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=====================================
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Cross Site Request Forgery protection
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=====================================
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The CsrfMiddleware class provides easy-to-use protection against
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`Cross Site Request Forgeries`_. This type of attack occurs when a malicious
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web site creates a link or form button that is intended to perform some action
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on your web site, using the credentials of a logged-in user who is tricked
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into clicking on the link in their browser.
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The first defense against CSRF attacks is to ensure that GET requests
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are side-effect free. POST requests can then be protected by adding this
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middleware into your list of installed middleware.
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.. _Cross Site Request Forgeries: http://www.squarefree.com/securitytips/web-developers.html#CSRF
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How to use it
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=============
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Add the middleware ``'django.contrib.csrf.middleware.CsrfMiddleware'`` to
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your list of middleware classes, ``MIDDLEWARE_CLASSES``. It needs to process
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the response after the SessionMiddleware, so must come before it in the
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list. It also must process the response before things like compression
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happen to the response, so it must come after GZipMiddleware in the list.
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How it works
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============
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CsrfMiddleware does two things:
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1. It modifies outgoing requests by adding a hidden form field to all
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'POST' forms, with the name 'csrfmiddlewaretoken' and a value which is
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a hash of the session ID plus a secret. If there is no session ID set,
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this modification of the response isn't done, so there is very little
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performance penalty for those requests that don't have a session.
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2. On all incoming POST requests that have the session cookie set, it
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checks that the 'csrfmiddlewaretoken' is present and correct. If it
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isn't, the user will get a 403 error.
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This ensures that only forms that have originated from your web site
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can be used to POST data back.
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It deliberately only targets HTTP POST requests (and the corresponding
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POST forms). GET requests ought never to have side effects (if you are
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using HTTP GET and POST correctly), and so a CSRF attack with a GET
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request will always be harmless.
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POST requests that are not accompanied by a session cookie are not protected,
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but they do not need to be protected, since the 'attacking' web site
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could make these kind of requests anyway.
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The Content-Type is checked before modifying the response, and only
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pages that are served as 'text/html' or 'application/xml+xhtml'
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are modified.
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Limitations
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===========
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CsrfMiddleware requires Django's session framework to work. If you have
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a custom authentication system that manually sets cookies and the like,
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it won't help you.
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If your app creates HTML pages and forms in some unusual way, (e.g.
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it sends fragments of HTML in javascript document.write statements)
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you might bypass the filter that adds the hidden field to the form,
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in which case form submission will always fail. It may still be possible
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to use the middleware, provided you can find some way to get the
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CSRF token and ensure that is included when your form is submitted. |