django/docs/ref/contrib/csrf.txt

.. _ref-contrib-csrf:

=====================================
Cross Site Request Forgery protection
=====================================

.. module:: django.middleware.csrf
   :synopsis: Protects against Cross Site Request Forgeries

The CSRF middleware and template tag provides easy-to-use protection against
`Cross Site Request Forgeries`_.  This type of attack occurs when a malicious
Web site contains a link, a form button or some javascript that is intended to
perform some action on your Web site, using the credentials of a logged-in user
who visits the malicious site in their browser.  A related type of attack,
'login CSRF', where an attacking site tricks a user's browser into logging into
a site with someone else's credentials, is also covered.

The first defense against CSRF attacks is to ensure that GET requests are
side-effect free.  POST requests can then be protected by following the steps
below.

.. versionadded:: 1.2
    The 'contrib' apps, including the admin, use the functionality described
    here. Because it is security related, a few things have been added to core
    functionality to allow this to happen without any required upgrade steps.

.. _Cross Site Request Forgeries: http://www.squarefree.com/securitytips/web-developers.html#CSRF

How to use it
=============

.. versionchanged:: 1.2
    The template tag functionality (the recommended way to use this) was added
    in version 1.2. The previous method (still available) is described under
    `Legacy method`_.

To enable CSRF protection for your views, follow these steps:

    1. Add the middleware
       ``'django.middleware.csrf.CsrfViewMiddleware'`` to your list of
       middleware classes, :setting:`MIDDLEWARE_CLASSES`.  (It should come
       before ``CsrfResponseMiddleware`` if that is being used, and before any
       view middleware that assume that CSRF attacks have been dealt with.)

       Alternatively, you can use the decorator
       ``django.views.decorators.csrf.csrf_protect`` on particular views you
       want to protect.  This is **not recommended** by itself, since if you
       forget to use it, you will have a security hole.  The 'belt and braces'
       strategy of using both is fine, and will incur minimal overhead.

    2. In any template that uses a POST form, use the ``csrf_token`` tag inside
       the ``<form>`` element if the form is for an internal URL, e.g.::

           <form action="" method="POST">{% csrf_token %}

       This should not be done for POST forms that target external URLs, since
       that would cause the CSRF token to be leaked, leading to a vulnerability.

    3. In the corresponding view functions, ensure that the
       ``'django.core.context_processors.csrf'`` context processor is
       being used. Usually, this can be done in one of two ways:

       1. Use RequestContext, which always uses
          ``'django.core.context_processors.csrf'`` (no matter what your
          TEMPLATE_CONTEXT_PROCESSORS setting).  If you are using
          generic views or contrib apps, you are covered already, since these
          apps use RequestContext throughout.

       2. Manually import and use the processor to generate the CSRF token and
          add it to the template context. e.g.::

              from django.core.context_processors import csrf
              from django.shortcuts import render_to_response

              def my_view(request):
                  c = {}
                  c.update(csrf(request))
                  # ... view code here
                  return render_to_response("a_template.html", c)

          You may want to write your own ``render_to_response`` wrapper that
          takes care of this step for you.

The utility script ``extras/csrf_migration_helper.py`` can help to automate the
finding of code and templates that may need to be upgraded.  It contains full
help on how to use it.

Legacy method
-------------

In Django 1.1, the template tag did not exist.  Instead, a post-processing
middleware that re-wrote POST forms to include the CRSF token was used.  If you
are upgrading a site from version 1.1 or earlier, please read this section and
the `Upgrading notes`_ below.  The post-processing middleware is still available
as ``CsrfResponseMiddleware``, and it can be used by following these steps:

    1. Follow step 1 above to install ``CsrfViewMiddleware``.

    2. Add ``'django.middleware.csrf.CsrfResponseMiddleware'`` to your
       :setting:`MIDDLEWARE_CLASSES` setting.

       ``CsrfResponseMiddleware`` needs to process the response before things
       like compression or setting ofETags happen to the response, so it must
       come after ``GZipMiddleware``, ``CommonMiddleware`` and 
       ``ConditionalGetMiddleware`` in the list. It also must come after
       ``CsrfViewMiddleware``.

Use of the ``CsrfResponseMiddleware`` is not recommended because of the
performance hit it imposes, and because of a potential security problem (see
below).  It can be used as an interim measure until applications have been
updated to use the ``{% crsf_token %}`` tag.  It is deprecated and will be
removed in Django 1.4.

Django 1.1 and earlier provided a single ``CsrfMiddleware`` class.  This is also
still available for backwards compatibility.  It combines the functions of the
two middleware.

Note also that previous versions of these classes depended on the sessions
framework, but this dependency has now been removed, with backward compatibility
support so that upgrading will not produce any issues.

Security of legacy method
~~~~~~~~~~~~~~~~~~~~~~~~~

The post-processing ``CsrfResponseMiddleware`` adds the CSRF token to all POST
forms (unless the view has been decorated with ``csrf_response_exempt``).  If
the POST form has an external untrusted site as its target, rather than an
internal page, that site will be sent the CSRF token when the form is submitted.
Armed with this leaked information, that site will then be able to successfully
launch a CSRF attack on your site against that user.  The
``@csrf_response_exempt`` decorator can be used to fix this, but only if the
page doesn't also contain internal forms that require the token.

Upgrading notes
---------------

When upgrading to version 1.2 or later, you may have applications that rely on
the old post-processing functionality for CSRF protection, or you may not have
enabled any CSRF protection.  This section outlines the steps necessary for a
smooth upgrade, without having to fix all the applications to use the new
template tag method immediately.

First of all, the location of the middleware and related functions have
changed.  There are backwards compatible stub files so that old imports will
continue to work for now, but they are deprecated and will be removed in Django
1.4.  The following changes have been made:

 * Middleware have been moved to ``django.middleware.csrf``
 * Decorators have been moved to ``django.views.decorators.csrf``

======================================================  ==============================================
   Old                                                       New
======================================================  ==============================================
django.contrib.csrf.middleware.CsrfMiddleware           django.middleware.csrf.CsrfMiddleware
django.contrib.csrf.middleware.CsrfViewMiddleware       django.middleware.csrf.CsrfViewMiddleware
django.contrib.csrf.middleware.CsrfResponseMiddleware   django.middleware.csrf.CsrfResponseMiddleware
django.contrib.csrf.middleware.csrf_exempt              django.views.decorators.csrf_exempt
django.contrib.csrf.middleware.csrf_view_exempt         django.views.decorators.csrf_view_exempt
django.contrib.csrf.middleware.csrf_response_exempt     django.views.decorators.csrf_response_exempt
======================================================  ==============================================

You should update any imports, and also the paths in your
:setting:`MIDDLEWARE_CLASSES`.

If you have ``CsrfMiddleware`` in your :setting:`MIDDLEWARE_CLASSES`, you will now
have a working installation with CSRF protection.  It is recommended at this
point that you replace ``CsrfMiddleware`` with its two components,
``CsrfViewMiddleware`` and ``CsrfResponseMiddleware`` (in that order).

If you do not have any of the middleware in your :setting:`MIDDLEWARE_CLASSES`,
you will have a working installation but without any CSRF protection for your
views (just as you had before). It is strongly recommended to install
``CsrfViewMiddleware`` and ``CsrfResponseMiddleware``, as described above.

(Note that contrib apps, such as the admin, have been updated to use the
``csrf_protect`` decorator, so that they are secured even if you do not add the
``CsrfViewMiddleware`` to your settings).

Assuming you have followed the above, all views in your Django site will now be
protected by the ``CsrfViewMiddleware``.  Contrib apps meet the requirements
imposed by the ``CsrfViewMiddleware`` using the template tag, and other
applications in your project will meet its requirements by virtue of the
``CsrfResponseMiddleware``.

The next step is to update all your applications to use the template tag, as
described in `How to use it`_, steps 2-3.  This can be done as soon as is
practical. Any applications that are updated will now require Django 1.2 or
later, since they will use the CSRF template tag which was not available in
earlier versions.

The utility script ``extras/csrf_migration_helper.py`` can help to automate the
finding of code and templates that may need to be upgraded.  It contains full
help on how to use it.

Finally, once all applications are upgraded, ``CsrfResponseMiddleware`` can be
removed from your settings.

While ``CsrfResponseMiddleware`` is still in use, the ``csrf_response_exempt``
decorator, described in `Exceptions`_, may be useful.  The post-processing
middleware imposes a performance hit and a potential vulnerability, and any
views that have been upgraded to use the new template tag method no longer need
it.

Exceptions
----------

.. versionadded:: 1.1

To manually exclude a view function from being handled by either of the two CSRF
middleware, you can use the ``csrf_exempt`` decorator, found in the
``django.views.decorators.csrf`` module. For example::

    from django.views.decorators.csrf import csrf_exempt

    @csrf_exempt
    def my_view(request):
        return HttpResponse('Hello world')

Like the middleware, the ``csrf_exempt`` decorator is composed of two parts: a
``csrf_view_exempt`` decorator and a ``csrf_response_exempt`` decorator, found
in the same module.  These disable the view protection mechanism
(``CsrfViewMiddleware``) and the response post-processing
(``CsrfResponseMiddleware``) respectively.  They can be used individually if
required.

You don't have to worry about doing this for most AJAX views. Any request sent
with "X-Requested-With: XMLHttpRequest" is automatically exempt. (See the `How
it works`_ section.)

Subdomains
----------

By default, CSRF cookies are specific to the subdomain they are set for.  This
means that a form served from one subdomain (e.g. server1.example.com) will not
be able to have a target on another subdomain (e.g. server2.example.com).  This
restriction can be removed by setting :setting:`CSRF_COOKIE_DOMAIN` to be
something like ``".example.com"``.

Please note that, with or without use of this setting, this CSRF protection
mechanism is not safe against cross-subdomain attacks -- see `Limitations`_.

Rejected requests
=================

By default, a '403 Forbidden' response is sent to the user if an incoming
request fails the checks performed by ``CsrfViewMiddleware``.  This should
usually only be seen when there is a genuine Cross Site Request Forgery, or
when, due to a programming error, the CSRF token has not been included with a
POST form.

No logging is done, and the error message is not very friendly, so you may want
to provide your own page for handling this condition.  To do this, simply set
the :setting:`CSRF_FAILURE_VIEW` setting to a dotted path to your own view
function, which should have the following signature::

    def csrf_failure(request, reason="")

where ``reason`` is a short message (intended for developers or logging, not for
end users) indicating the reason the request was rejected.

How it works
============

The CSRF protection is based on the following things:

1. A CSRF cookie that is set to a random value (a session independent nonce, as
   it is called), which other sites will not have access to.

   This cookie is set by ``CsrfViewMiddleware``.  It is meant to be permanent,
   but since there is no way to set a cookie that never expires, it is sent with
   every response that has called ``django.middleware.csrf.get_token()``
   (the function used internally to retrieve the CSRF token).

2. A hidden form field with the name 'csrfmiddlewaretoken' present in all
   outgoing POST forms.  The value of this field is the value of the CSRF
   cookie.

   This part is done by the template tag (and with the legacy method, it is done
   by ``CsrfResponseMiddleware``).

3. For all incoming POST requests, a CSRF cookie must be present, and the
   'csrfmiddlewaretoken' field must be present and correct. If it isn't, the
   user will get a 403 error.

   This check is done by ``CsrfViewMiddleware``.

4. In addition, for HTTPS requests, strict referer checking is done by
   ``CsrfViewMiddleware``.  This is necessary to address a Man-In-The-Middle
   attack that is possible under HTTPS when using a session independent nonce,
   due to the fact that HTTP 'Set-Cookie' headers are (unfortunately) accepted
   by clients that are talking to a site under HTTPS.  (Referer checking is not
   done for HTTP requests because the presence of the Referer header is not
   reliable enough under HTTP.)

This ensures that only forms that have originated from your Web site can be used
to POST data back.

It deliberately only targets HTTP POST requests (and the corresponding POST
forms). GET requests ought never to have any potentially dangerous side effects
(see `9.1.1 Safe Methods, HTTP 1.1, RFC 2616`_), and so a CSRF attack with a GET
request ought to be harmless.

``CsrfResponseMiddleware`` checks the Content-Type before modifying the
response, and only pages that are served as 'text/html' or
'application/xml+xhtml' are modified.

AJAX
----

The middleware tries to be smart about requests that come in via AJAX. Most
modern JavaScript toolkits send an "X-Requested-With: XMLHttpRequest" HTTP
header; these requests are detected and automatically *not* handled by this
middleware.  We can do this safely because, in the context of a browser, the
header can only be added by using ``XMLHttpRequest``, and browsers already
implement a same-domain policy for ``XMLHttpRequest``.

For the more recent browsers that relax this same-domain policy, custom headers
like "X-Requested-With" are only allowed after the browser has done a
'preflight' check to the server to see if the cross-domain request is allowed,
using a strictly 'opt in' mechanism, so the exception for AJAX is still safe—if
the developer has specifically opted in to allowing cross-site AJAX POST
requests on a specific URL, they obviously don't want the middleware to disallow
exactly that.

.. _9.1.1 Safe Methods, HTTP 1.1, RFC 2616: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html

Caching
=======

If the ``csrf_token`` template tag is used by a template (or the ``get_token``
function is called some other way), ``CsrfViewMiddleware`` will add a cookie and
a ``Vary: Cookie`` header to the response.  Similarly,
``CsrfResponseMiddleware`` will send the ``Vary: Cookie`` header if it inserted
a token.  This means that these middleware will play well with the cache
middleware if it is used as instructed (``UpdateCacheMiddleware`` goes before
all other middleware).

However, if you use cache decorators on individual views, the CSRF middleware
will not yet have been able to set the Vary header.  In this case, on any views
that will require a CSRF token to be inserted you should use the
:func:`django.views.decorators.vary.vary_on_cookie` decorator first::

  from django.views.decorators.cache import cache_page
  from django.views.decorators.vary import vary_on_cookie

  @cache_page(60 * 15)
  @vary_on_cookie
  def my_view(request):
      # ...


Testing
=======

The ``CsrfViewMiddleware`` will usually be a big hindrance to testing view
functions, due to the need for the CSRF token which must be sent with every POST
request.  For this reason, Django's HTTP client for tests has been modified to
set a flag on requests which relaxes the middleware and the ``csrf_protect``
decorator so that they no longer rejects requests.  In every other respect
(e.g. sending cookies etc.), they behave the same.

Limitations
===========

Subdomains within a site will be able to set cookies on the client for the whole
domain.  By setting the cookie and using a corresponding token, subdomains will
be able to circumvent the CSRF protection.  The only way to avoid this is to
ensure that subdomains are controlled by trusted users (or, are at least unable
to set cookies).  Note that even without CSRF, there are other vulnerabilities,
such as session fixation, that make giving subdomains to untrusted parties a bad
idea, and these vulnerabilities cannot easily be fixed with current browsers.

If you are using ``CsrfResponseMiddleware`` and your app creates HTML pages and
forms in some unusual way, (e.g.  it sends fragments of HTML in JavaScript
document.write statements) you might bypass the filter that adds the hidden
field to the form, in which case form submission will always fail.  You should
use the template tag or :meth:`django.middleware.csrf.get_token` to get
the CSRF token and ensure it is included when your form is submitted.

Contrib and reusable apps
=========================

Because it is possible for the developer to turn off the ``CsrfViewMiddleware``,
all relevant views in contrib apps use the ``csrf_protect`` decorator to ensure
the security of these applications against CSRF.  It is recommended that the
developers of other reusable apps that want the same guarantees also use the
``csrf_protect`` decorator on their views.