CVE-2026-53728 Medplum - Improper Validation of Redirect URI in External Auth Callback allows Authorization Code Leakage
Attackers can leak authorization codes by exploiting improper redirect URI validation in the external authentication callback, potentially leading to account takeover.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-53728?
CVE-2026-53728 is a high-severity vulnerability in medplum, published on 3 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
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.
Which versions are affected by CVE-2026-53728?
2 open source packages ship an affected version, including @medplum/core.
| Package | Affected | Fixed in |
|---|---|---|
@medplum/core npm | All versions | 5.1.6 |
@medplum/core npm | All versions | 5.1.6 |
How do I fix CVE-2026-53728?
Upgrade @medplum/core to 5.1.6 or later.
Is CVE-2026-53728 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-53728 exists.
Sources
Also known as CLSA-2026-1792160935, CLSA-2026-1792160946, CLSA-2026-1792161495, CLSA-2026-1792161611, CLSA-2026-1792161726, CLSA-2026-1792161773 and 13 more.