Ruby's OpenSSL bindings do not check the length of the supplied authentication tag when decrypting an authenticated encryption mode such as AES-GCM, leaving this up to the authors of a gem/app to implement for properly validating the message.

json-jwt was not checking for the authentication tag length, meaning that with a one byte tag the JWT would be considered not tampered with. This means that with an average of 128 (max 256) attempts an attacker can forge a valid signature.