The default configuration of ASP.NET in Mono before 2.6.4 has a value of FALSE for the EnableViewStateMac property, which allows remote attackers to conduct cross-site scripting (XSS) attacks, as demonstrated by the __VIEWSTATE parameter to 2.0/menu/menu1.aspx in the XSP sample project.
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.