This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Apache Struts.
The vulnerability is caused by an error when the REST plugin with XStream handler deserialises XML requests. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
Apache Software Foundation Struts 2.5 to 2.5.12
System Compromise: Remote attackers can gain control of vulnerable systems.
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