Threats tagged 'brew-fastmcp-cve-2026-114616'

Authlib versions up to 1.7.2 accept an empty HMAC key for JWS/JWT signature verification using HS256, HS384, and HS512 algorithms. This allows an attacker to forge arbitrary tokens that pass signature verification if the application verifies tokens with an empty key. The vulnerability arises because empty keys produce deterministic HMAC digests that anyone can reproduce, and Authlib does not reject zero-length keys. This flaw can lead to token forgery and authentication bypass in OAuth 2.0 resource servers and OIDC relying parties using Authlib. A fix is available in Authlib version 4.1.1.