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: ["*"]`.
History

Mon, 06 Apr 2026 13:00:00 +0000

Type Values Removed Values Added
References
Metrics threat_severity

None

ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

threat_severity

Low


Mon, 06 Apr 2026 09:15:00 +0000

Type Values Removed Values Added
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: ["*"]`.
Title Keycloak: org.keycloak.protocol.oidc.grants.ciba: keycloak: information disclosure via cors header injection due to unvalidated jwt azp claim
First Time appeared Redhat
Redhat build Keycloak
Weaknesses CWE-346
CPEs cpe:/a:redhat:build_keycloak:
Vendors & Products Redhat
Redhat build Keycloak
References
Metrics cvssV3_1

{'score': 3.7, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N'}


cve-icon MITRE

Status: PUBLISHED

Assigner: redhat

Published:

Updated: 2026-04-06T11:57:17.383Z

Reserved: 2026-04-06T07:48:39.721Z

Link: CVE-2026-37977

cve-icon Vulnrichment

Updated: 2026-04-06T11:57:12.995Z

cve-icon NVD

Status : Received

Published: 2026-04-06T09:16:17.850

Modified: 2026-04-06T09:16:17.850

Link: CVE-2026-37977

cve-icon Redhat

Severity : Low

Publid Date: 2026-04-06T08:34:01Z

Links: CVE-2026-37977 - Bugzilla

cve-icon OpenCVE Enrichment

No data.