Django/Forms: How can i validate initial data against received data (if match)?

Question

Let's say that I'm creating a form and I'm passing some hidden values through bound, that must not be changed. My question is how can I test if a malicious user has changed this hidden values? I'm not sure what exactly does bound data in a form and the difference between initial.

In the Django's forms.py is a property called changed_data but I don't know if can help or not.

Code for demonstration:

forms.py

class ConfirmForm(forms.Form):
    client_id = forms.CharField(widget=forms.HiddenInput())
    identifier = forms.CharField(widget=forms.HiddenInput())

    def clean(self):
        # Maybe here the validation process of cliend_id and identifier like:
        clean_client_id = self.cleaned_data.get('client_id')
        clean_identifier = self.cleaned_data.get('identifier')
        if last_client_id == clean_client_id and 
           last_identifier == clean_identifier:
            return self.cleaned_data
        else:
            raise forms.ValidationError("False data.")

views.py

def form_confirm_handler(request):
    if request.method == 'POST':
        form = ConfirmForm(request.POST)
        if form.is_valid():
            #Do something...
            return redirect('home:index')

    #The following values are not fixed. Are generated variables!
    bound_data = {'client_id':'123456','identifier':'wuiy5895'}
    form = ConfirmForm(bound_data)
    return render(request, 'client/theform.html', {'form':form})

html template

<form action="{% url 'client:confirm' %}" method="post">
    <p>Do you really want to proceed?</p>
    {% csrf_token %}
    {{ form.client_id }}
    {{ form.identifier }}
    <button id="submit" type="submit" name="submit" class="btn" value="accept">Accept</button>
    <button id="cancel" type="submit" name="cancel" class="btn btn-primary" value="cancel">Cancel</button>
</form>

Thanks in advance!

Answer 1

I found 4 (easy) possible solutions to this problem.

The most valid solution for Django is this:

class TheFormName():
    client_id = forms.CharField(show_hidden_initial=True, widget=forms.HiddenInput())
    identifier = forms.CharField(show_hidden_initial=True, widget=forms.HiddenInput())

def clean(self):
    if self.has_changed():
        raise forms.ValidationError('Fields are not valid.')

    return self.cleaned_data

The second solution is by using the changed_data to see what has changed:

def clean(self):
    for field_name in self.changed_data:
        # loop through the fields which have changed
        print "field {} has changed. New value {}".format(field_name, cleaned_data['field_name']

For my case is translated to this, but is exactly the same as the has_changed() method:

def clean(self):
    if len(self.changed_data) > 0:
        raise forms.ValidationError('Fields are not valid.')

    return self.cleaned_data

Another solution that looks more like a hack is:

self.cleaned_data['cliend_id'] == self.instance.cliend_id
self.cleaned_data['identifier'] == self.instance.identifier

And the final solution a bit more complex is by using sessions inside clean() method (and outside of view). Example from Django Docs:

from django.contrib.sessions.backends.db import SessionStore
import datetime
s = SessionStore()
s['last_login'] = datetime.datetime(2005, 8, 20, 13, 35, 10)
s.save()
s.session_key
>>> '2b1189a188b44ad18c35e113ac6ceead'

s = SessionStore(session_key='2b1189a188b44ad18c35e113ac6ceead')
s['last_login']

Also useful is this post In Django 1.4, do Form.has_changed() and Form.changed_data, which are undocumented, work as expected? provided by @LarsVegas

Answer 2

Create a new hidden field called temper_seal

temper_seal = forms.CharField(widget=forms.HiddenInput())

Set the initial value of temper_seal to a hash of client_id and identifier and some random constant string only known to your server.

When the form comes back with user data, hash the values of client_id and identifier and the constant string used earlier. Compare this value to the value submitted in the hidden temper_seal field. If they are the same, the user did not change the data in client_id and identifier.

Answer 3

I been thinking if you can use a session variables.

def form_confirm_handler(request):
    if request.method == 'POST':
        form = ConfirmForm(request.POST)
        if form.is_valid():

            if form.cleaned_data.get['client_id'] == request.session.get('client_id'):
                //.....
            else:
                //.....

            //delete the session after comparing
            del request.session['client_id']
            del request.session['identifier']

            return redirect('home:index')

    #The following values are not fixed. Are generated variables!

    request.session['client_id'] = '123456'
    request.session['identifier'] = 'wuiy5895'

    bound_data = {
        'client_id': request.session.get('client_id'), 
        'identifier': request.session.get('identifier')
    }

    form = ConfirmForm(bound_data)
    return render(request, 'client/theform.html', {'form':form})