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exploiting-jwt-algorithm-confusion-attack

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Exploits JWT algorithm confusion where the server's verification library trusts the alg named in the token header, by switching RS256 to HS256 (signing with the RSA public key as HMAC secret), setting alg to none, or injecting kid/jku/x5u headers to supply an attacker-controlled key. Use when testing RS256 JWT auth for algorithm downgrade, alg:none bypass, or key-confusion signature forgery.

When to use

• Testing APIs that use RS256 (asymmetric) JWT tokens for authentication to check for algorithm downgrade to HS256
• Assessing JWT implementations for alg:none bypass where the server skips signature verification
• Evaluating JWT libraries for key confusion vulnerabilities where the public key is used as HMAC secret
• Testing kid (Key ID), jku (JWK Set URL), and x5u (X.509 URL) header parameters for injection
• Validating that the API server enforces a specific algorithm and does not trust the JWT header

When not to use

• Use without written authorization

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