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 crafted HTTP request containing a malicious "Content-Type" or "Content-Dispostion" field. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
Struts 2.3.5 - Struts 2.3.31
Struts 2.5 - Struts 2.5.10
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.