public class FbAuthorize : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        if (!Service.SignedIn())
            RouteToAction("Account", "Index"); // Fictional method (I wish it existed)
    }
}

public static class ServerHelper {

  public static void Transfer(ActionExecutingContext filterContext, string url) {

    // Rewrite path
    HttpContext.Current.RewritePath(GetPath(filterContext, url), false);

    IHttpHandler httpHandler = new System.Web.Mvc.MvcHttpHandler();

    // Process request
    httpHandler.ProcessRequest(HttpContext.Current);
    filterContext.HttpContext.Response.End();
  }

  private static string GetPath(ActionExecutingContext filterContext, string url) {
      HttpRequestBase request = filterContext.HttpContext.Request;

      UriBuilder uriBuilder = new UriBuilder(request.Url.Scheme, request.Url.Host, request.Url.Port, request.ApplicationPath);

      uriBuilder.Path += url;

      return filterContext.HttpContext.Server.UrlDecode(uriBuilder.Uri.PathAndQuery);
  }
}

ServerHelper.Transfer(filterContext, "Account/Index");