REDHAT-BUG-2503138: High severity Keycloak keycloak-services vulnerability

A vulnerability was found in the keycloak-services component where the SAML IdP-initiated broker login endpoint fails to enforce the link-only identity provider restriction. When a SAML identity provider is configured with linkOnly=true, it is intended to only allow users to link their accounts, not to use that provider for logging in. However, the endpoint at /realms/{realm}/broker/{alias}/endpoint/clients/{clientid} does not validate this setting. An attacker who controls an upstream SAML identity already linked to a Keycloak user can submit a valid signed SAML response directly to this endpoint. This allows the attacker to bypass the restriction and obtain a full authenticated session as the linked local user.