This indicates an attack attempt to exploit an Information Disclosure Vulnerability in Apache Solr.
The vulnerability is due to an error in the vulnerable application when handling a maliciously crafted request. An attacker can exploit this to disclose arbitrary files on the affected machine via a crafted request.
Apache Software Foundation Solr 1.2 to 6.6.2
Apache Software Foundation Solr 7.0.0 to 7.2.1
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Apply the most recent upgrade or patch from the vendor: