CVE-2026-65625: Attacker-controlled AES-GCM authentication tag length allows tag forgery in jose

The pure-Erlang AES-GCM fallback in josejwaaes derives the number of verified authentication tag bits from the attacker-supplied tag itself. In the {aesgcm, Bits} clause of blockdecrypt/4, the incoming tag is guarded only by isbinary/1, with no check on its length. Verification in gcmblockdecrypt/6 then computes TBits = bitsize(T) and compares only those bits of the real 128-bit tag. An attacker who controls a JWE token can truncate the authentication tag to as little as 8 bits, so only 8 bits are ever checked. This reduces forgery resistance from 2^-128 to 2^-8, and a forged ciphertext is reliably accepted as authentic within 256 attempts without knowledge of the key (an authentication tag forgery and integrity bypass). Supplying a tag longer than 128 bits instead causes a badmatch that crashes the decrypting process. The fallback is reached through the public JWE decryption path whenever AES-GCM is not provided by a native crypto NIF, for example on FIPS or stripped or older OTP builds.

This issue affects jose: from 1.2.0 before 1.11.13.