Authlib versions up to 1.7.2 accept an empty HMAC key for JWS/JWT HS256/HS384/HS512 signature verification, allowing attackers to forge tokens with arbitrary claims. The empty key produces a deterministic digest that anyone can reproduce, bypassing signature verification. This vulnerability affects OAuth 2.0 resource servers and OIDC relying parties using Authlib if the verification key can be empty at runtime. The issue is fixed in Authlib 1.9.1.