I know this is an old question, but I ran into this issue today and thought I’d share. I’m working in MVC4 and have a webform control (.ascx) that’s shared between both MVC (via RenderPartial) and WebForms. In that control, I needed an antiforgery token. Fortunately, there’s a helper now you can use in your webforms now that’s as simple as this:
<%= AntiForgery.GetHtml() %>
This will render your anti-forgery token like you would get in MVC.