The external identity provider callback at "GET /auth/external" accepts attacker-controlled redirect URIs that only need to start with a registered client redirect URI, rather than matching exactly. After a successful external IdP login, the server appends Medplum "login" and "code" values to that attacker-supplied URL and issues a redirect. Because the external login request "state" is serialized as raw JSON and later trusted by the callback, an attacker who can tamper with "state.redirectUri" can cause Medplum to redirect authorization artifacts to an attacker-controlled endpoint. When the registered redirect URI is a bare origin or another prefix that can be extended into a different hostname, this becomes a cross-origin authorization code leak.