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.

CVE-2026-53728 affects Medplum, a healthcare developer platform, prior to version 5.1.6. The vulnerability exists in the external identity provider callback endpoint (GET /auth/external), which accepts attacker-controlled redirect URIs that only need to start with a registered client redirect URI rather than matching exactly. An attacker can tamper with the state.redirectUri field (serialized as raw JSON and trusted by the callback) to redirect authorization artifacts to an attacker-controlled endpoint. This can result in a cross-origin authorization code leak when the registered redirect URI is a bare origin or extendable prefix. The flaw enables theft of Medplum login codes and tokens after a successful external IdP authentication flow. The issue has been fully patched in Medplum version 5.1.6. Healthcare application developers using Medplum should upgrade immediately to mitigate the risk of credential and authorization token theft.