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.
http://www.securityfocus.com/bid/40351
http://www.mono-project.com/Vulnerabilities#ASP.NET_View_State_Cross-Site_Scripting
http://lists.opensuse.org/opensuse-security-announce/2010-08/msg00001.html
http://lists.opensuse.org/opensuse-security-announce/2010-06/msg00001.html
http://lists.opensuse.org/opensuse-security-announce/2010-05/msg00002.html