django1/docs/ref/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
website contains a link, a form button or some JavaScript that is intended to
perform some action on your website, 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 :rfc:`7231#section-4.2.1`) 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: https://www.squarefree.com/securitytips/web-developers.html#CSRF

.. _using-csrf:

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

To take advantage of CSRF protection in your views, follow these steps:

1. The CSRF middleware is activated by default in the :setting:`MIDDLEWARE`
   setting. If you override that setting, remember that
   ``'django.middleware.csrf.CsrfViewMiddleware'`` should come before any view
   middleware that assume that CSRF attacks have been dealt with.

   If you disabled it, which is not recommended, you can use
   :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
   :class:`~django.template.RequestContext` is used to render the response so
   that ``{% csrf_token %}`` will work properly. If you're using the
   :func:`~django.shortcuts.render` function, generic views, or contrib apps,
   you are covered already since these all use ``RequestContext``.

.. _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.

    The CSRF header name is ``HTTP_X_CSRFTOKEN`` by default, but you can
    customize it using the :setting:`CSRF_HEADER_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 `JavaScript Cookie library
<https://github.com/js-cookie/js-cookie/>`_ to replace ``getCookie``:

.. code-block:: javascript

    var csrftoken = Cookies.get('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 using
`settings.crossDomain <https://api.jquery.com/jQuery.ajax>`_ in jQuery 1.5.1 and
newer:

.. code-block:: javascript

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

If you're using AngularJS 1.1.3 and newer, it's sufficient to configure the
``$http`` provider with the cookie and header names:

.. code-block:: javascript

    $httpProvider.defaults.xsrfCookieName = 'csrftoken';
    $httpProvider.defaults.xsrfHeaderName = 'X-CSRFToken';

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 Jinja2 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)

    If you are using class-based views, you can refer to
    :ref:`Decorating class-based views<decorating-class-based-views>`.

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.

CSRF failures are logged as warnings to the :ref:`django-request-logger`
logger.

.. _how-csrf-works:

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

The CSRF protection is based on the following things:

1. A CSRF cookie that is based on a random secret value, which other sites
   will not have access to.

   This cookie is set by ``CsrfViewMiddleware``. It is sent with every
   response that has called ``django.middleware.csrf.get_token()`` (the
   function used internally to retrieve the CSRF token), if it wasn't already
   set on the request.

   In order to protect against `BREACH`_ attacks, the token is not simply the
   secret; a random salt is prepended to the secret and used to scramble it.

   For security reasons, the value of the secret is changed each time a
   user logs in.

2. A hidden form field with the name 'csrfmiddlewaretoken' present in all
   outgoing POST forms. The value of this field is, again, the value of the
   secret, with a salt which is both added to it and used to scramble it. The
   salt is regenerated on every call to ``get_token()`` so that the form field
   value is changed in every such response.

   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.

   When validating the 'csrfmiddlewaretoken' field value, only the secret,
   not the full token, is compared with the secret in the cookie value.
   This allows the use of ever-changing tokens. While each request may use its
   own token, the secret remains common to all.

   This check is done by ``CsrfViewMiddleware``.

4. In addition, for HTTPS requests, strict referer checking is done by
   ``CsrfViewMiddleware``. This means that even if a subdomain can set or
   modify cookies on your domain, it can't force a user to post to your
   application since that request won't come from your own exact domain.

   This also addresses a man-in-the-middle attack that's possible under HTTPS
   when using a session independent secret, due to the fact that HTTP
   ``Set-Cookie`` headers are (unfortunately) accepted by clients even when
   they are talking to a site under HTTPS. (Referer checking is not done for
   HTTP requests because the presence of the ``Referer`` header isn't reliable
   enough under HTTP.)

   If the :setting:`CSRF_COOKIE_DOMAIN` setting is set, the referer is compared
   against it. This setting supports subdomains. For example,
   ``CSRF_COOKIE_DOMAIN = '.example.com'`` will allow POST requests from
   ``www.example.com`` and ``api.example.com``. If the setting is not set, then
   the referer must match the HTTP ``Host`` header.

   Expanding the accepted referers beyond the current host or cookie domain can
   be done with the :setting:`CSRF_TRUSTED_ORIGINS` setting.

This ensures that only forms that have originated from trusted domains can be
used to POST data back.

It deliberately ignores GET requests (and other requests that are defined as
'safe' by :rfc:`7231`). 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:`7231` defines POST, PUT, and DELETE as 'unsafe', and all other
methods are also assumed to be unsafe, for maximum protection.

The CSRF protection cannot protect against man-in-the-middle attacks, so use
:ref:`HTTPS <security-recommendation-ssl>` with
:ref:`http-strict-transport-security`. It also assumes :ref:`validation of
the HOST header <host-headers-virtual-hosting>` and that there aren't any
:ref:`cross-site scripting vulnerabilities <cross-site-scripting>` on your site
(because XSS vulnerabilities already let an attacker do anything a CSRF
vulnerability allows and much worse).

.. versionchanged:: 1.10

   Added salting to the token and started changing it with each request
   to protect against `BREACH`_ attacks.

.. _BREACH: http://breachattack.com/

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):
      ...

If you are using class-based views, you can refer to :ref:`Decorating
class-based views<decorating-class-based-views>`.

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
---------

The examples below assume you are using function-based views. If you
are working with class-based views, you can refer to :ref:`Decorating
class-based views<decorating-class-based-views>`.

.. 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
        from django.http import HttpResponse

        @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)

    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 CSRF 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:

* :setting:`CSRF_COOKIE_AGE`
* :setting:`CSRF_COOKIE_DOMAIN`
* :setting:`CSRF_COOKIE_HTTPONLY`
* :setting:`CSRF_COOKIE_NAME`
* :setting:`CSRF_COOKIE_PATH`
* :setting:`CSRF_COOKIE_SECURE`
* :setting:`CSRF_FAILURE_VIEW`
* :setting:`CSRF_HEADER_NAME`
* :setting:`CSRF_TRUSTED_ORIGINS`

Frequently Asked Questions
==========================

Is posting an arbitrary CSRF token pair (cookie and POST data) a vulnerability?
-------------------------------------------------------------------------------

No, this is by design. Without a man-in-the-middle attack, there is no way for
an attacker to send a CSRF token cookie to a victim's browser, so a successful
attack would need to obtain the victim's browser's cookie via XSS or similar,
in which case an attacker usually doesn't need CSRF attacks.

Some security audit tools flag this as a problem but as mentioned before, an
attacker cannot steal a user's browser's CSRF cookie. "Stealing" or modifying
*your own* token using Firebug, Chrome dev tools, etc. isn't a vulnerability.

Is the fact that Django's CSRF protection isn't linked to a session a problem?
------------------------------------------------------------------------------

No, this is by design. Not linking CSRF protection to a session allows using
the protection on sites such as a `pastebin` that allow submissions from
anonymous users which don't have a session.

Why might a user encounter a CSRF validation failure after logging in?
----------------------------------------------------------------------

For security reasons, CSRF tokens are rotated each time a user logs in. Any
page with a form generated before a login will have an old, invalid CSRF token
and need to be reloaded. This might happen if a user uses the back button after
a login or if they log in in a different browser tab.