Weak Cryptographic Signature: User-Controlled Key Source

JSON Web Token implementations that do not carefully select a key source to verify a token received from client might be susceptible to key confusion attacks and can enable an attacker to forge token payload.

Vulnerable implementations of JSON Web Token (JWT) could be susceptible to an attack known as key confusion attack where an attacker can modify a token signing algorithm to a symmetric algorithm (such as HMAC) and re-sign the token to generate a new token. This attack utilizes the fact that some libraries use the same variable name for the secret that signs/verifies the HMAC symmetric encryption, and the secret that contains the public key used for verifying an RSA-signed token. By tweaking the algorithm to an HMAC variant (HS256/HS384/HS512) and signing it using the available public key, an attacker can trick the service into verifying the HMAC token using the hard-coded public key in the secret variable. An attacker might be able to get the public key that server recognizes as valid from claims such as jwk, jku, x5u, and x5c.