HAPI FHIR is a complete implementation of the HL7 FHIR standard for healthcare interoperability in Java. Prior to 6.9.11, XhtmlParser.java imposes no maximum element nesting depth, so a deeply nested text.div narrative triggers unbounded recursion between parseElementInner() and parseElement(), raising a StackOverflowError. An attacker who can submit FHIR resources containing such narratives can thus crash a parsing or validation worker thread, affecting validator services and any application that parses attacker-supplied FHIR JSON or XML. This issue is fixed in version 6.9.11.
https://github.com/hapifhir/org.hl7.fhir.core/security/advisories/GHSA-5v24-q6x8-hc38
https://github.com/hapifhir/org.hl7.fhir.core/commit/396f447500407693d6ae1e64db59782862ca7506