This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Apache Struts.
The vulnerability is due to insufficient validation of user supplied inputs in the application. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
Apache Software Foundation Struts 2.3.20 to 18.104.22.168
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the latest update from the vendor