I've searched around quite a bit to find the definitive cause of the issue.
This post from Microsoft really helped explain all the different causes.
http://support.microsoft.com/kb/2915218
Cause 4 is what we have landed on which is an invalid ViewStateUserKeyValue
Setting ViewStateUserKey to Session.SessionID or User.Identity.Name did not work for us.
We intermittently got the validation error due to the following.
When the application pool is reset by IIS, the session is renewed in effect causing the error.
We drop the Session on login to avoid session fixation, also resulting in the error on login.
What finally worked for us was a cookie based solution, which is now provided in VS2012.
public partial class SiteMaster : MasterPage
{
private const string AntiXsrfTokenKey = "__AntiXsrfToken";
private const string AntiXsrfUserNameKey = "__AntiXsrfUserName";
private string _antiXsrfTokenValue;
protected void Page_Init(object sender, EventArgs e)
{
//First, check for the existence of the Anti-XSS cookie
var requestCookie = Request.Cookies[AntiXsrfTokenKey];
Guid requestCookieGuidValue;
//If the CSRF cookie is found, parse the token from the cookie.
//Then, set the global page variable and view state user
//key. The global variable will be used to validate that it matches in the view state form field in the Page.PreLoad
//method.
if (requestCookie != null
&& Guid.TryParse(requestCookie.Value, out requestCookieGuidValue))
{
//Set the global token variable so the cookie value can be
//validated against the value in the view state form field in
//the Page.PreLoad method.
_antiXsrfTokenValue = requestCookie.Value;
//Set the view state user key, which will be validated by the
//framework during each request
Page.ViewStateUserKey = _antiXsrfTokenValue;
}
//If the CSRF cookie is not found, then this is a new session.
else
{
//Generate a new Anti-XSRF token
_antiXsrfTokenValue = Guid.NewGuid().ToString("N");
//Set the view state user key, which will be validated by the
//framework during each request
Page.ViewStateUserKey = _antiXsrfTokenValue;
//Create the non-persistent CSRF cookie
var responseCookie = new HttpCookie(AntiXsrfTokenKey)
{
//Set the HttpOnly property to prevent the cookie from
//being accessed by client side script
HttpOnly = true,
//Add the Anti-XSRF token to the cookie value
Value = _antiXsrfTokenValue
};
//If we are using SSL, the cookie should be set to secure to
//prevent it from being sent over HTTP connections
if (FormsAuthentication.RequireSSL &&
Request.IsSecureConnection)
responseCookie.Secure = true;
//Add the CSRF cookie to the response
Response.Cookies.Set(responseCookie);
}
Page.PreLoad += master_Page_PreLoad;
}
protected void master_Page_PreLoad(object sender, EventArgs e)
{
//During the initial page load, add the Anti-XSRF token and user
//name to the ViewState
if (!IsPostBack)
{
//Set Anti-XSRF token
ViewState[AntiXsrfTokenKey] = Page.ViewStateUserKey;
//If a user name is assigned, set the user name
ViewState[AntiXsrfUserNameKey] =
Context.User.Identity.Name ?? String.Empty;
}
//During all subsequent post backs to the page, the token value from
//the cookie should be validated against the token in the view state
//form field. Additionally user name should be compared to the
//authenticated users name
else
{
//Validate the Anti-XSRF token
if ((string)ViewState[AntiXsrfTokenKey] != _antiXsrfTokenValue
|| (string)ViewState[AntiXsrfUserNameKey] !=
(Context.User.Identity.Name ?? String.Empty))
{
throw new InvalidOperationException("Validation of
Anti-XSRF token failed.");
}
}
}
}
Source
