Invicti identified a code evaluation (Apache Struts) which occurs when using an unintentional expression in Freemarker tag instead of string literals.

This is a highly critical issue and should be addressed as soon as possible.

An attacker can execute arbitrary code by using wrong construction in Freemarker tags. The attacker may also be able to execute arbitrary system commands.

Do not accept input from end users which will be directly interpreted as source code. If this is a business requirement, validate all input to the application by removing any data that could be directly interpreted as code.

This vulnerability is not difficult to leverage. There are vast resources available about FreeMarker tags. Successful exploitation requires knowledge of the programming language, access to or the ability to produce source code for use in such attacks, and minimal attack skills.

Update your Struts framework version to latest version.