Medplum is a developer platform that enables development of healthcare apps. Prior to version 5.1.6, 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. This issue has been patched in version 5.1.6.

Affected: npm: @medplum/core

## Summary

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.

## Technical Explanation

The vulnerable flow is exposed on the unauthenticated callback route `GET /auth/external`.

In `externalCallbackHandler()`, the server parses the external auth `state` and uses the decoded `clientId` and `redirectUri` after completing the IdP code exchange. If login succeeds and a client is found, the handler calls:

- `getClientRedirectUri(client, body.redirectUri, true)`

The third argument explicitly enables partial matching. In `getClientRedirectUri()`, the function returns the attacker-supplied `requestedUri` whenever:

- `requestedUri.startsWith(uri)`

As a result, any redirect URI beginning with a registered value is accepted. The returned URL is then passed into `new URL(redirectUri)…