The Apache Struts REST Plugin is using XStream library which is vulnerable and allow perform a DoS attack when using a malicious request with specially crafted XML payload. Upgrade to the Apache Struts version 2.5.16 and switch to an optional Jackson XML handler as described here http://struts.apache.org/plugins/rest/#custom-contenttypehandlers. Another option is to implement a custom XML handler based on the Jackson XML handler from the Apache Struts 2.5.16.
https://security.netapp.com/advisory/ntap-20180330-0001/
https://cwiki.apache.org/confluence/display/WW/S2-056
http://www.securitytracker.com/id/1040575
http://www.securityfocus.com/bid/103516
http://www.oracle.com/technetwork/security-advisory/cpujul2018-4258247.html