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Cross Site Request Forgery protection

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:

How to use it

Add the middleware ``'django.contrib.csrf.middleware.CsrfMiddleware'`` to
your list of middleware classes, ``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.

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.

.. _9.1.1 Safe Methods, HTTP 1.1, RFC 2616:


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.