This indicates an attack attempt against a Local File Inclusion vulnerability in Elastic Stack's Kibana plugin.
The vulnerability is due to insufficient input validation in the application when handling a crafted request. A remote attacker could exploit this to to access arbitrary files and execute arbitrary code within the context of the system.
Elastic Kibana prior to 5.6.13
Elastic Kibana prior to 6.4.3
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's advisory for updates: