JPA Server in HAPI FHIR before 5.4.0 allows a user to deny service (e.g., disable access to the database after the attack stops) via history requests. This occurs because of a SELECT COUNT statement that requires a full index scan, with an accompanying large amount of server resources if there are many simultaneous history requests.

XSS exists in the HAPI FHIR testpage overlay module of the HAPI FHIR library before 3.8.0. The attack involves unsanitized HTTP parameters being output in a form page, allowing attackers to leak cookies and other sensitive information from ca/uhn/fhir/to/BaseController.java via a specially crafted URL. (This module is not generally used in production systems so the attack surface is expected to be low, but affected systems are recommended to upgrade immediately.)