A flaw was found in Keycloak. A remote attacker can exploit a Cross-Origin Resource Sharing (CORS) header injection vulnerability in Keycloak's User-Managed Access (UMA) token endpoint. This flaw occurs because the "azp" claim from a client-supplied JSON Web Token (JWT) is used to set the "Access-Control-Allow-Origin" header before the JWT signature is validated. When a specially crafted JWT with an attacker-controlled "azp" value is processed, this value is reflected as the CORS origin, even if the grant is later rejected. This can lead to the exposure of low-sensitivity information from authorization server error responses, weakening origin isolation, but only when a target client is misconfigured with "webOrigins: ["*"]".