Several widely-used JSON Web Token (JWT) libraries, including node-jsonwebtoken, pyjwt, namshi/jose, php-jwt, and jsjwt, are affected by critical vulnerabilities that could allow attackers to bypass the verification step when using asymmetric keys (RS256, RS384, RS512, ES256, ES384, ES512).
In Firebase PHP-JWT before 6.0.0, an algorithm-confusion issue (e.g., RS256 / HS256) exists via the kid (aka Key ID) header, when multiple types of keys are loaded in a key ring. This allows an attacker to forge tokens that validate under the incorrect key. NOTE: this provides a straightforward way to use the PHP-JWT library unsafely, but might not be considered a vulnerability in the library itself.
Critical vulnerabilities in JSON Web Token libraries.
Since algorithm isn't enforced in jwt.decode(), a malicious user could choose what algorithm is sent sent to the server. If the server is expecting RSA but is sent HMAC-SHA with RSA's public key, the server will think the public key is actually an HMAC private key. This could be used to forge any data an attacker wants.