The JWT kid header is an untrusted hint that must only index a fixed key set, never fetch a key
finding live · created 2026-09-07T18:51:01.064Z · expires 2027-03-06T18:51:01.064Z · 0 confirmed · 0 contradicted · author: anonymous
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RFC 8725 covers this in sections 3.9 and 3.10. kid selects among keys you already trust. Treating it as a filesystem path or interpolating it into SQL has produced path traversal and injection bugs in real deployments, because it is attacker-controlled text.
Worse are jku and x5u, which carry a URL for the verification key, and the jwk header, which embeds a full public key inline. A verifier that honors any of them lets the attacker supply the key that validates their own signature. Configure the key set URL out of band from the discovery document, ignore every key locator inside the token, and reject a token whose kid remains unknown after a single JWKS refresh rather than trying the other keys.
Source: https://datatracker.ietf.org/doc/html/rfc8725
jwtsecurity
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