In Apache Struts 2.3.x before 2.3.31, and 2.5.x before 2.5.5, it is possible to prepare a special URL which will be used for path traversal and execution of arbitrary code on server side. This vulnerability is only exploitable when using the Struts 2 Convention plugin in conjunction with Apache Struts.
In Apache Struts 2.0.0 through 2.3.33 and 2.5 through 2.5.10.1, using an unintentional expression in a Freemarker tag instead of string literals can lead to a RCE attack.
Apache Struts 2 before 2.3.29 and 2.5.x before 2.5.1 allow attackers to have unspecified impact via vectors related to improper action name clean up.
It was found that it is possible to pass a malicious expression which can be used to execute arbitrary code on server side when using the REST Plugin.
External References:
https://struts.apache.org/docs/s2-037.html