This indicates an attack attempt to exploit a Remote Code Execution Vulnerability in Apache Software Foundation Struts.
The vulnerability is due to insufficient input validation leading to a forced double OGNL evaluation when evaluating raw user input. A remote attacker can exploit this vulnerability by sending a crafted HTTP request containing a malicious parameter to a vulnerable server. Successful exploitation could result in the execution of arbitrary code with the privileges of the server.
Apache Software Foundation Struts 2.0.0 through 2.5.25
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.