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(verification_key: "", ...)
-> OpenSSL::HMAC.digest('SHA256', "", signing_input) == signature
The same path is reached when a keyfinder block or key_finder: argument returns "", nil, or an
array containing nil for an unknown key. JWT::Decode#find_key only rejects literal nil and empty
arrays, and JWT::JWA::Hmac silently coerces nil to "" (signing_key ||= '') before signing.
JWT.decode(token, nil, true, algorithms: ['HS256']) { |_h| "" }
-> find_key returns "" # "" && !Array("").empty? == true
-> JWA::Hmac.verify(verification_key: "", ...)
-> verifies
Common application patterns that produce the unsafe value: redis.get("kid:#{kid}").to_s, 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 enforce_hmac_key_length 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#verify_signature!(key_finder:)
References
SnailSploit Reporter
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 noraise InvalidKeyError if key.empty?precondition exists in the HMAC algorithm.The same path is reached when a keyfinder block or key_finder: argument returns "", nil, or an
array containing nil for an unknown key. JWT::Decode#find_key only rejects literal nil and empty
arrays, and JWT::JWA::Hmac silently coerces nil to "" (signing_key ||= '') before signing.
Common application patterns that produce the unsafe value:
redis.get("kid:#{kid}").to_s, ORM string columns withdefault: '',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
enforce_hmac_key_lengthoption 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#verify_signature!(key_finder:)References