory for mitigations.

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

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