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 sent to showcase plugin. 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.x with Struts 1 plugin and Struts 1 action
System Compromise: Remote attackers can gain control of vulnerable systems.
Refer to the vendor's website for suggested workaround.