django-safeform 2.0.0

Revision history

v2.0.0 - 18 September 2009

Breaks backward compatibility with previous release - no longer changes the form constructor signature to take just the request object and decide whether or not to bind the form based on the request method. You now need to make that decision yourself in your view code (just as you do with regular Django forms). All examples and tests have been updated.

Added CsrfTestCase to test_utils, to simplify testing of CSRF protected forms.

v1.0.1 - 17 September 2009

Documentation fixes.

v1.0.0 - 17 September 2009

Initial release.

Installation

django-safeform is in PyPI: http://pypi.python.org/pypi/django-safeform

pip install django-safeform
- OR -
easy_install django-safeform

Custom form templates

If your template uses one of the form rendering helper methods such as {{ form.as_p }} the hidden csrf_token field will be output automatically. If you are rendering the form using a custom template you will need to remember to output that field in your template explicitly. Here’s an example:

<form action="/change-password/" method="post">
    {{ form.non_field_errors }}
    <div{% if form.password.errors %} class="errors"{% endif %}>
        <label for="id_password">New password</label>
        {{ form.password }}
    </div>
    <div{% if form.password2.errors %} class="errors"{% endif %}>
        <label for="id_password2">Confirm password</label>
        {{ form.password2 }}
    </div>
    <div>{{ form.csrf_token }}<input type="submit" value="Change"></div>
</form>

Note the {{ form.csrf_token }} replacement variable just before the submit button - this will output a hidden form field containing the correct value.

You should also be sure to include {{ form.non_field_errors }} somewhere in your template - this is where the “Form session expired - please resubmit” message will be displayed should the CSRF check fail for some reason.

Protecting forms that do not use django.forms

If you are not using the django.forms framework - for example you are writing forms with hand-written HTML and pulling submitted data directly from request.POST - you can still add CSRF protection to your forms using the @csrf_protect decorator in conjunction with the csrf_utils module:

from django_safeform import csrf_protect, csrf_utils

@csrf_protect
def hand_rolled(request):
    if request.method == 'POST':
        csrf_token = request.POST.get('csrf_token', '')
        if not csrf_utils.validate_csrf_token(csrf_token, request):
            return HttpResponse('Invalid CSRF token')
        else:
            return HttpResponse('OK')
    else:
        return HttpResponse("""
        <form action="." method="post">
        <input type="text" name="name">
        <input type="hidden" name="csrf_token" value="%s">
        </form>
        """ % csrf_utils.new_csrf_token(request))

It is your responsibility to include a hidden form field with the value from csrf_utils.new_csrf_token(request) in your form, and to check that token when the form is submitted using csrf_utils.validate_csrf_token.

You could also use CsrfForm to protect hand-written forms, as explained in the next section.

Protecting formsets / multiple forms on the same page

If you have multiple forms on the page and they are each contained in separate <form> elements, you should ensure each one has a csrf_token field, most likely by configuring each one using the SafeForm class decorator.

If you have multiple django.forms forms within a single <form> element (for example, if you are using formsets) you still only need to include a single csrf_token field for the overall form. In this case, rather than applying the SafeForm decorator to each of the form classes, it makes more sense to have a single standalone SafeForm instance within the overall form. The CsrfForm class is designed to handle this exact use-case. Here’s how to use it:

from django import forms
from django.forms.formsets import formset_factory
from django_safeform import csrf_protect, CsrfForm
from django.http import HttpResponse

class PersonForm(forms.Form):
    name = forms.CharField(max_length = 100)
    email = forms.EmailField()

PersonFormSet = formset_factory(PersonForm, extra=3)

@csrf_protect
def formset(request):
    csrf_form = CsrfForm(request)
    formset = PersonFormSet()
    if request.method == 'POST':
        csrf_form = CsrfForm(request, request.POST)
        formset = PersonFormSet(request.POST)
        if csrf_form.is_valid() and formset.is_valid():
            return HttpResponse('Valid: %s' % ', '.join([
                '%(name)s [%(email)s]' % form.cleaned_data
                for form in formset.forms
                if form.cleaned_data
            ]))
    return render_to_response('formset.html', {
        'csrf_form': csrf_form,
        'formset': formset,
    })

The formset is used in the same way as usual. The CSRF protection is implemented entirely by the separate CsrfForm instance. In the template, the CsrfForm should be output using {{ csrf_form }} like this:

<form action="." method="post">
    {{ csrf_form }}
    {% for form in formset.forms %}
        {{ form.as_p }}
    {% endfor %}
    <p>
        {{ formset.management_form }}
        <input type="submit">
    </p>
</form>

The {{ csrf_form }} template tag specifies where the hidden input field containing the form token should be included. Should a CSRF failure occur, it also specifies where the <ul class=”errorlist”> containing the CSRF failure message should be displayed.

If you want to include the hidden input field in a different location to the error message, you can use {{ csrf_form.csrf_token }} to output the hidden field and {{ csrf_form.non_field_errors }} to output the error message:

<form action="." method="post">
    {{ csrf_form.non_field_errors }}
    {% for form in formset.forms %}
        {{ form.as_p }}
    {% endfor %}
    <p>
        {{ formset.management_form }}
        {{ csrf_form.csrf_token }}
        <input type="submit">
    </p>
</form>

Changing the CSRF error message

The default message shown to the user if the CSRF check fails is:

Form session expired - please resubmit

The wording here is deliberately a bit vague - most users will have no idea what a “CSRF failure” is, but users have probably seen “session expired” messages before. A “form session” seems like a reasonable metaphor for what is going on under the hood.

If you dislike this message, you can over-ride it in your call to the SafeForm class decorator:

ChangePasswordForm = SafeForm(ChangePasswordForm,
    invalid_message='CSRF check failed'
)

Handling Ajax

By default, Ajax requests do NOT have CSRF protection applied to them - they will be ignored by the form validator, which looks out for any requests where request.is_ajax() returns True (i.e. requests which have a X-Requested_With header set to XMLHttpRequest). This custom header is set by most common Ajax libraries. Protection is not needed here because it is not possible to forge HTTP headers when committing a CSRF attack using an HTML form.

If you are ultra-paranoid and want to apply CSRF protection even to requests with that header, you can disable the Ajax special case like this:

ChangePasswordForm = SafeForm(ChangePasswordForm,
    ajax_skips_check=False
)

If you do this, you will need to ensure the csrf_token is included in Ajax POST requests yourself. One way to do this would be to read the token out of the first hidden input field with name=”csrf_token” using the JavaScript DOM.

Enhancing security

By default, the form tokens served up in hidden fields are valid for POST submissions to any form on the site (for the user with that cookie) and never expire. You can limit the scope of the tokens in two ways - by tying them to a specific form, or by causing them to expire after a number of seconds.

To tie a token to one specific form, simply pass an identifier argument to the SafeForm decorator:

ChangePasswordForm = SafeForm(ChangePasswordForm,
    identifier='change-password'
)

Tokens generated for that form will now only allow submissions back to the same form. If you are using the csrf_utils module directly (in a hand-rolled form for example) you should pass identifier arguments to both the new_csrf_token and validate_csrf_token functions:

token = csrf_utils.new_csrf_token(request, identifier='my-custom-form')
# ... later ...
token_ok = csrf_utils.validate_csrf_token(
    token, request, identifier='my-custom-form'
)
if not token_ok:
    return HttpResponse('Invalid CSRF token')

To cause your form tokens to expire, use the expire_after argument in your call to the SafeForm class decorator:

ChangePasswordForm = SafeForm(ChangePasswordForm,
    identifier='change-password',
    expire_after=24 * 60 * 60 # Expire after 24 hours
)

If using csrf_utils directly, pass that argument to the validate_csrf_token function:

token_ok = csrf_utils.validate_csrf_token(
    token, request, identifier='my-custom-form', expire_after=24 * 60 * 60
)

The default expire_after value is set by the CSRF_TOKENS_EXPIRE_AFTER setting, which defaults to None. If you want all of your CSRF tokens to expire, add this to your settings.py file:

CSRF_TOKENS_EXPIRE_AFTER = 24 * 60 * 60

When a token expires, the user will see the CSRF error message described above but will not lose their form submission, so don’t worry too much about the consequences of setting a strict timeout.

Protecting GET forms

The examples so far have all been for forms submitted using the POST verb. It is also possible to protect GET forms against CSRF, but you should very rarely need to do this. HTTP specifies that actions submitted via GET should be idempotent, which is generally interpreted as ruling that they should not cause state changes to your system. As such, an authenticated GET request should not be able to cause any damage.

Many GET forms are specifically designed to allow links from other sites to trigger an action - search forms for example. If your application has GET requests that require CSRF protection you should probably rethink the design of your application.

If you decide you do need to add CSRF protection to a GET form, you should be aware that it is much easier for csrf_tokens used in GET requests to “leak” to an external attacker. URLs show up in Browser referral headers, so links to external sites from your CSRF protected GET page will inadvertently pass the token on to those sites. Your users may also accidentally share their CSRF tokens by pasting them in to e-mails or bookmarking them on link sharing sites.

Consequently, any CSRF tokens used in GET forms should take advantage of the extra security features documented above - they should use an identifier to lock the token down just to that form and should specify a strict expiry time to limit the damage that can be caused should the token accidentally leak.

Unit testing

Properly unit testing CSRF protection is significantly more complicated than a regular unit test, as you need to first GET the initial form, then extract the csrf_token field from it, then submit that as part of the POST. The unit tests that ship with django_safeform show how to do this.

You can shortcut this process by using CsrfTestCase as the base class for your unit tests. This swaps in an alternative Client implementation which causes POST requests using client.post() to automatically include a valid CSRF token.

Here’s an example test using CsrfTestCase:

from django_safeform import test_utils

class SubmitTestCase(test_utils.CsrfTestCase):
    def test_submission_with_correct_csrf_token_works(self):
        response = self.client.post('/safe-basic-form/', {
            'name': 'Test',
        })
        self.assertEqual(response.content, 'Valid: Test')

If you are using the CsrfTestCase subclass but you do NOT wish to include the csrf_token automatically in one of your tests, pass a csrf=False argument to the client.post() method:

def test_submission_without_csrf_token(self):
    response = self.client.post('/safe-basic-form/', {
        'name': 'Test',
    }, csrf=False)
    self.assert_(CSRF_INVALID_MESSAGE in response.content)

If you are protecting your forms with a custom form identifier, you should pass that identifier as the csrf argument:

def test_change_password(self):
    response = self.client.post('/change-password/', {
        'password': 'new-password',
        'password2': 'new-password',
    }, csrf='change-password')
    # ...

If you are already using your own custom TestCase subclass and do not wish to use CsrfTestCase, you can instantiate the special client in your own setUp method:

from django_safeform import test_utils
from django.test import TestCase

class MyTestCaseSubclass(TestCase):
    def setUp(self):
        self.client = test_utils.CsrfClient()

Design notes

Apps shipped with Django, in particular the admin, MUST be secure against CSRF no matter what the user’s configuration is (so dependency on middleware alone is a problem).

Secure by default for user code would be nice, but in its absence explicitly raising developer awareness of CSRF is probably a good thing.

Should not be tied to sessions - some developers might not be using them.

Should not require the form framework - hand-rolled forms should be easy to protect too.

The original idea was to have an alternative Form base class called SafeForm - this was replaced with a class decorator when we realised that we would otherwise also need to provide SafeModelForm, SafeFormSet and so on.

Credits

This library was developed from a discussion with Russell Keith-Magee, Andrew Godwin and Armin Ronacher at the DjangoCon 2009 sprints, and improved based on extensive feedback from Luke Plant on the django-developers mailing list.

TODO

• Figure out what to do about protecting forms which already alter the Form constructor signature themselves.

• _csrf_token_from_request should throw error if cookie has not been set by the csrf_protect decorator.

• Improved support for unit testing CSRF protected forms.

Alternative approaches

Pure middleware:

• breaks with etags

• rewrites HTML

• doesn’t work with streaming

• you have to decorate things as exempt

• potential leakage of external forms

• XHTML v.s. HTML

Middleware and template tags and RequestContext:

• uses a view middleware

• applying by default is error prone

• if user disables middleware, admin becomes insecure

• requires RequestContext

• form submissions are lost on CSRF failure