Kibana versions 6.7.0 to 6.8.8 and 7.0.0 to 7.6.2 contain a prototype pollution flaw in the Upgrade Assistant. An authenticated attacker with privileges to write to the Kibana index could insert data that would cause Kibana to execute arbitrary code. This could possibly lead to an attacker executing code with the permissions of the Kibana process on the host system.
The vulnerable functionality was introduced in Kibana 6.7, we ship Kibana 5.6 and it has not been back ported. Additionally, it is a component of the X-Pack, paid additions for Kibana.