CVE-2026-18203 Keycloak-services: keycloak-services: group policy extendchildren matches sibling group path prefixes
Attackers can bypass group-based access controls by exploiting a flawed prefix check, gaining unauthorized access to administrative functions.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-18203?
CVE-2026-18203 is a medium-severity authentication bypass vulnerability in Redhat Build of Keycloak, published on 31 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official 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.
Weakness: CWE-863
Which versions are affected by CVE-2026-18203?
It affects 1 product.
Products include Redhat Build of Keycloak.
How do I fix CVE-2026-18203?
No fix has been published for CVE-2026-18203 yet. Limit exposure of the affected software until one is available.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-18203 being exploited?
There is no known exploitation of CVE-2026-18203 in the wild and no public exploit code.
Other vulnerabilities in Redhat Build of Keycloak
- CVE-2026-15572 Keycloak-services: keycloak-services: dcr protocol mapper type-swap policy bypass allows privilege escalation
- CVE-2026-3047 Keycloak SAML Broken has Authentication Bypass by Primary Weakness
- CVE-2026-15573 Keycloak-services: keycloak-services: authorization bypass via unnormalized uri matching in pathmatcher
- CVE-2026-16102 Keycloak-services: keycloak-services: default dcr policy allows role forgery via user property mappers
- CVE-2026-1609 Org.keycloak/keycloak-quarkus-server: keycloak: unauthorized access via jwt authorization grant with disabled users
Sources
Also known as GHSA-wx5m-whr8-48h5.