CVE-2026-18203 Keycloak-services: keycloak-services: group policy extendchildren matches sibling group path prefixes

Description

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.

Exploitable remotely over the network, with an ordinary account and without any user action; an attacker can seriously compromise confidentiality, integrity or availability.

Attack vector

Network

Attack complexity

Low

Privileges required

Low

Interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

High

Availability

None

Affected products and versions