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310 lines
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310 lines
12 KiB
Plaintext
.. _using-csrf:
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===================================
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How to use Django's CSRF protection
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===================================
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To take advantage of CSRF protection in your views, follow these steps:
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#. The CSRF middleware is activated by default in the :setting:`MIDDLEWARE`
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setting. If you override that setting, remember that
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``'django.middleware.csrf.CsrfViewMiddleware'`` should come before any view
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middleware that assume that CSRF attacks have been dealt with.
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If you disabled it, which is not recommended, you can use
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:func:`~django.views.decorators.csrf.csrf_protect` on particular views
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you want to protect (see below).
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#. In any template that uses a POST form, use the :ttag:`csrf_token` tag inside
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the ``<form>`` element if the form is for an internal URL, e.g.:
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.. code-block:: html+django
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<form method="post">{% csrf_token %}
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This should not be done for POST forms that target external URLs, since
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that would cause the CSRF token to be leaked, leading to a vulnerability.
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#. In the corresponding view functions, ensure that
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:class:`~django.template.RequestContext` is used to render the response so
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that ``{% csrf_token %}`` will work properly. If you're using the
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:func:`~django.shortcuts.render` function, generic views, or contrib apps,
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you are covered already since these all use ``RequestContext``.
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.. _csrf-ajax:
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Using CSRF protection with AJAX
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===============================
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While the above method can be used for AJAX POST requests, it has some
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inconveniences: you have to remember to pass the CSRF token in as POST data with
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every POST request. For this reason, there is an alternative method: on each
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XMLHttpRequest, set a custom ``X-CSRFToken`` header (as specified by the
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:setting:`CSRF_HEADER_NAME` setting) to the value of the CSRF token. This is
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often easier because many JavaScript frameworks provide hooks that allow
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headers to be set on every request.
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First, you must get the CSRF token. How to do that depends on whether or not
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the :setting:`CSRF_USE_SESSIONS` and :setting:`CSRF_COOKIE_HTTPONLY` settings
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are enabled.
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.. _acquiring-csrf-token-from-cookie:
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Acquiring the token if :setting:`CSRF_USE_SESSIONS` and :setting:`CSRF_COOKIE_HTTPONLY` are ``False``
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-----------------------------------------------------------------------------------------------------
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The recommended source for the token is the ``csrftoken`` cookie, which will be
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set if you've enabled CSRF protection for your views as outlined above.
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The CSRF token cookie is named ``csrftoken`` by default, but you can control
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the cookie name via the :setting:`CSRF_COOKIE_NAME` setting.
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You can acquire the token like this:
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.. code-block:: javascript
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function getCookie(name) {
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let cookieValue = null;
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if (document.cookie && document.cookie !== '') {
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const cookies = document.cookie.split(';');
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for (let i = 0; i < cookies.length; i++) {
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const cookie = cookies[i].trim();
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// Does this cookie string begin with the name we want?
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if (cookie.substring(0, name.length + 1) === (name + '=')) {
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cookieValue = decodeURIComponent(cookie.substring(name.length + 1));
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break;
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}
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}
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}
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return cookieValue;
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}
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const csrftoken = getCookie('csrftoken');
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The above code could be simplified by using the `JavaScript Cookie library
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<https://github.com/js-cookie/js-cookie/>`_ to replace ``getCookie``:
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.. code-block:: javascript
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const csrftoken = Cookies.get('csrftoken');
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.. note::
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The CSRF token is also present in the DOM in a masked form, but only if
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explicitly included using :ttag:`csrf_token` in a template. The cookie
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contains the canonical, unmasked token. The
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:class:`~django.middleware.csrf.CsrfViewMiddleware` will accept either.
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However, in order to protect against `BREACH`_ attacks, it's recommended to
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use a masked token.
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.. warning::
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If your view is not rendering a template containing the :ttag:`csrf_token`
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template tag, Django might not set the CSRF token cookie. This is common in
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cases where forms are dynamically added to the page. To address this case,
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Django provides a view decorator which forces setting of the cookie:
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:func:`~django.views.decorators.csrf.ensure_csrf_cookie`.
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.. _BREACH: https://www.breachattack.com/
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.. _acquiring-csrf-token-from-html:
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Acquiring the token if :setting:`CSRF_USE_SESSIONS` or :setting:`CSRF_COOKIE_HTTPONLY` is ``True``
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--------------------------------------------------------------------------------------------------
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If you activate :setting:`CSRF_USE_SESSIONS` or
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:setting:`CSRF_COOKIE_HTTPONLY`, you must include the CSRF token in your HTML
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and read the token from the DOM with JavaScript:
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.. code-block:: html+django
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{% csrf_token %}
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<script>
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const csrftoken = document.querySelector('[name=csrfmiddlewaretoken]').value;
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</script>
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Setting the token on the AJAX request
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-------------------------------------
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Finally, you'll need to set the header on your AJAX request. Using the
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`fetch()`_ API:
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.. code-block:: javascript
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const request = new Request(
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/* URL */,
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{
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method: 'POST',
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headers: {'X-CSRFToken': csrftoken},
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mode: 'same-origin' // Do not send CSRF token to another domain.
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}
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);
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fetch(request).then(function(response) {
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// ...
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});
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.. _fetch(): https://developer.mozilla.org/en-US/docs/Web/API/fetch
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Using CSRF protection in Jinja2 templates
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=========================================
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Django's :class:`~django.template.backends.jinja2.Jinja2` template backend
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adds ``{{ csrf_input }}`` to the context of all templates which is equivalent
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to ``{% csrf_token %}`` in the Django template language. For example:
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.. code-block:: html+jinja
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<form method="post">{{ csrf_input }}
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Using the decorator method
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==========================
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Rather than adding ``CsrfViewMiddleware`` as a blanket protection, you can use
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the :func:`~django.views.decorators.csrf.csrf_protect` decorator, which has
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exactly the same functionality, on particular views that need the protection.
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It must be used **both** on views that insert the CSRF token in the output, and
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on those that accept the POST form data. (These are often the same view
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function, but not always).
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Use of the decorator by itself is **not recommended**, since if you forget to
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use it, you will have a security hole. The 'belt and braces' strategy of using
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both is fine, and will incur minimal overhead.
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.. _csrf-rejected-requests:
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Handling rejected requests
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==========================
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By default, a '403 Forbidden' response is sent to the user if an incoming
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request fails the checks performed by ``CsrfViewMiddleware``. This should
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usually only be seen when there is a genuine Cross Site Request Forgery, or
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when, due to a programming error, the CSRF token has not been included with a
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POST form.
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The error page, however, is not very friendly, so you may want to provide your
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own view for handling this condition. To do this, set the
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:setting:`CSRF_FAILURE_VIEW` setting.
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CSRF failures are logged as warnings to the :ref:`django.security.csrf
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<django-security-logger>` logger.
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Using CSRF protection with caching
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==================================
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If the :ttag:`csrf_token` template tag is used by a template (or the
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``get_token`` function is called some other way), ``CsrfViewMiddleware`` will
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add a cookie and a ``Vary: Cookie`` header to the response. This means that the
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middleware will play well with the cache middleware if it is used as instructed
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(``UpdateCacheMiddleware`` goes before all other middleware).
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However, if you use cache decorators on individual views, the CSRF middleware
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will not yet have been able to set the Vary header or the CSRF cookie, and the
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response will be cached without either one. In this case, on any views that
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will require a CSRF token to be inserted you should use the
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:func:`django.views.decorators.csrf.csrf_protect` decorator first::
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from django.views.decorators.cache import cache_page
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from django.views.decorators.csrf import csrf_protect
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@cache_page(60 * 15)
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@csrf_protect
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def my_view(request):
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...
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If you are using class-based views, you can refer to :ref:`Decorating
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class-based views<decorating-class-based-views>`.
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Testing and CSRF protection
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===========================
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The ``CsrfViewMiddleware`` will usually be a big hindrance to testing view
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functions, due to the need for the CSRF token which must be sent with every POST
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request. For this reason, Django's HTTP client for tests has been modified to
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set a flag on requests which relaxes the middleware and the ``csrf_protect``
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decorator so that they no longer rejects requests. In every other respect
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(e.g. sending cookies etc.), they behave the same.
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If, for some reason, you *want* the test client to perform CSRF
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checks, you can create an instance of the test client that enforces
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CSRF checks::
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>>> from django.test import Client
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>>> csrf_client = Client(enforce_csrf_checks=True)
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Edge cases
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==========
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Certain views can have unusual requirements that mean they don't fit the normal
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pattern envisaged here. A number of utilities can be useful in these
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situations. The scenarios they might be needed in are described in the following
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section.
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Disabling CSRF protection for just a few views
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----------------------------------------------
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Most views requires CSRF protection, but a few do not.
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Solution: rather than disabling the middleware and applying ``csrf_protect`` to
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all the views that need it, enable the middleware and use
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:func:`~django.views.decorators.csrf.csrf_exempt`.
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Setting the token when ``CsrfViewMiddleware.process_view()`` is not used
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------------------------------------------------------------------------
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There are cases when ``CsrfViewMiddleware.process_view`` may not have run
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before your view is run - 404 and 500 handlers, for example - but you still
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need the CSRF token in a form.
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Solution: use :func:`~django.views.decorators.csrf.requires_csrf_token`
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Including the CSRF token in an unprotected view
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-----------------------------------------------
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There may be some views that are unprotected and have been exempted by
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``csrf_exempt``, but still need to include the CSRF token.
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Solution: use :func:`~django.views.decorators.csrf.csrf_exempt` followed by
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:func:`~django.views.decorators.csrf.requires_csrf_token`. (i.e. ``requires_csrf_token``
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should be the innermost decorator).
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Protecting a view for only one path
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-----------------------------------
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A view needs CSRF protection under one set of conditions only, and mustn't have
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it for the rest of the time.
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Solution: use :func:`~django.views.decorators.csrf.csrf_exempt` for the whole
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view function, and :func:`~django.views.decorators.csrf.csrf_protect` for the
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path within it that needs protection. Example::
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from django.views.decorators.csrf import csrf_exempt, csrf_protect
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@csrf_exempt
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def my_view(request):
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@csrf_protect
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def protected_path(request):
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do_something()
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if some_condition():
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return protected_path(request)
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else:
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do_something_else()
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Protecting a page that uses AJAX without an HTML form
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-----------------------------------------------------
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A page makes a POST request via AJAX, and the page does not have an HTML form
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with a :ttag:`csrf_token` that would cause the required CSRF cookie to be sent.
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Solution: use :func:`~django.views.decorators.csrf.ensure_csrf_cookie` on the
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view that sends the page.
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CSRF protection in reusable applications
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========================================
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Because it is possible for the developer to turn off the ``CsrfViewMiddleware``,
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all relevant views in contrib apps use the ``csrf_protect`` decorator to ensure
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the security of these applications against CSRF. It is recommended that the
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developers of other reusable apps that want the same guarantees also use the
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``csrf_protect`` decorator on their views.
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