Elastic X-Pack Security versions prior to 5.4.1 and 5.3.3 did not always correctly apply Document Level Security to index aliases. This bug could allow a user with restricted permissions to view data they should not have access to when performing certain operations against an index alias.
https://www.elastic.co/community/security
https://www.elastic.co/blog/elasticsearch-5-4-1-and-5-3-3-released
https://discuss.elastic.co/t/elastic-stack-5-4-1-and-5-3-3-security-updates/87952