This indicates an attack attempt to exploit a Security Bypass vulnerability in Apache Struts.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when handling a crafted HTTP request A remote attacker may be able to exploit this to run arbitrary script code within the context of the application, via a crafted HTTP request.
Struts 1.x through 1.3.10
System Compromise: Remote attackers can execute arbitrary script code within the context of the target user's browser
Upgrade to the latest Struts 2