GHSA-5v8v-xvjv-57x7 Keycloak vulnerable to information disclosure via CORS header injection due to unvalidated JWT azp claim

• Maven org.keycloak:keycloak-services: < 26.4.13 → fixed in 26.4.13 • Maven org.keycloak:keycloak-services: >= 26.5.0, < 26.6.3 → fixed in 26.6.3

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: ["*"]`.