JSON Web Token Signature Bypass via Unvalidated x5c Parameter

high Web App Scanning Plugin ID 115302

Synopsis

Description

JSON Web Tokens can be signed to protect against data tampering. The 'x5c' (X.509 Certificate Chain) header parameter can embed the certificate whose public key must be used to verify the token signature.

When the application trusts the certificate embedded in the 'x5c' header instead of a key it controls, an attacker can embed a self-signed certificate in the token header and sign a forged token with the matching private key, which will be accepted by the application.

Depending on the token usage, attackers could leverage this vulnerability to forge valid tokens and impersonate other users, or gain further privileges.

Solution

Ensure that the application validates every JSON Web Token against a trusted key it controls. Header parameters that influence key resolution (kid, jwk, jku, x5u, x5c) must never be trusted as-is: pin verification to a known key or a strict allow-list, reject tokens whose header selects an unexpected key, and sanitize any value used to look up or load a key.