The vulnerability exists in Medplum's external authentication callback handler, which accepts redirect URIs that only need to start with a registered client redirect URI rather than matching exactly. The server appends authorization artifacts (login and code) to the attacker-controlled redirect URI and issues a redirect. Because the state parameter is serialized as raw JSON and can be tampered with, an attacker can supply a redirect URI that prefixes a registered URI but points to a different origin, resulting in cross-origin leakage of authorization codes. The exact-match check is only used for logging and does not prevent the redirect. This flaw allows an attacker to capture authorization codes and redeem them if the PKCE verifier is known, leading to full account takeover.

This vulnerability has a high impact as it enables an attacker to leak authorization codes to an attacker-controlled endpoint by exploiting improper redirect URI validation. Successful exploitation can lead to full account takeover (ATO) of users authenticating via the external identity provider flow, compromising confidentiality and integrity of user accounts. The CVSS score is 7.1 (High), reflecting network attack vector with high confidentiality and integrity impact but requiring user interaction and high attack complexity.

A patch is available for this vulnerability. Users should upgrade Medplum to version 5.1.6 or later, where the redirect URI validation enforces exact matching rather than prefix matching. Until patched, avoid registering redirect URIs that can be prefix-matched and extended into attacker-controlled domains. Review client redirect URI configurations to ensure they do not allow prefix overlaps with untrusted domains. Patch status is confirmed as a fix is available.