jwt-simple prior to 0.5.3 are vulnerable to Signature Verification Bypass. If no algorithm is specified in the
decode() function, the packages uses the algorithm in the JWT to decode tokens. This allows an attacker to create a HS256 (symmetric algorithm) JWT with the server's public key as secret, and the package will verify it as HS256 instead of RS256 (asymmetric algorithm).
Upgrade to version 0.5.3 or later.