JWT::EncodedToken calls key_finder before checking the token's algorithm

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Since Nov 2, 2026.

  • #784 by @anakinj — open

Assessment

Difficulty
3/5
Estimated time
Half a day
Newbie friendliness
42/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
ruby, security
Domain
authentication, security

Research direction

The bug sits in lib/jwt/encoded_token.rb: in valid_signature?, key_finder.call(self) runs before the verifiers are built from the allowed algorithms. Reproduce with the snippet in the issue, which should show the finder called once before JWT::VerificationError. Done means a disallowed alg raises JWT::IncorrectAlgorithm without calling the finder, while JWK-supplied algorithm calls still pass the existing specs.

Written by the indexing model from the issue text.

Description

enhancement

JWT.decode rejects a token with a disallowed alg before looking up a key. JWT::EncodedToken#verify_signature! calls key_finder first, so tokens that could never pass still trigger the lookup.

Signature verification still fails correctly, so this isn't a bypass. The concern is that key finders often fetch a remote JWKS, which lets an unauthenticated alg: none or HS256 token cause outbound requests when the application expects only RS256.

rsa = OpenSSL::PKey::RSA.generate(2048)
token = JWT.encode({ 'sub' => 'x' }, 'secret', 'HS256')
calls = 0
finder = lambda { |_token|
  calls += 1
  rsa.public_key
}

JWT::EncodedToken.new(token).verify_signature!(algorithm: 'RS256', key_finder: finder)
# => JWT::VerificationError: Signature verification failed (calls == 1)

JWT.decode(token, nil, true, algorithm: 'RS256') { |_header| rsa.public_key }
# => JWT::IncorrectAlgorithm: Expected a different algorithm (block not called)

Reproduced on 3.3.1, and an alg: none token behaves the same way. lib/jwt/encoded_token.rb is unchanged on main.

Cause: EncodedToken#valid_signature? evaluates key_finder.call(self) before creating the verifiers from the allowed algorithms.

Suggestion: When algorithm: is given, check the token's alg against it before calling key_finder, and raise JWT::IncorrectAlgorithm if it isn't allowed. Calls that rely on a JWK to supply the algorithm would need to keep working.

Posted by Claude on behalf of @anakinj.

Dominant language
Ruby
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Forks
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Avg merge
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Merged PRs (30d)
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