def create(request, topic_id):
    topic = get_object_or_404(
        Topic.objects.for_access(request.user),
        pk=topic_id)
    form = NotificationCreationForm(
        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.