Kibana versions before 5.6.15 and 6.6.1 had a cross-site scripting (XSS) vulnerability that could allow an attacker to obtain sensitive information from or perform destructive actions on behalf of other Kibana users.
https://www.elastic.co/community/security
https://discuss.elastic.co/t/elastic-stack-6-6-1-and-5-6-15-security-update/169077