CVE-2026-18203 in Keycloak
Summary
by MITRE • 07/31/2026
A flaw was found in the group policy evaluation logic of Keycloak, an identity and access management solution. When a group policy is set to extend permissions to child groups, the system incorrectly uses a simple text-based prefix check to verify group membership. This allows a user who belongs to a different group with a similar starting name to bypass security checks and gain unauthorized access to administrative functions or protected resources.
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Analysis
by VulDB Data Team • 07/31/2026
This vulnerability exists within Keycloak's group policy evaluation mechanism, specifically in how the system handles permission inheritance across nested group structures. The flaw stems from an insufficient validation approach that relies on basic string prefix matching rather than proper hierarchical group membership verification. When administrators configure group policies to extend permissions to child groups, the system should verify legitimate parent-child relationships through structured group hierarchies. However, the implementation uses a naive text-based comparison that merely checks if a user's group name begins with the parent group identifier. This approach fundamentally violates security principles by creating an attack surface where malicious actors can exploit naming similarities to bypass intended access controls.
The technical implementation of this vulnerability demonstrates a clear failure in access control validation logic that aligns with CWE-284 Access Control Bypass Through User-Controlled Key patterns. The system's group membership verification process operates without proper authorization checking mechanisms, creating a privilege escalation vector where users can manipulate their group assignments to match prefix criteria of higher-privileged groups. This flaw particularly affects organizations that rely on hierarchical group structures for role-based access control, as it undermines the fundamental security model that assumes proper group membership validation. The vulnerability is especially concerning because it operates at the policy evaluation layer, meaning it affects all applications and services that depend on Keycloak's authentication and authorization decisions.
Operational impacts of this vulnerability extend beyond simple unauthorized access to encompass potential data breaches, privilege escalation attacks, and complete compromise of administrative functions within Keycloak-managed environments. Attackers can exploit this flaw by creating group names that match the prefix criteria of legitimate administrative groups, thereby gaining access to sensitive resources without proper authentication or authorization. The attack requires minimal technical skill since it relies on simple naming conventions rather than complex exploitation techniques, making it particularly dangerous for organizations with less sophisticated security monitoring capabilities. This vulnerability effectively nullifies the security benefits of hierarchical group policies, potentially allowing unauthorized users to access confidential data, modify system configurations, or perform administrative actions that should be restricted to authorized personnel only.
Organizations utilizing Keycloak must implement immediate mitigations to address this vulnerability while awaiting official patches from Red Hat. The most effective immediate solution involves implementing additional verification layers beyond simple prefix matching, such as requiring explicit group membership validation through proper hierarchical tree traversal mechanisms. Administrators should also consider implementing stricter group naming conventions and monitoring for suspicious group creation patterns that might indicate exploitation attempts. Organizations should conduct comprehensive audits of their group policy configurations to identify any instances where child group permissions are improperly extended without proper access control verification. The mitigation approach should align with ATT&CK technique T1078 Valid Accounts to ensure that the solution addresses the root cause rather than merely patching symptoms, requiring a fundamental redesign of how group membership is validated within the Keycloak authorization framework.