This indicates an attack attempt to exploit an Arbitrary File Read Vulnerability in Jenkins.
The vulnerability is due to an error in the vulnerable application when handling a maliciously crafted request. An attacker can exploit this to disclose sensitive information on the affected machine via a crafted request.
Jenkins LTS prior to 2.121.2
Jenkins weekly prior to 2.133
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Apply the most recent upgrade or patch from the vendor: