JWT x5c Header

The x5c header parameter carries an X.509 certificate chain as an array of base64-encoded DER certificates. The first certificate should correspond to the key that signed the JWS compact serialization. Verifiers that accept x5c must validate the chain against trusted roots and confirm the leaf key matches the signature, not merely parse PEM-looking data and move on. Embedding certificates in tokens increases size and can encourage insecure shortcuts like trusting any self-presented chain without PKIX checks. Prefer JWKS managed by your identity provider when possible for rotation. If x5c is required for interoperability, combine real certificate path validation with algorithm allowlists and audience checks on every request.