The vulnerability stems from insecure XML parsing in jbpmmigration's XmlUtils class, as explicitly stated in the advisory. The removal of JbpmMigration usage in jbpm-designer's commit (including the deletion of JbpmMigration.transform() calls in TransformerServlet.java) confirms these functions were the attack surface. The XXE occurs when parsing XML without disabling external entities, a hallmark of CWE-611. The high confidence derives from the explicit linkage between the advisory's description, the CWE, and the code changes in the referenced commit.