def create(request, topic_id):
    topic = get_object_or_404(Topic, pk=topic_id)
    form = FavoriteForm(user=request.user, topic=topic, data=request.POST)

    if form.is_valid():
        form.save()
    else:
        messages.error(request, utils.render_form_errors(form))

    return redirect(request.POST.get('next', topic.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.