This indicates an attack attempt against a Denial-Of-Service vulnerability in Apache Struts.
The vulnerability is caused by an error when the vulnerable software handles maliciously crafted packets. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system, via a crafted packet.
Apache Software Foundation Struts 2.5 through 2.5.12
Apache Software Foundation Struts 2.3.7 through 2.3.33
XStream XStream prior to 1.4.10
Denial of Service: Remote attackers can crash vulnerable systems.