High, CVSS 7.1npmPublished
Medplum: Improper Validation of Redirect URI in External Auth Callback allows Authorization Code Leakage
CVE-2026-53728 · GHSA-m44r-7c5h-m6mj
- Affected
- npm @medplum/core <= 5.1.5
- Fixed in
- 5.1.6· Vendor fix available
- Issue
- 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…
Recommended action: Upgrade to 5.1.6 or later, then redeploy and verify.A vendor fix does not resolve your finding until you verify it in your own systems.