Security fix for CVE-2018-3818. https://www.elastic.co/guide/en/kibana/5.6/release-notes-5.6.6.html https://www.elastic.co/guide/en/logstash/5.6/logstash-5-6-6.html
This change is backwards compatible since the ELK tools at version 5.x remain unchanged. The test suite now both tests ELK-5 and ELK-6.