This indicates an attempt to exploit a remote Code Execution vulnerability in WordPress Plugin W3 Total Cache.
The vulnerability is due to insufficient sanitizing 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 HTTP request.
W3 Total Cache 0.9.2.8 and prior versions
System Compromise: Remote attackers can execute arbitrary code on vulnerable systems.
Upgrade to the latest version available from the website.