Medium [CVE-2026-18215] Microsoft external access-token exchange bypasses configured tenant

Keycloak provides a way to let users log in using Microsoft accounts while restricting access to a specific organization (tenant). A flaw was discovered where this restriction is ignored when using the token exchange feature.

This means an attacker with a valid Microsoft token from a completely different organization could gain access to the Keycloak realm, potentially accessing sensitive data or performing unauthorized actions.

The Red Hat Product Security team has assessed the severity of this vulnerability as Moderate, given that exploitation requires the attacker to possess a valid Microsoft Graph access token from a different tenant. Successful exploitation allows an attacker to bypass configured tenant restrictions and gain unauthorized access to a Keycloak realm.

The vulnerability's root cause is the lack of tenant ID verification during the external access-token exchange flow when interacting with the Microsoft Graph API. Weakness: CWE-287.

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.