XXE vulnerability in XSLT parsing in `org.hl7.fhir.core`

HAPI FHIR is a complete implementation of the HL7 FHIR standard for healthcare interoperability in Java. XSLT parsing performed by various components are vulnerable to XML external entity injections. A processed XML file with a malicious DTD tag ( <!DOCTYPE foo [<!ENTITY example SYSTEM "/etc/passwd"> ]> could produce XML containing data from the host system. This impacts use cases where org.hl7.fhir.core is being used to within a host where external clients can submit XML. This is related to GHSA-6cr6-ph3p-f5rf, in which its fix (#1571 & #1717) was incomplete. This issue has been addressed in release version 6.4.0 and all users are advised to upgrade. There are no known workarounds for this vulnerability.