def create_access(request, topic_id):
    topic_private = TopicPrivate.objects.for_create_or_404(topic_id, request.user)
    form = TopicPrivateInviteForm(
        topic=topic_private.topic,
        data=post_data(request))

    if form.is_valid():
        form.save()
        notify_access(user=form.get_user(), topic_private=topic_private)
    else:
        messages.error(request, utils.render_form_errors(form))

    return redirect(request.POST.get('next', topic_private.get_absolute_url()))
A web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a Redirect. This simplifies phishing attacks.