CVE-2026-0976 identifies an improper input validation vulnerability in the Red Hat Build of Keycloak, a widely used open-source identity and access management solution. The issue arises because Keycloak accepts RFC-compliant matrix parameters embedded within URL path segments. While these parameters are valid per RFC standards, many common reverse proxy configurations do not properly recognize or handle them, leading to a mismatch in URL interpretation between the proxy and Keycloak. An attacker can exploit this discrepancy by crafting URLs that include matrix parameters to mask or alter path segments, effectively bypassing proxy-level path filtering rules. This can result in exposure of administrative or sensitive endpoints that operators believe are shielded from external access. The vulnerability is remotely exploitable without authentication or user interaction, but the complexity of crafting effective requests and the limited impact on confidentiality (only partial information disclosure without integrity or availability compromise) contribute to its low CVSS score of 3.7. No known exploits have been reported in the wild, and no patches were linked at the time of publication, indicating that mitigation relies heavily on configuration adjustments and monitoring. This vulnerability highlights the importance of consistent URL parsing between proxies and backend services, especially in security-critical components like identity management platforms.

For European organizations, the primary risk is unauthorized exposure of administrative or sensitive Keycloak endpoints that are assumed to be protected by reverse proxies. This could lead to information disclosure about internal configurations or user data, potentially aiding further attacks. Although the vulnerability does not directly allow code execution or denial of service, the exposure of sensitive endpoints can weaken the overall security posture. Organizations relying on Keycloak for authentication and authorization may face increased risk of targeted attacks if attackers gain insight into administrative interfaces. The impact is more pronounced in environments where reverse proxies are misconfigured or do not properly handle matrix parameters. Given Keycloak's role in identity management, any compromise or information leakage can have cascading effects on access control across multiple applications and services. However, the low CVSS score and lack of known exploits suggest the immediate threat level is limited but should not be ignored.

European organizations should take the following specific actions: 1) Review and update reverse proxy configurations (e.g., Apache, NGINX, HAProxy) to ensure they correctly parse and handle RFC-compliant matrix parameters in URL paths, preventing bypass of path filtering rules. 2) Implement strict access controls and network segmentation to limit exposure of Keycloak administrative endpoints to trusted internal networks only. 3) Monitor web server and proxy logs for unusual URL patterns containing matrix parameters that could indicate exploitation attempts. 4) Apply any vendor-provided patches or updates for Red Hat Build of Keycloak as soon as they become available. 5) Conduct security testing and validation of proxy and Keycloak integration to confirm consistent URL interpretation. 6) Educate DevOps and security teams about this vulnerability to raise awareness of subtle URL parsing issues. These targeted mitigations go beyond generic advice by focusing on proxy configuration and monitoring specific to matrix parameter handling.