A flaw was found in the Microsoft social identity provider in Keycloak. When a Microsoft identity provider is configured with a specific tenant ID, the restriction is correctly enforced for standard browser-based logins. However, the external access-token exchange endpoint fails to perform this validation.
The root cause is that Keycloak calls the tenant-agnostic Microsoft Graph /me endpoint during the exchange process. It accepts the returned user profile and issues Keycloak tokens without verifying that the tenant associated with the original Microsoft token matches the configured tenant ID in the identity provider settings.
An attacker with a valid Microsoft Graph access token from an arbitrary tenant can submit it to the Keycloak token exchange endpoint. Successful exploitation allows the attacker to bypass tenant restrictions, assume an identity within the Keycloak realm, and perform actions with the privileges of that identity, such as accessing protected resources or modifying user data.