In Apache NiFi 1.2.0 to 1.11.4, the NiFi UI and API were protected by mandating TLS v1.2, as well as listening connections established by processors like ListenHTTP, HandleHttpRequest, etc. However intracluster communication such as cluster request replication, Site-to-Site, and load balanced queues continued to support TLS v1.0 or v1.1.
In Apache NiFi 1.0.0 to 1.11.4, the notification service manager and various policy authorizer and user group provider objects allowed trusted administrators to inadvertently configure a potentially malicious XML file. The XML file has the ability to make external calls to services (via XXE).