Medium [CVE-2026-18203] Group policy extendChildren matches sibling group path prefixes

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.

The Red Hat Product Security team has assessed the severity of this vulnerability as Moderate, given that exploitation requires a specific group naming collision and the extendChildren policy setting to be active. Successful exploitation allows an attacker to gain unauthorized integrity-impacting permissions within the realm.

The vulnerability's root cause is an improper string prefix comparison during group path validation in the authorization services. Weakness: CWE-863.

Affected Red Hat products: Red Hat Build of Keycloak. Red Hat lists Red Hat Data Grid 8; Red Hat JBoss Enterprise Application Platform Expansion Pack; Red Hat Single Sign-On 7 as not affected.

Red Hat does not currently list a fixing RHSA for this CVE.