LibJWT, a C JSON Web Token library, incorrectly accepted an RSA JSON Web Key that lacked an "alg" property as a key for HMAC verification of HS256/HS384/HS512 tokens. The OpenSSL backend interprets the missing alg as a zero‑length key, permitting an attacker to create a valid JWT without possessing the secret or RSA private key. This algorithm‑confusion flaw enables authentication bypass and can give attackers full access to any resource that relies on JWT validation for authorization.

The vulnerability affects Benmcollins libjwt versions 3.0.0 through 3.3.2. Any application that loads RSA keys from a JWKS where the algorithm is omitted and then selects the verification algorithm from the JWT header is impacted. Systems using libjwt in authentication or session handling should scrutinize their implementation for the presence of this flaw.

The CVSS score of 9.1 indicates a high‑severity authentication bypass. The EPSS score is not available, and the issue is not listed in CISA KEV. An attacker who can supply a forged JWT—typically via network observation or API integration—can exploit this weakness without needing privileged credentials. The likelihood of exploitation is significant in environments where libjwt is used without hard‑coded algorithm checks, and the impact can range from unauthorized user access to full system compromise depending on the application’s security model.