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: ["*"]`.
Problem types
Product status
Timeline
| 2026-04-06: | Reported to Red Hat. |
| 2026-04-06: | Made public. |
References
access.redhat.com/security/cve/CVE-2026-37977
bugzilla.redhat.com/show_bug.cgi?id=2455324 (RHBZ#2455324)