This indicates an attack attempt to exploit a Security Bypass vulnerability in Jenkins.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted script. A remote attacker may be able to exploit this to bypass the sandbox protection on vulnerable systems to execute arbitrary code.
Pipeline: Declarative Plugin up to and including 1.3.4
Pipeline: Groovy Plugin up to and including 2.61
Script Security Plugin up to and including 1.49
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's advisory for updates: