Bug 2455324 (CVE-2026-37977)

Summary: CVE-2026-37977 keycloak: org.keycloak.protocol.oidc.grants.ciba: Keycloak: Information disclosure via CORS header injection due to unvalidated JWT azp claim
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: low Docs Contact:
Priority: low    
Version: unspecifiedCC: aschwart, boliveir, mposolda, pjindal, rmartinc, security-response-team, ssilvert, sthorger, vmuzikar
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
Whiteboard:
Fixed In Version: Doc Type: ---
Doc Text:
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: ["*"]`.
 Story Points: ---
Clone Of: Environment:
Last Closed: Type: ---
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-04-06 07:50:01 UTC 
CORS header injection vulnerability in Keycloak’s UMA token endpoint. The flaw is caused by reading the azp claim from a client-supplied JWT 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, that value is reflected as the CORS origin even though the grant is later rejected. This can be exploited remotely without authentication when a target client is misconfigured with webOrigins: ["*"]. Attackers can then read UMA error responses cross-origin, weakening origin isolation and exposing low-sensitivity information from the authorization server.