CVE-2026-37977: Red Hat Build Of Keycloak

Medium severity, CVSS 5.3. EPSS: 0.3% chance of exploitation in the next 30 days.

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: ["*"]`.

Affected products

• Red Hat Build Of Keycloak: affected versions not specified

Published 2026-04-06. Last modified 2026-06-26.