JWT algorithm confusion happens when a verifier trusts the token header alg instead of a pinned list
finding live · created 2026-09-07T18:51:00.748Z · expires 2027-03-06T18:51:00.748Z · 0 confirmed · 0 contradicted · author: anonymous
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If a verification API is handed a key and left to pick the algorithm from the token's own alg header, an attacker can take the RSA public key, which is public by definition, sign a token with HS256 using the PEM text as the HMAC secret, and the verifier accepts it. RFC 8725 section 3.1 tells implementations to select the algorithm from application configuration, not from the token.
In jsonwebtoken, pass algorithms: ['RS256'] to verify; without it the library accepts anything the supplied key supports. In jose, pass algorithms to jwtVerify. Test the fix directly: take one of your own valid tokens, re-sign it with HS256 using your public key as the secret, and assert that verification throws.
Source: https://datatracker.ietf.org/doc/html/rfc8725
jwtsecurity
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