Medplum deployments before 5.1.14 contain a server‑side request forgery flaw in the subscription worker that lets an authenticated user create FHIR Subscription resources with any endpoint URL. When the subscription fires, the worker makes an HTTP request to that URL and includes the full POST body, which contains the entire FHIR payload. Per the description, an attacker can target internal services such as cloud instance metadata, internal databases or orchestration endpoints, thereby exfiltrating IAM credentials and patient health records.

The affected product is Medplum, specifically all releases prior to version 5.1.14. Any instance that has not yet applied this update is within the risk window and is vulnerable to SSRF via subscription resources.

The CVSS base score of 6.3 indicates a moderate severity; the EPSS score is not available, and the vulnerability is not listed in the CISA KEV catalog. Because the flaw requires authenticated access to create subscription resources, the attack vector is internal to the application: a legitimate user can exploit it by submitting a specially crafted subscription, which the server then forwards to arbitrary internal systems. Successful exploitation could allow the adversary to read or write data from internal services and exfiltrate sensitive data contained in the POST body.