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Medplum - Improper Validation of Redirect URI in External Auth Callback allows Authorization Code Leakage

Type: advisorySources: OSV

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.

Products
Not specified
Affected versions
Not specified
Fixed versions
Not specified
CVEs
CVE-2026-53728
Confirmed fixed CVEs
Not specified
Updated in this feed
8 Nov 2026, 03:27 UTC · Revision 1

Source evidence

Medplum - Improper Validation of Redirect URI in External Auth Callback allows Authorization Code Leakage

core-osv · Original source

Published 3 Sept 2026, 19:04 UTC

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.

osv:CVE-2026-53728
Complete source record
{
  "id": "68bd51ce34f1f3a5b27d47dc48bd14fb8966c0b892bb0d5d771dce4a110fc512",
  "url": "https://osv.dev/vulnerability/CVE-2026-53728",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Medplum - Improper Validation of Redirect URI in External Auth Callback allows Authorization Code Leakage",
    "summary": "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.",
    "evidence": "osv:CVE-2026-53728",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-53728",
      "aliases": [
        "CVE-2026-53728",
        "GHSA-m44r-7c5h-m6mj"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:L"
        }
      ],
      "affected": [
        {
          "ranges": [
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              "repo": "https://github.com/medplum/medplum",
              "type": "GIT",
              "events": [
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                {
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              "database_specific": {
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                  {
                    "range": [
                      {
                        "introduced": "0"
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                        "fixed": "5.1.6"
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                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
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          }
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      ],
      "modified": "2026-10-28T02:30:53.345950667Z",
      "provider": "osv",
      "references": [
        "https://github.com/medplum/medplum/releases/tag/v5.1.6",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53728.json",
        "https://github.com/medplum/medplum/security/advisories/GHSA-m44r-7c5h-m6mj",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-53728",
        "https://github.com/medplum/medplum/commit/7ae10035ddadde4dba7b18d3156553940465b3a1",
        "https://github.com/medplum/medplum/pull/8749"
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        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/53xxx/CVE-2026-53728.json"
      }
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    "related_cves": [
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        "evidence": "/id"
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    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 522206,
  "published_at": "2026-09-03T19:04:30.358+00:00"
}