ASP.NET MVC Forms Authentication + Authorize Attribute + Simple Roles

I think I’ve implemented something similar.
My solution, based on NerdDinner tutorial, is following.

When you sign the user in, add code like this:

var authTicket = new FormsAuthenticationTicket(
    1,                             // version
    userName,                      // user name
    DateTime.Now,                  // created
    DateTime.Now.AddMinutes(20),   // expires
    rememberMe,                    // persistent?
    "Moderator;Admin"                        // can be used to store roles
    );

string encryptedTicket = FormsAuthentication.Encrypt(authTicket);

var authCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encryptedTicket);
System.Web.HttpContext.Current.Response.Cookies.Add(authCookie);

Add following code to Global.asax.cs:

protected void Application_AuthenticateRequest(Object sender, EventArgs e)
{
    HttpCookie authCookie = Context.Request.Cookies[FormsAuthentication.FormsCookieName];
    if (authCookie == null || authCookie.Value == "")
        return;

    FormsAuthenticationTicket authTicket;
    try
    {
        authTicket = FormsAuthentication.Decrypt(authCookie.Value);
    }
    catch
    {
        return;
    }

    // retrieve roles from UserData
    string[] roles = authTicket.UserData.Split(';');

    if (Context.User != null)
        Context.User = new GenericPrincipal(Context.User.Identity, roles);
}

After you’ve done this, you can use [Authorize] attribute in your controller action code:

[Authorize(Roles="Admin")]
public ActionResult AdminIndex ()

Please let me know if you have further questions.