CVE-2026-37977

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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: [\"*\"]`."}, {"lang": "es", "value": "Se encontr\u00f3 un defecto en Keycloak. Un atacante remoto puede explotar una vulnerabilidad de inyecci\u00f3n de encabezado de Cross-Origin Resource Sharing (CORS) en el endpoint de token de User-Managed Access (UMA) de Keycloak. Este defecto ocurre porque la declaraci\u00f3n \u201cazp\u201d de un JSON Web Token (JWT) proporcionado por el cliente se utiliza para establecer el encabezado \u201cAccess-Control-Allow-Origin\u201d antes de que se valide la firma del JWT. Cuando un JWT especialmente dise\u00f1ado con un valor \u201cazp\u201d controlado por el atacante es procesado, este valor se refleja como el origen CORS, incluso si la concesi\u00f3n es rechazada posteriormente. Esto puede llevar a la exposici\u00f3n de informaci\u00f3n de baja sensibilidad de las respuestas de error del servidor de autorizaci\u00f3n, debilitando el aislamiento de origen, pero solo cuando un cliente objetivo est\u00e1 mal configurado con \u201cwebOrigins: [\u2019*\u2019]\u201d."}], "lastModified": "2026-10-26T21:10:00.170", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:a:redhat:build_of_keycloak:-:*:*:*:-:*:*:*", "vulnerable": true, "matchCriteriaId": "E5C930CB-4EAD-497B-A44B-D880F2A1F85B"}], "operator": "OR"}]}], "sourceIdentifier": "secalert@redhat.com"}