CVE-2026-37977
Full vulnerability record from the local CVENow database.
Description
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: ["*"]`.
Severity
MEDIUM
CVSS score
5.3
(v3.1)
Vulnerability status
Modified
Source identifier
Published
2026-04-06 09:16:17.850000
Last modified
2026-10-26 21:10:00.170000
View original CVE JSON
{
"id": "CVE-2026-37977",
"sourceIdentifier": "[email protected]",
"published": "2026-04-06T09:16:17.850",
"lastModified": "2026-10-26T21:10:00.170",
"vulnStatus": "Modified",
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"value": "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: [\"*\"]`."
},
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"value": "Se encontró un defecto en Keycloak. Un atacante remoto puede explotar una vulnerabilidad de inyección 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 declaración “azp” de un JSON Web Token (JWT) proporcionado por el cliente se utiliza para establecer el encabezado “Access-Control-Allow-Origin” antes de que se valide la firma del JWT. Cuando un JWT especialmente diseñado con un valor “azp” controlado por el atacante es procesado, este valor se refleja como el origen CORS, incluso si la concesión es rechazada posteriormente. Esto puede llevar a la exposición de información de baja sensibilidad de las respuestas de error del servidor de autorización, debilitando el aislamiento de origen, pero solo cuando un cliente objetivo está mal configurado con “webOrigins: [’*’]”."
}
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"affected": [
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"description": [
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"references": [
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"source": "[email protected]"
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{
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{
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}