CVE-2026-27962: Authlib JWS JWK Header Injection: Signature Verification Bypass

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 behavior violates RFC 7515 §4.1.3 and the validation algorithm defined in RFC 7515 §5.2.