This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Apache Struts.
The vulnerability is caused error handling issue when the application handles a malicious HTTP multipart request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
Struts 1.x through 1.3.10
System Compromise: Remote attackers can gain control of vulnerable systems.
Upgrade to the latest Struts 2