1 =====================================

     2 Cross Site Request Forgery protection

     3 =====================================

     4 

     5 The CsrfMiddleware class provides easy-to-use protection against

     6 `Cross Site Request Forgeries`_.  This type of attack occurs when a malicious

     7 web site creates a link or form button that is intended to perform some action

     8 on your web site, using the credentials of a logged-in user who is tricked

     9 into clicking on the link in their browser.

    10 

    11 The first defense against CSRF attacks is to ensure that GET requests

    12 are side-effect free.  POST requests can then be protected by adding this

    13 middleware into your list of installed middleware.

    14 

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

    16 

    17 How to use it

    18 =============

    19 

    20 Add the middleware ``'django.contrib.csrf.middleware.CsrfMiddleware'`` to

    21 your list of middleware classes, ``MIDDLEWARE_CLASSES``. It needs to process

    22 the response after the SessionMiddleware, so must come before it in the

    23 list. It also must process the response before things like compression

    24 happen to the response, so it must come after GZipMiddleware in the list.

    25 

    26 How it works

    27 ============

    28 

    29 CsrfMiddleware does two things:

    30 

    31 1. It modifies outgoing requests by adding a hidden form field to all

    32    'POST' forms, with the name 'csrfmiddlewaretoken' and a value which is

    33    a hash of the session ID plus a secret. If there is no session ID set,

    34    this modification of the response isn't done, so there is very little

    35    performance penalty for those requests that don't have a session.

    36 

    37 2. On all incoming POST requests that have the session cookie set, it

    38    checks that the 'csrfmiddlewaretoken' is present and correct. If it

    39    isn't, the user will get a 403 error.

    40 

    41 This ensures that only forms that have originated from your web site

    42 can be used to POST data back.

    43 

    44 It deliberately only targets HTTP POST requests (and the corresponding

    45 POST forms). GET requests ought never to have side effects (if you are

    46 using HTTP GET and POST correctly), and so a CSRF attack with a GET

    47 request will always be harmless.

    48 

    49 POST requests that are not accompanied by a session cookie are not protected,

    50 but they do not need to be protected, since the 'attacking' web site

    51 could make these kind of requests anyway.

    52 

    53 The Content-Type is checked before modifying the response, and only

    54 pages that are served as 'text/html' or 'application/xml+xhtml'

    55 are modified.

    56 

    57 Limitations

    58 ===========

    59 

    60 CsrfMiddleware requires Django's session framework to work. If you have

    61 a custom authentication system that manually sets cookies and the like,

    62 it won't help you.

    63 

    64 If your app creates HTML pages and forms in some unusual way, (e.g.

    65 it sends fragments of HTML in javascript document.write statements)

    66 you might bypass the filter that adds the hidden field to the form,

    67 in which case form submission will always fail.  It may still be possible

    68 to use the middleware, provided you can find some way to get the

    69 CSRF token and ensure that is included when your form is submitted.