Elasticsearch Security versions 6.5.0 and 6.5.1 contain an XXE flaw in Machine Learning's find_file_structure API. If a policy allowing external network access has been added to Elasticsearch's Java Security Manager then an attacker could send a specially crafted request capable of leaking content of local files on the Elasticsearch node. This could allow a user to access information that they should not have access to.
https://www.elastic.co/community/security
https://discuss.elastic.co/t/elastic-stack-6-5-2-security-update/159594