CVE-2026-27962: Authlib

Critical severity, CVSS 9.1. EPSS: 0.5% chance of exploitation in the next 30 days.

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a JWK Header Injection vulnerability in authlib's JWS implementation allows an unauthenticated attacker to forge arbitrary JWT tokens that pass signature verification. When key=None is passed to any JWS deserialization function, the library extracts and uses the cryptographic key embedded in the attacker-controlled JWT jwk header field. An attacker can sign a token with their own private key, embed the matching public key in the header, and have the server accept the forged token as cryptographically valid — bypassing authentication and authorization entirely. This issue has been patched in version 1.6.9.

Affected products

• Authlib Authlib: before 1.6.9 (fixed in 1.6.9)

Published 2026-03-16. Last modified 2026-09-10.