This indicates an attack attempt against a Directory Traversal vulnerability in Elastic ElasticSearch.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application while handling maliciously crafted requests. A remote attacker can exploit this to access arbitrary file on the targeted server, via a crafted request.
Elastic Elasticsearch prior to 1.6.1
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Apply the most recent upgrade or patch from the vendor