CVE-2026-45363: `jwt` (Ruby gem) - empty-key HMAC bypass

JWT.decode(token, '', true, algorithm: 'HS256') accepts an attacker-forged token. OpenSSL::HMAC.digest('SHA256', '', payload) returns a valid digest under an empty key, and no raise InvalidKeyError if key.empty? precondition exists in the HMAC algorithm.

JWT.decode(token, "", true, algorithm: 'HS256') -> JWA::Hmac.verify(verificationkey: "", ...) -> OpenSSL::HMAC.digest('SHA256', "", signinginput) == signature

The same path is reached when a keyfinder block or keyfinder: argument returns "", nil, or an array containing nil for an unknown key. JWT::Decode#findkey only rejects literal nil and empty arrays, and JWT::JWA::Hmac silently coerces nil to "" (signingkey ||= '') before signing.

JWT.decode(token, nil, true, algorithms: ['HS256']) { |h| "" } -> findkey returns "" # "" && !Array("").empty? == true -> JWA::Hmac.verify(verificationkey: "", ...) -> verifies Common application patterns that produce the unsafe value: redis.get("kid:#{kid}").tos, ORM string columns with default: '', ENV['SECRET'] || '', Hash.new('') lookups, [primary, fallback] where fallback may be nil. Applications passing a non-empty static key:, or whose keyfinder returns nil / raises on miss, are not affected.

The existing enforcehmackeylength option would block this but defaults to false. On OpenSSL ≥ 3.5 the empty-key HMAC.digest call no longer raises, so the OpenSSL-3.0 rescue in JWA::Hmac#sign does not fire.

Affects HS256/HS384/HS512 via both JWT.decode (positional key and block keyfinder) and JWT::EncodedToken#verifysignature!(keyfinder:)