CVE-2026-16093: Keycloak-services: keycloak-services: required signed-jwt assertion policy can be bypassed with unsigned assertion headers

A flaw was found in the Secure Client Registration executor within the keycloak-services component. The vulnerability exists in the logic that enforces the signed-JWT assertion policy for client registration and updates. When the SecureSigningAlgorithmForSignedJwtExecutor is configured to require a client assertion, it only checks the alg field in the raw client assertion JWT header. It fails to validate that the client assertion type is present or that the client's actual authenticator is assertion-based. By providing a specially crafted request with an unsigned assertion header containing a valid alg field, an attacker with valid client credentials can bypass the signature verification check. This allows the attacker to successfully authenticate or update a client without providing the required cryptographic proof of identity. Successful exploitation allows an attacker to circumvent administrative security policies intended to restrict client authentication to trusted assertion-based methods.