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For the case where someone is using contrib views with custom templates that they cannot update to use the template tag, it should be possible to use CsrfResponseMiddleware. This requires that 'csrf_response_exempt' is not used for the admin views. git-svn-id: http://code.djangoproject.com/svn/django/trunk@11683 bcc190cf-cafb-0310-a4f2-bffc1f526a37
397 lines
19 KiB
Plaintext
397 lines
19 KiB
Plaintext
.. _ref-contrib-csrf:
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=====================================
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Cross Site Request Forgery protection
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=====================================
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.. module:: django.middleware.csrf
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:synopsis: Protects against Cross Site Request Forgeries
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The CSRF middleware and template tag provides easy-to-use protection against
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`Cross Site Request Forgeries`_. This type of attack occurs when a malicious
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Web site contains a link, a form button or some javascript that is intended to
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perform some action on your Web site, using the credentials of a logged-in user
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who visits the malicious site in their browser. A related type of attack,
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'login CSRF', where an attacking site tricks a user's browser into logging into
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a site with someone else's credentials, is also covered.
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The first defense against CSRF attacks is to ensure that GET requests are
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side-effect free. POST requests can then be protected by following the steps
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below.
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.. versionadded:: 1.2
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The 'contrib' apps, including the admin, use the functionality described
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here. Because it is security related, a few things have been added to core
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functionality to allow this to happen without any required upgrade steps.
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.. _Cross Site Request Forgeries: http://www.squarefree.com/securitytips/web-developers.html#CSRF
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How to use it
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=============
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.. versionchanged:: 1.2
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The template tag functionality (the recommended way to use this) was added
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in version 1.2. The previous method (still available) is described under
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`Legacy method`_.
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To enable CSRF protection for your views, follow these steps:
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1. Add the middleware
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``'django.middleware.csrf.CsrfViewMiddleware'`` to your list of
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middleware classes, :setting:`MIDDLEWARE_CLASSES`. (It should come
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before ``CsrfResponseMiddleware`` if that is being used, and before any
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view middleware that assume that CSRF attacks have been dealt with.)
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Alternatively, you can use the decorator
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``django.views.decorators.csrf.csrf_protect`` on particular views you
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want to protect. This is **not recommended** by itself, since if you
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forget to use it, you will have a security hole. The 'belt and braces'
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strategy of using both is fine, and will incur minimal overhead.
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2. In any template that uses a POST form, use the ``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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<form action="" 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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3. In the corresponding view functions, ensure that the
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``'django.core.context_processors.csrf'`` context processor is
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being used. Usually, this can be done in one of two ways:
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1. Use RequestContext, which always uses
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``'django.core.context_processors.csrf'`` (no matter what your
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TEMPLATE_CONTEXT_PROCESSORS setting). If you are using
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generic views or contrib apps, you are covered already, since these
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apps use RequestContext throughout.
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2. Manually import and use the processor to generate the CSRF token and
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add it to the template context. e.g.::
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from django.core.context_processors import csrf
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from django.shortcuts import render_to_response
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def my_view(request):
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c = {}
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c.update(csrf(request))
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# ... view code here
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return render_to_response("a_template.html", c)
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You may want to write your own ``render_to_response`` wrapper that
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takes care of this step for you.
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The utility script ``extras/csrf_migration_helper.py`` can help to automate the
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finding of code and templates that may need to be upgraded. It contains full
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help on how to use it.
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Legacy method
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-------------
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In Django 1.1, the template tag did not exist. Instead, a post-processing
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middleware that re-wrote POST forms to include the CSRF token was used. If you
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are upgrading a site from version 1.1 or earlier, please read this section and
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the `Upgrading notes`_ below. The post-processing middleware is still available
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as ``CsrfResponseMiddleware``, and it can be used by following these steps:
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1. Follow step 1 above to install ``CsrfViewMiddleware``.
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2. Add ``'django.middleware.csrf.CsrfResponseMiddleware'`` to your
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:setting:`MIDDLEWARE_CLASSES` setting.
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``CsrfResponseMiddleware`` needs to process the response before things
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like compression or setting ofETags happen to the response, so it must
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come after ``GZipMiddleware``, ``CommonMiddleware`` and
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``ConditionalGetMiddleware`` in the list. It also must come after
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``CsrfViewMiddleware``.
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Use of the ``CsrfResponseMiddleware`` is not recommended because of the
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performance hit it imposes, and because of a potential security problem (see
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below). It can be used as an interim measure until applications have been
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updated to use the ``{% csrf_token %}`` tag. It is deprecated and will be
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removed in Django 1.4.
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Django 1.1 and earlier provided a single ``CsrfMiddleware`` class. This is also
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still available for backwards compatibility. It combines the functions of the
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two middleware.
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Note also that previous versions of these classes depended on the sessions
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framework, but this dependency has now been removed, with backward compatibility
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support so that upgrading will not produce any issues.
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Security of legacy method
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The post-processing ``CsrfResponseMiddleware`` adds the CSRF token to all POST
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forms (unless the view has been decorated with ``csrf_response_exempt``). If
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the POST form has an external untrusted site as its target, rather than an
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internal page, that site will be sent the CSRF token when the form is submitted.
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Armed with this leaked information, that site will then be able to successfully
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launch a CSRF attack on your site against that user. The
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``@csrf_response_exempt`` decorator can be used to fix this, but only if the
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page doesn't also contain internal forms that require the token.
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Upgrading notes
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---------------
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When upgrading to version 1.2 or later, you may have applications that rely on
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the old post-processing functionality for CSRF protection, or you may not have
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enabled any CSRF protection. This section outlines the steps necessary for a
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smooth upgrade, without having to fix all the applications to use the new
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template tag method immediately.
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First of all, the location of the middleware and related functions have
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changed. There are backwards compatible stub files so that old imports will
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continue to work for now, but they are deprecated and will be removed in Django
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1.4. The following changes have been made:
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* Middleware have been moved to ``django.middleware.csrf``
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* Decorators have been moved to ``django.views.decorators.csrf``
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====================================================== ==============================================
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Old New
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====================================================== ==============================================
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django.contrib.csrf.middleware.CsrfMiddleware django.middleware.csrf.CsrfMiddleware
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django.contrib.csrf.middleware.CsrfViewMiddleware django.middleware.csrf.CsrfViewMiddleware
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django.contrib.csrf.middleware.CsrfResponseMiddleware django.middleware.csrf.CsrfResponseMiddleware
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django.contrib.csrf.middleware.csrf_exempt django.views.decorators.csrf_exempt
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django.contrib.csrf.middleware.csrf_view_exempt django.views.decorators.csrf_view_exempt
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django.contrib.csrf.middleware.csrf_response_exempt django.views.decorators.csrf_response_exempt
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====================================================== ==============================================
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You should update any imports, and also the paths in your
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:setting:`MIDDLEWARE_CLASSES`.
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If you have ``CsrfMiddleware`` in your :setting:`MIDDLEWARE_CLASSES`, you will now
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have a working installation with CSRF protection. It is recommended at this
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point that you replace ``CsrfMiddleware`` with its two components,
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``CsrfViewMiddleware`` and ``CsrfResponseMiddleware`` (in that order).
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If you do not have any of the middleware in your :setting:`MIDDLEWARE_CLASSES`,
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you will have a working installation but without any CSRF protection for your
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views (just as you had before). It is strongly recommended to install
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``CsrfViewMiddleware`` and ``CsrfResponseMiddleware``, as described above.
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Note that contrib apps, such as the admin, have been updated to use the
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``csrf_protect`` decorator, so that they are secured even if you do not add the
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``CsrfViewMiddleware`` to your settings. However, if you have supplied
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customised templates to any of the view functions of contrib apps (whether
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explicitly via a keyword argument, or by overriding built-in templates), **you
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MUST update them** to include the ``csrf_token`` template tag as described
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above, or they will stop working. (If you cannot update these templates for
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some reason, you will be forced to use ``CsrfResponseMiddleware`` for these
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views to continue working).
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Assuming you have followed the above, all views in your Django site will now be
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protected by the ``CsrfViewMiddleware``. Contrib apps meet the requirements
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imposed by the ``CsrfViewMiddleware`` using the template tag, and other
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applications in your project will meet its requirements by virtue of the
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``CsrfResponseMiddleware``.
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The next step is to update all your applications to use the template tag, as
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described in `How to use it`_, steps 2-3. This can be done as soon as is
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practical. Any applications that are updated will now require Django 1.1.2 or
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later, since they will use the CSRF template tag which was not available in
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earlier versions. (The template tag in 1.1.2 is actually a no-op that exists
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solely to ease the transition to 1.2 — it allows apps to be created that have
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CSRF protection under 1.2 without requiring users of the apps to upgrade to the
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Django 1.2.X series).
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The utility script ``extras/csrf_migration_helper.py`` can help to automate the
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finding of code and templates that may need to be upgraded. It contains full
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help on how to use it.
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Finally, once all applications are upgraded, ``CsrfResponseMiddleware`` can be
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removed from your settings.
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While ``CsrfResponseMiddleware`` is still in use, the ``csrf_response_exempt``
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decorator, described in `Exceptions`_, may be useful. The post-processing
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middleware imposes a performance hit and a potential vulnerability, and any
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views that have been upgraded to use the new template tag method no longer need
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it.
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Exceptions
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----------
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.. versionadded:: 1.1
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To manually exclude a view function from being handled by either of the two CSRF
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middleware, you can use the ``csrf_exempt`` decorator, found in the
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``django.views.decorators.csrf`` module. For example::
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from django.views.decorators.csrf import csrf_exempt
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@csrf_exempt
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def my_view(request):
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return HttpResponse('Hello world')
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Like the middleware, the ``csrf_exempt`` decorator is composed of two parts: a
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``csrf_view_exempt`` decorator and a ``csrf_response_exempt`` decorator, found
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in the same module. These disable the view protection mechanism
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(``CsrfViewMiddleware``) and the response post-processing
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(``CsrfResponseMiddleware``) respectively. They can be used individually if
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required.
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You don't have to worry about doing this for most AJAX views. Any request sent
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with "X-Requested-With: XMLHttpRequest" is automatically exempt. (See the `How
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it works`_ section.)
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Subdomains
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----------
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By default, CSRF cookies are specific to the subdomain they are set for. This
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means that a form served from one subdomain (e.g. server1.example.com) will not
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be able to have a target on another subdomain (e.g. server2.example.com). This
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restriction can be removed by setting :setting:`CSRF_COOKIE_DOMAIN` to be
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something like ``".example.com"``.
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Please note that, with or without use of this setting, this CSRF protection
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mechanism is not safe against cross-subdomain attacks -- see `Limitations`_.
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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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No logging is done, and the error message is not very friendly, so you may want
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to provide your own page for handling this condition. To do this, simply set
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the :setting:`CSRF_FAILURE_VIEW` setting to a dotted path to your own view
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function, which should have the following signature::
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def csrf_failure(request, reason="")
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where ``reason`` is a short message (intended for developers or logging, not for
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end users) indicating the reason the request was rejected.
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How it works
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============
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The CSRF protection is based on the following things:
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1. A CSRF cookie that is set to a random value (a session independent nonce, as
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it is called), which other sites will not have access to.
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This cookie is set by ``CsrfViewMiddleware``. It is meant to be permanent,
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but since there is no way to set a cookie that never expires, it is sent with
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every response that has called ``django.middleware.csrf.get_token()``
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(the function used internally to retrieve the CSRF token).
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2. A hidden form field with the name 'csrfmiddlewaretoken' present in all
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outgoing POST forms. The value of this field is the value of the CSRF
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cookie.
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This part is done by the template tag (and with the legacy method, it is done
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by ``CsrfResponseMiddleware``).
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3. For all incoming POST requests, a CSRF cookie must be present, and the
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'csrfmiddlewaretoken' field must be present and correct. If it isn't, the
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user will get a 403 error.
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This check is done by ``CsrfViewMiddleware``.
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4. In addition, for HTTPS requests, strict referer checking is done by
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``CsrfViewMiddleware``. This is necessary to address a Man-In-The-Middle
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attack that is possible under HTTPS when using a session independent nonce,
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due to the fact that HTTP 'Set-Cookie' headers are (unfortunately) accepted
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by clients that are talking to a site under HTTPS. (Referer checking is not
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done for HTTP requests because the presence of the Referer header is not
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reliable enough under HTTP.)
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This ensures that only forms that have originated from your Web site can be used
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to POST data back.
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It deliberately only targets HTTP POST requests (and the corresponding POST
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forms). GET requests ought never to have any potentially dangerous side effects
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(see `9.1.1 Safe Methods, HTTP 1.1, RFC 2616`_), and so a CSRF attack with a GET
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request ought to be harmless.
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``CsrfResponseMiddleware`` checks the Content-Type before modifying the
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response, and only pages that are served as 'text/html' or
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'application/xml+xhtml' are modified.
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AJAX
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----
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The middleware tries to be smart about requests that come in via AJAX. Most
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modern JavaScript toolkits send an "X-Requested-With: XMLHttpRequest" HTTP
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header; these requests are detected and automatically *not* handled by this
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middleware. We can do this safely because, in the context of a browser, the
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header can only be added by using ``XMLHttpRequest``, and browsers already
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implement a same-domain policy for ``XMLHttpRequest``.
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For the more recent browsers that relax this same-domain policy, custom headers
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like "X-Requested-With" are only allowed after the browser has done a
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'preflight' check to the server to see if the cross-domain request is allowed,
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using a strictly 'opt in' mechanism, so the exception for AJAX is still safe—if
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the developer has specifically opted in to allowing cross-site AJAX POST
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requests on a specific URL, they obviously don't want the middleware to disallow
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exactly that.
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.. _9.1.1 Safe Methods, HTTP 1.1, RFC 2616: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html
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Caching
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=======
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If the ``csrf_token`` template tag is used by a template (or the ``get_token``
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function is called some other way), ``CsrfViewMiddleware`` will add a cookie and
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a ``Vary: Cookie`` header to the response. Similarly,
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``CsrfResponseMiddleware`` will send the ``Vary: Cookie`` header if it inserted
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a token. This means that these middleware will play well with the cache
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middleware if it is used as instructed (``UpdateCacheMiddleware`` goes before
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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. In this case, on any views
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that will require a CSRF token to be inserted you should use the
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:func:`django.views.decorators.vary.vary_on_cookie` decorator first::
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from django.views.decorators.cache import cache_page
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from django.views.decorators.vary import vary_on_cookie
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@cache_page(60 * 15)
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@vary_on_cookie
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def my_view(request):
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# ...
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Testing
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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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Limitations
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===========
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Subdomains within a site will be able to set cookies on the client for the whole
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domain. By setting the cookie and using a corresponding token, subdomains will
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be able to circumvent the CSRF protection. The only way to avoid this is to
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ensure that subdomains are controlled by trusted users (or, are at least unable
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to set cookies). Note that even without CSRF, there are other vulnerabilities,
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such as session fixation, that make giving subdomains to untrusted parties a bad
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idea, and these vulnerabilities cannot easily be fixed with current browsers.
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If you are using ``CsrfResponseMiddleware`` and your app creates HTML pages and
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forms in some unusual way, (e.g. it sends fragments of HTML in JavaScript
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document.write statements) you might bypass the filter that adds the hidden
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field to the form, in which case form submission will always fail. You should
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use the template tag or :meth:`django.middleware.csrf.get_token` to get
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the CSRF token and ensure it is included when your form is submitted.
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Contrib and reusable apps
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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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