source: http://www.securityfocus.com/bid/47784/info

Apache Struts is prone to a cross-site scripting vulnerability because it fails to sufficiently sanitize user-supplied data.

Successful exploitation requires 'Dynamic Method Invocation' to be enabled by default.

An attacker may leverage this issue to execute arbitrary script code in the browser of an unsuspecting user in the context of the affected site. This may allow the attacker to steal cookie-based authentication credentials and to launch other attacks.

Apache Struts versions 2.0.0 through 2.2.1.1 are vulnerable. 

http://www.example.com/struts2-blank/home.action!login:cantLogin<script>alert(document.cookie)</script>=some_value