The REST Plugin in Apache Struts 2.1.x, 2.3.7 through 2.3.33 and 2.5 through 2.5.12 is using an outdated XStream library which is vulnerable and allow perform a DoS attack using malicious request with specially crafted XML payload.
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20170907-struts2
https://struts.apache.org/docs/s2-051.html
https://security.netapp.com/advisory/ntap-20180629-0001/
http://www.securitytracker.com/id/1039262
http://www.securityfocus.com/bid/100611
http://www.oracle.com/technetwork/security-advisory/alert-cve-2017-9805-3889403.html