The __VIEWSTATE functionality in Microsoft ASP.NET 1.x allows remote attackers to conduct replay attacks to (1) apply a ViewState generated from one view to a different view, (2) reuse ViewState information after the application's state has changed, or (3) use the ViewState to conduct attacks or expose content to third parties.
https://exchange.xforce.ibmcloud.com/vulnerabilities/20409
http://secunia.com/advisories/15241
http://scottonwriting.net/sowblog/posts/3747.aspx