This indicates an attack attempt to exploit a remote Code Injection vulnerability in Apache Solar.
The vulnerability is due to insufficient handling of user supplied inputs in the application when handling crafted HTTP requests. A remote attacker can exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
Apache Software Foundation Solr 6.x up to 6.6.1
Apache Software Foundation Solr 7.x up to 7.0.1
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's advisory for updates: