Inserire questo codice in global.asax:

    Sub Application_BeginRequest(ByVal sender As Object, ByVal e As EventArgs)
        Dim path As String = HttpContext.Current.Request.Url.PathAndQuery
        Dim pagequery As String = path.Substring(path.LastIndexOf("/") + 1)
        Dim pagequery_Elements As String() = pagequery.Split("?")
        Dim ReturnUrl As String = pagequery_Elements(pagequery_Elements.Length - 1)
        If ReturnUrl.Contains("ReturnUrl") Then Response.Redirect("~/Login.aspx", True)
    End Sub