django/docs/ref/contrib/csrf.txt

.. _ref-contrib-csrf:

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

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

The CsrfMiddleware class provides easy-to-use protection against
`Cross Site Request Forgeries`_.  This type of attack occurs when a malicious
Web site creates a link or form button that is intended to perform some action
on your Web site, using the credentials of a logged-in user who is tricked
into clicking on the link in their browser.

The first defense against CSRF attacks is to ensure that GET requests
are side-effect free.  POST requests can then be protected by adding this
middleware into your list of installed middleware.

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

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

Add the middleware ``'django.contrib.csrf.middleware.CsrfMiddleware'`` to
your list of middleware classes, :setting:`MIDDLEWARE_CLASSES`. It needs to process
the response after the SessionMiddleware, so must come before it in the
list. It also must process the response before things like compression
happen to the response, so it must come after GZipMiddleware in the
list.

Exceptions
----------

To manually exclude a view function from being handled by the
CsrfMiddleware, you can use the ``csrf_exempt`` decorator (found in
the ``django.contrib.csrf.middleware`` module).

AJAX requests sent with "X-Requested-With: XMLHttpRequest" are
automatically exempt (see below).

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

CsrfMiddleware does two things:

1. It modifies outgoing requests by adding a hidden form field to all
   'POST' forms, with the name 'csrfmiddlewaretoken' and a value which is
   a hash of the session ID plus a secret. If there is no session ID set,
   this modification of the response isn't done, so there is very little
   performance penalty for those requests that don't have a session.

2. On all incoming POST requests that have the session cookie set, it
   checks that the 'csrfmiddlewaretoken' is present and correct. If it
   isn't, the user will get a 403 error.

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.

POST requests that are not accompanied by a session cookie are not protected,
but they do not need to be protected, since the 'attacking' Web site
could make these kind of requests anyway.

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

AJAX requests sent with "X-Requested-With: XMLHttpRequest", as done by
many AJAX toolkits, are detected and automatically excepted from this
mechanism.  This is because in the context of a browser, this header
can only be added by using XMLHttpRequest, and browsers already
implement a same-domain policy for XMLHttpRequest.  This is not secure
if you do not trust content within the same domain or sub-domains.

The above two functions of ``CsrfMiddleware`` are split between two
classes: ``CsrfResponseMiddleware`` and ``CsrfViewMiddleware``
respectively.  This allows the individual components to be used and/or
replaced instead of using ``CsrfMiddleware``.

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

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

CsrfMiddleware requires Django's session framework to work. If you have
a custom authentication system that manually sets cookies and the like,
it won't help you.

If 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.  It may still be possible
to use the middleware, provided you can find some way to get the
CSRF token and ensure that is included when your form is submitted.