lib/django-1.4/docs/ref/contrib/csrf.txt

=====================================
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 (and other
'safe' methods, as defined by 9.1.1 Safe Methods, HTTP 1.1,
:rfc:`2616#section-9.1.1`) are side-effect free. Requests via 'unsafe' methods,
such as POST, PUT and DELETE, can then be protected by following the steps
below.

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

.. _using-csrf:

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

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 any view middleware that assume that CSRF attacks have
   been dealt with.)

   Alternatively, you can use the decorator
   :func:`~django.views.decorators.csrf.csrf_protect` on particular views
   you want to protect (see below).

2. In any template that uses a POST form, use the :ttag:`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
      :func:`~django.shortcuts.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 these steps. It contains full help
on how to use it.

.. _csrf-ajax:

AJAX
----

While the above method can be used for AJAX POST requests, it has some
inconveniences: you have to remember to pass the CSRF token in as POST data with
every POST request. For this reason, there is an alternative method: on each
XMLHttpRequest, set a custom ``X-CSRFToken`` header to the value of the CSRF
token. This is often easier, because many javascript frameworks provide hooks
that allow headers to be set on every request.

As a first step, you must get the CSRF token itself. The recommended source for
the token is the ``csrftoken`` cookie, which will be set if you've enabled CSRF
protection for your views as outlined above.

.. note::

    The CSRF token cookie is named ``csrftoken`` by default, but you can control
    the cookie name via the :setting:`CSRF_COOKIE_NAME` setting.

Acquiring the token is straightforward:

.. code-block:: javascript

    // using jQuery
    function getCookie(name) {
        var cookieValue = null;
        if (document.cookie && document.cookie != '') {
            var cookies = document.cookie.split(';');
            for (var i = 0; i < cookies.length; i++) {
                var cookie = jQuery.trim(cookies[i]);
                // Does this cookie string begin with the name we want?
                if (cookie.substring(0, name.length + 1) == (name + '=')) {
                    cookieValue = decodeURIComponent(cookie.substring(name.length + 1));
                    break;
                }
            }
        }
        return cookieValue;
    }
    var csrftoken = getCookie('csrftoken');

The above code could be simplified by using the `jQuery cookie plugin
<http://plugins.jquery.com/cookie/>`_ to replace ``getCookie``:

.. code-block:: javascript

    var csrftoken = $.cookie('csrftoken');

.. note::

    The CSRF token is also present in the DOM, but only if explicitly included
    using :ttag:`csrf_token` in a template. The cookie contains the canonical
    token; the ``CsrfViewMiddleware`` will prefer the cookie to the token in
    the DOM. Regardless, you're guaranteed to have the cookie if the token is
    present in the DOM, so you should use the cookie!

.. warning::

    If your view is not rendering a template containing the :ttag:`csrf_token`
    template tag, Django might not set the CSRF token cookie. This is common in
    cases where forms are dynamically added to the page. To address this case,
    Django provides a view decorator which forces setting of the cookie:
    :func:`~django.views.decorators.csrf.ensure_csrf_cookie`.

Finally, you'll have to actually set the header on your AJAX request, while
protecting the CSRF token from being sent to other domains.

.. code-block:: javascript

    function csrfSafeMethod(method) {
        // these HTTP methods do not require CSRF protection
        return (/^(GET|HEAD|OPTIONS|TRACE)$/.test(method));
    }
    function sameOrigin(url) {
        // test that a given url is a same-origin URL
        // url could be relative or scheme relative or absolute
        var host = document.location.host; // host + port
        var protocol = document.location.protocol;
        var sr_origin = '//' + host;
        var origin = protocol + sr_origin;
        // Allow absolute or scheme relative URLs to same origin
        return (url == origin || url.slice(0, origin.length + 1) == origin + '/') ||
            (url == sr_origin || url.slice(0, sr_origin.length + 1) == sr_origin + '/') ||
            // or any other URL that isn't scheme relative or absolute i.e relative.
            !(/^(\/\/|http:|https:).*/.test(url));
    }
    $.ajaxSetup({
        beforeSend: function(xhr, settings) {
            if (!csrfSafeMethod(settings.type) && sameOrigin(settings.url)) {
                // Send the token to same-origin, relative URLs only.
                // Send the token only if the method warrants CSRF protection
                // Using the CSRFToken value acquired earlier
                xhr.setRequestHeader("X-CSRFToken", csrftoken);
            }
        }
    });

.. note::

    Due to a bug introduced in jQuery 1.5, the example above will not work
    correctly on that version. Make sure you are running at least jQuery 1.5.1.

You can use `settings.crossDomain <http://api.jquery.com/jQuery.ajax>`_ in
jQuery 1.5 and newer in order to replace the `sameOrigin` logic above:

.. code-block:: javascript

    function csrfSafeMethod(method) {
        // these HTTP methods do not require CSRF protection
        return (/^(GET|HEAD|OPTIONS|TRACE)$/.test(method));
    }
    $.ajaxSetup({
        crossDomain: false, // obviates need for sameOrigin test
        beforeSend: function(xhr, settings) {
            if (!csrfSafeMethod(settings.type)) {
                xhr.setRequestHeader("X-CSRFToken", csrftoken);
            }
        }
    });

.. note::

    In a `security release blogpost`_, a simpler "same origin test" example
    was provided which only checked for a relative URL. The ``sameOrigin``
    test above supersedes that example—it works for edge cases like
    scheme-relative or absolute URLs for the same domain.

.. _security release blogpost: https://www.djangoproject.com/weblog/2011/feb/08/security/

Other template engines
----------------------

When using a different template engine than Django's built-in engine, you can
set the token in your forms manually after making sure it's available in the
template context.

For example, in the Cheetah template language, your form could contain the
following:

.. code-block:: html

    <div style="display:none">
        <input type="hidden" name="csrfmiddlewaretoken" value="$csrf_token"/>
    </div>

You can use JavaScript similar to the :ref:`AJAX code <csrf-ajax>` above to get
the value of the CSRF token.

The decorator method
--------------------

.. module:: django.views.decorators.csrf

Rather than adding ``CsrfViewMiddleware`` as a blanket protection, you can use
the ``csrf_protect`` decorator, which has exactly the same functionality, on
particular views that need the protection. It must be used **both** on views
that insert the CSRF token in the output, and on those that accept the POST form
data. (These are often the same view function, but not always).

Use of the decorator by itself is **not recommended**, 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.

.. function:: csrf_protect(view)

    Decorator that provides the protection of ``CsrfViewMiddleware`` to a view.

    Usage::

        from django.views.decorators.csrf import csrf_protect
        from django.shortcuts import render

        @csrf_protect
        def my_view(request):
            c = {}
            # ...
            return render(request, "a_template.html", c)

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.

The error page, however, is not very friendly, so you may want to provide your
own view for handling this condition.  To do this, simply set the
:setting:`CSRF_FAILURE_VIEW` setting.

.. _how-csrf-works:

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.

3. For all incoming requests that are not using HTTP GET, HEAD, OPTIONS or
   TRACE, 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 ignores GET requests (and other requests that are defined as
'safe' by :rfc:`2616`). These requests ought never to have any potentially
dangerous side effects , and so a CSRF attack with a GET request ought to be
harmless. :rfc:`2616` defines POST, PUT and DELETE as 'unsafe', and all other
methods are assumed to be unsafe, for maximum protection.

Caching
=======

If the :ttag:`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. This means that the
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 or the CSRF cookie, and the
response will be cached without either one. In this case, on any views that
will require a CSRF token to be inserted you should use the
:func:`django.views.decorators.csrf.csrf_protect` decorator first::

  from django.views.decorators.cache import cache_page
  from django.views.decorators.csrf import csrf_protect

  @cache_page(60 * 15)
  @csrf_protect
  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.

If, for some reason, you *want* the test client to perform CSRF
checks, you can create an instance of the test client that enforces
CSRF checks::

    >>> from django.test import Client
    >>> csrf_client = Client(enforce_csrf_checks=True)

.. _csrf-limitations:

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.

Edge cases
==========

Certain views can have unusual requirements that mean they don't fit the normal
pattern envisaged here. A number of utilities can be useful in these
situations. The scenarios they might be needed in are described in the following
section.

Utilities
---------

.. function:: csrf_exempt(view)

    This decorator marks a view as being exempt from the protection ensured by
    the middleware. Example::

        from django.views.decorators.csrf import csrf_exempt

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

.. function:: requires_csrf_token(view)

    Normally the :ttag:`csrf_token` template tag will not work if
    ``CsrfViewMiddleware.process_view`` or an equivalent like ``csrf_protect``
    has not run. The view decorator ``requires_csrf_token`` can be used to
    ensure the template tag does work. This decorator works similarly to
    ``csrf_protect``, but never rejects an incoming request.

    Example::

        from django.views.decorators.csrf import requires_csrf_token
        from django.shortcuts import render

        @requires_csrf_token
        def my_view(request):
            c = {}
            # ...
            return render(request, "a_template.html", c)

.. function:: ensure_csrf_cookie(view)

    .. versionadded:: 1.4

    This decorator forces a view to send the CSRF cookie.

Scenarios
---------

CSRF protection should be disabled for just a few views
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Most views requires CSRF protection, but a few do not.

Solution: rather than disabling the middleware and applying ``csrf_protect`` to
all the views that need it, enable the middleware and use
:func:`~django.views.decorators.csrf.csrf_exempt`.

CsrfViewMiddleware.process_view not used
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There are cases when ``CsrfViewMiddleware.process_view`` may not have run
before your view is run - 404 and 500 handlers, for example - but you still
need the CSRF token in a form.

Solution: use :func:`~django.views.decorators.csrf.requires_csrf_token`

Unprotected view needs the CSRF token
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There may be some views that are unprotected and have been exempted by
``csrf_exempt``, but still need to include the CSRF token.

Solution: use :func:`~django.views.decorators.csrf.csrf_exempt` followed by
:func:`~django.views.decorators.csrf.requires_csrf_token`. (i.e. ``requires_csrf_token``
should be the innermost decorator).

View needs protection for one path
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A view needs CRSF protection under one set of conditions only, and mustn't have
it for the rest of the time.

Solution: use :func:`~django.views.decorators.csrf.csrf_exempt` for the whole
view function, and :func:`~django.views.decorators.csrf.csrf_protect` for the
path within it that needs protection. Example::

    from django.views.decorators.csrf import csrf_exempt, csrf_protect

    @csrf_exempt
    def my_view(request):

        @csrf_protect
        def protected_path(request):
            do_something()

        if some_condition():
           return protected_path(request)
        else:
           do_something_else()

Page uses AJAX without any HTML form
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A page makes a POST request via AJAX, and the page does not have an HTML form
with a :ttag:`csrf_token` that would cause the required CSRF cookie to be sent.

Solution: use :func:`~django.views.decorators.csrf.ensure_csrf_cookie` on the
view that sends the page.

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.

Settings
========

A number of settings can be used to control Django's CSRF behavior.

CSRF_COOKIE_DOMAIN
------------------

.. versionadded:: 1.2

Default: ``None``

The domain to be used when setting the CSRF cookie.  This can be useful for
easily allowing cross-subdomain requests to be excluded from the normal cross
site request forgery protection.  It should be set to a string such as
``".lawrence.com"`` to allow a POST request from a form on one subdomain to be
accepted by a view served from another subdomain.

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

CSRF_COOKIE_NAME
----------------

.. versionadded:: 1.2

Default: ``'csrftoken'``

The name of the cookie to use for the CSRF authentication token. This can be
whatever you want.

CSRF_COOKIE_PATH
----------------

.. versionadded:: 1.4

Default: ``'/'``

The path set on the CSRF cookie. This should either match the URL path of your
Django installation or be a parent of that path.

This is useful if you have multiple Django instances running under the same
hostname. They can use different cookie paths, and each instance will only see
its own CSRF cookie.

CSRF_COOKIE_SECURE
------------------

.. versionadded:: 1.4

Default: ``False``

Whether to use a secure cookie for the CSRF cookie. If this is set to ``True``,
the cookie will be marked as "secure," which means browsers may ensure that the
cookie is only sent under an HTTPS connection.

CSRF_FAILURE_VIEW
-----------------

.. versionadded:: 1.2

Default: ``'django.views.csrf.csrf_failure'``

A dotted path to the view function to be used when an incoming request
is rejected by the CSRF protection.  The function should have this 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.