mirror of
https://github.com/django/django.git
synced 2024-11-30 07:06:18 +01:00
9f592ecced
git-svn-id: http://code.djangoproject.com/svn/django/trunk@12618 bcc190cf-cafb-0310-a4f2-bffc1f526a37
429 lines
20 KiB
Plaintext
429 lines
20 KiB
Plaintext
.. _ref-contrib-csrf:
|
|
|
|
=====================================
|
|
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
|
|
Web site contains a link, a form button or some javascript that is intended to
|
|
perform some action on your Web site, 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 are
|
|
side-effect free. POST requests can then be protected by following the steps
|
|
below.
|
|
|
|
.. versionadded:: 1.2
|
|
The 'contrib' apps, including the admin, use the functionality described
|
|
here. Because it is security related, a few things have been added to core
|
|
functionality to allow this to happen without any required upgrade steps.
|
|
|
|
.. _Cross Site Request Forgeries: http://www.squarefree.com/securitytips/web-developers.html#CSRF
|
|
|
|
How to use it
|
|
=============
|
|
|
|
.. versionchanged:: 1.2
|
|
The template tag functionality (the recommended way to use this) was added
|
|
in version 1.2. The previous method (still available) is described under
|
|
`Legacy method`_.
|
|
|
|
To enable CSRF protection for your views, follow these steps:
|
|
|
|
1. Add the middleware
|
|
``'django.middleware.csrf.CsrfViewMiddleware'`` to your list of
|
|
middleware classes, :setting:`MIDDLEWARE_CLASSES`. (It should come
|
|
before ``CsrfResponseMiddleware`` if that is being used, and before any
|
|
view middleware that assume that CSRF attacks have been dealt with.)
|
|
|
|
Alternatively, you can use the decorator
|
|
``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 the
|
|
``'django.core.context_processors.csrf'`` context processor is
|
|
being used. Usually, this can be done in one of two ways:
|
|
|
|
1. Use RequestContext, which always uses
|
|
``'django.core.context_processors.csrf'`` (no matter what your
|
|
TEMPLATE_CONTEXT_PROCESSORS setting). If you are using
|
|
generic views or contrib apps, you are covered already, since these
|
|
apps use RequestContext throughout.
|
|
|
|
2. Manually import and use the processor to generate the CSRF token and
|
|
add it to the template context. e.g.::
|
|
|
|
from django.core.context_processors import csrf
|
|
from django.shortcuts import render_to_response
|
|
|
|
def my_view(request):
|
|
c = {}
|
|
c.update(csrf(request))
|
|
# ... view code here
|
|
return render_to_response("a_template.html", c)
|
|
|
|
You may want to write your own ``render_to_response`` wrapper that
|
|
takes care of this step for you.
|
|
|
|
The utility script ``extras/csrf_migration_helper.py`` can help to automate the
|
|
finding of code and templates that may need to be upgraded. It contains full
|
|
help on how to use it.
|
|
|
|
The decorator method
|
|
--------------------
|
|
|
|
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). It is used like
|
|
this::
|
|
|
|
from django.views.decorators.csrf import csrf_protect
|
|
from django.template import RequestContext
|
|
|
|
@csrf_protect
|
|
def my_view(request):
|
|
c = {}
|
|
# ...
|
|
return render_to_response("a_template.html", c,
|
|
context_instance=RequestContext(request))
|
|
|
|
Use of the decorator is **not recommended** by itself, 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.
|
|
|
|
Legacy method
|
|
-------------
|
|
|
|
In Django 1.1, the template tag did not exist. Instead, a post-processing
|
|
middleware that re-wrote POST forms to include the CSRF token was used. If you
|
|
are upgrading a site from version 1.1 or earlier, please read this section and
|
|
the `Upgrading notes`_ below. The post-processing middleware is still available
|
|
as ``CsrfResponseMiddleware``, and it can be used by following these steps:
|
|
|
|
1. Follow step 1 above to install ``CsrfViewMiddleware``.
|
|
|
|
2. Add ``'django.middleware.csrf.CsrfResponseMiddleware'`` to your
|
|
:setting:`MIDDLEWARE_CLASSES` setting.
|
|
|
|
``CsrfResponseMiddleware`` needs to process the response before things
|
|
like compression or setting ofETags happen to the response, so it must
|
|
come after ``GZipMiddleware``, ``CommonMiddleware`` and
|
|
``ConditionalGetMiddleware`` in the list. It also must come after
|
|
``CsrfViewMiddleware``.
|
|
|
|
Use of the ``CsrfResponseMiddleware`` is not recommended because of the
|
|
performance hit it imposes, and because of a potential security problem (see
|
|
below). It can be used as an interim measure until applications have been
|
|
updated to use the :ttag:`csrf_token` tag. It is deprecated and will be
|
|
removed in Django 1.4.
|
|
|
|
Django 1.1 and earlier provided a single ``CsrfMiddleware`` class. This is also
|
|
still available for backwards compatibility. It combines the functions of the
|
|
two middleware.
|
|
|
|
Note also that previous versions of these classes depended on the sessions
|
|
framework, but this dependency has now been removed, with backward compatibility
|
|
support so that upgrading will not produce any issues.
|
|
|
|
Security of legacy method
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
The post-processing ``CsrfResponseMiddleware`` adds the CSRF token to all POST
|
|
forms (unless the view has been decorated with ``csrf_response_exempt``). If
|
|
the POST form has an external untrusted site as its target, rather than an
|
|
internal page, that site will be sent the CSRF token when the form is submitted.
|
|
Armed with this leaked information, that site will then be able to successfully
|
|
launch a CSRF attack on your site against that user. The
|
|
``@csrf_response_exempt`` decorator can be used to fix this, but only if the
|
|
page doesn't also contain internal forms that require the token.
|
|
|
|
.. _ref-csrf-upgrading-notes:
|
|
|
|
Upgrading notes
|
|
---------------
|
|
|
|
When upgrading to version 1.2 or later, you may have applications that rely on
|
|
the old post-processing functionality for CSRF protection, or you may not have
|
|
enabled any CSRF protection. This section outlines the steps necessary for a
|
|
smooth upgrade, without having to fix all the applications to use the new
|
|
template tag method immediately.
|
|
|
|
First of all, the location of the middleware and related functions have
|
|
changed. There are backwards compatible stub files so that old imports will
|
|
continue to work for now, but they are deprecated and will be removed in Django
|
|
1.4. The following changes have been made:
|
|
|
|
* Middleware have been moved to ``django.middleware.csrf``
|
|
* Decorators have been moved to ``django.views.decorators.csrf``
|
|
|
|
====================================================== ==============================================
|
|
Old New
|
|
====================================================== ==============================================
|
|
django.contrib.csrf.middleware.CsrfMiddleware django.middleware.csrf.CsrfMiddleware
|
|
django.contrib.csrf.middleware.CsrfViewMiddleware django.middleware.csrf.CsrfViewMiddleware
|
|
django.contrib.csrf.middleware.CsrfResponseMiddleware django.middleware.csrf.CsrfResponseMiddleware
|
|
django.contrib.csrf.middleware.csrf_exempt django.views.decorators.csrf.csrf_exempt
|
|
django.contrib.csrf.middleware.csrf_view_exempt django.views.decorators.csrf.csrf_view_exempt
|
|
django.contrib.csrf.middleware.csrf_response_exempt django.views.decorators.csrf.csrf_response_exempt
|
|
====================================================== ==============================================
|
|
|
|
You should update any imports, and also the paths in your
|
|
:setting:`MIDDLEWARE_CLASSES`.
|
|
|
|
If you have ``CsrfMiddleware`` in your :setting:`MIDDLEWARE_CLASSES`, you will now
|
|
have a working installation with CSRF protection. It is recommended at this
|
|
point that you replace ``CsrfMiddleware`` with its two components,
|
|
``CsrfViewMiddleware`` and ``CsrfResponseMiddleware`` (in that order).
|
|
|
|
If you do not have any of the middleware in your :setting:`MIDDLEWARE_CLASSES`,
|
|
you will have a working installation but without any CSRF protection for your
|
|
views (just as you had before). It is strongly recommended to install
|
|
``CsrfViewMiddleware`` and ``CsrfResponseMiddleware``, as described above.
|
|
|
|
Note that contrib apps, such as the admin, have been updated to use the
|
|
``csrf_protect`` decorator, so that they are secured even if you do not add the
|
|
``CsrfViewMiddleware`` to your settings. However, if you have supplied
|
|
customised templates to any of the view functions of contrib apps (whether
|
|
explicitly via a keyword argument, or by overriding built-in templates), **you
|
|
MUST update them** to include the :ttag:`csrf_token` template tag as described
|
|
above, or they will stop working. (If you cannot update these templates for
|
|
some reason, you will be forced to use ``CsrfResponseMiddleware`` for these
|
|
views to continue working).
|
|
|
|
Note also, if you are using the comments app, and you are not going to add
|
|
``CsrfViewMiddleware`` to your settings (not recommended), you will need to add
|
|
the ``csrf_protect`` decorator to any views that include the comment forms and
|
|
target the comment views (usually using the :ttag:`comment_form_target` template
|
|
tag).
|
|
|
|
Assuming you have followed the above, all views in your Django site will now be
|
|
protected by the ``CsrfViewMiddleware``. Contrib apps meet the requirements
|
|
imposed by the ``CsrfViewMiddleware`` using the template tag, and other
|
|
applications in your project will meet its requirements by virtue of the
|
|
``CsrfResponseMiddleware``.
|
|
|
|
The next step is to update all your applications to use the template tag, as
|
|
described in `How to use it`_, steps 2-3. This can be done as soon as is
|
|
practical. Any applications that are updated will now require Django 1.1.2 or
|
|
later, since they will use the CSRF template tag which was not available in
|
|
earlier versions. (The template tag in 1.1.2 is actually a no-op that exists
|
|
solely to ease the transition to 1.2 — it allows apps to be created that have
|
|
CSRF protection under 1.2 without requiring users of the apps to upgrade to the
|
|
Django 1.2.X series).
|
|
|
|
The utility script ``extras/csrf_migration_helper.py`` can help to automate the
|
|
finding of code and templates that may need to be upgraded. It contains full
|
|
help on how to use it.
|
|
|
|
Finally, once all applications are upgraded, ``CsrfResponseMiddleware`` can be
|
|
removed from your settings.
|
|
|
|
While ``CsrfResponseMiddleware`` is still in use, the ``csrf_response_exempt``
|
|
decorator, described in `Exceptions`_, may be useful. The post-processing
|
|
middleware imposes a performance hit and a potential vulnerability, and any
|
|
views that have been upgraded to use the new template tag method no longer need
|
|
it.
|
|
|
|
Exceptions
|
|
----------
|
|
|
|
.. versionadded:: 1.1
|
|
.. versionchanged:: 1.2
|
|
Import paths for the decorators below were changed.
|
|
|
|
To manually exclude a view function from being handled by either of the two CSRF
|
|
middleware, you can use the ``csrf_exempt`` decorator, found in the
|
|
``django.views.decorators.csrf`` module. For example::
|
|
|
|
from django.views.decorators.csrf import csrf_exempt
|
|
|
|
@csrf_exempt
|
|
def my_view(request):
|
|
return HttpResponse('Hello world')
|
|
|
|
Like the middleware, the ``csrf_exempt`` decorator is composed of two parts: a
|
|
``csrf_view_exempt`` decorator and a ``csrf_response_exempt`` decorator, found
|
|
in the same module. These disable the view protection mechanism
|
|
(``CsrfViewMiddleware``) and the response post-processing
|
|
(``CsrfResponseMiddleware``) respectively. They can be used individually if
|
|
required.
|
|
|
|
You don't have to worry about doing this for most AJAX views. Any request sent
|
|
with "X-Requested-With: XMLHttpRequest" is automatically exempt. (See the `How
|
|
it works`_ section.)
|
|
|
|
Subdomains
|
|
----------
|
|
|
|
By default, CSRF cookies are specific to the subdomain they are set for. This
|
|
means that a form served from one subdomain (e.g. server1.example.com) will not
|
|
be able to have a target on another subdomain (e.g. server2.example.com). This
|
|
restriction can be removed by setting :setting:`CSRF_COOKIE_DOMAIN` to be
|
|
something like ``".example.com"``.
|
|
|
|
Please note that, with or without use of this setting, this CSRF protection
|
|
mechanism is not safe against cross-subdomain attacks -- see `Limitations`_.
|
|
|
|
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.
|
|
|
|
No logging is done, and the error message is not very friendly, so you may want
|
|
to provide your own page for handling this condition. To do this, simply set
|
|
the :setting:`CSRF_FAILURE_VIEW` setting to a dotted path to your own view
|
|
function, which should have the following signature::
|
|
|
|
def csrf_failure(request, reason="")
|
|
|
|
where ``reason`` is a short message (intended for developers or logging, not for
|
|
end users) indicating the reason the request was rejected.
|
|
|
|
How it works
|
|
============
|
|
|
|
The CSRF protection is based on the following things:
|
|
|
|
1. A CSRF cookie that is set to a random value (a session independent nonce, as
|
|
it is called), which other sites will not have access to.
|
|
|
|
This cookie is set by ``CsrfViewMiddleware``. It is meant to be permanent,
|
|
but since there is no way to set a cookie that never expires, it is sent with
|
|
every response that has called ``django.middleware.csrf.get_token()``
|
|
(the function used internally to retrieve the CSRF token).
|
|
|
|
2. A hidden form field with the name 'csrfmiddlewaretoken' present in all
|
|
outgoing POST forms. The value of this field is the value of the CSRF
|
|
cookie.
|
|
|
|
This part is done by the template tag (and with the legacy method, it is done
|
|
by ``CsrfResponseMiddleware``).
|
|
|
|
3. For all incoming POST requests, 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.
|
|
|
|
This check is done by ``CsrfViewMiddleware``.
|
|
|
|
4. In addition, for HTTPS requests, strict referer checking is done by
|
|
``CsrfViewMiddleware``. This is necessary to address a Man-In-The-Middle
|
|
attack that is possible under HTTPS when using a session independent nonce,
|
|
due to the fact that HTTP 'Set-Cookie' headers are (unfortunately) accepted
|
|
by clients that are talking to a site under HTTPS. (Referer checking is not
|
|
done for HTTP requests because the presence of the Referer header is not
|
|
reliable enough under HTTP.)
|
|
|
|
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.
|
|
|
|
``CsrfResponseMiddleware`` checks the Content-Type before modifying the
|
|
response, and only pages that are served as 'text/html' or
|
|
'application/xml+xhtml' are modified.
|
|
|
|
AJAX
|
|
----
|
|
|
|
The middleware tries to be smart about requests that come in via AJAX. Most
|
|
modern JavaScript toolkits send an "X-Requested-With: XMLHttpRequest" HTTP
|
|
header; these requests are detected and automatically *not* handled by this
|
|
middleware. We can do this safely because, in the context of a browser, the
|
|
header can only be added by using ``XMLHttpRequest``, and browsers already
|
|
implement a same-domain policy for ``XMLHttpRequest``.
|
|
|
|
For the more recent browsers that relax this same-domain policy, custom headers
|
|
like "X-Requested-With" are only allowed after the browser has done a
|
|
'preflight' check to the server to see if the cross-domain request is allowed,
|
|
using a strictly 'opt in' mechanism, so the exception for AJAX is still safe—if
|
|
the developer has specifically opted in to allowing cross-site AJAX POST
|
|
requests on a specific URL, they obviously don't want the middleware to disallow
|
|
exactly that.
|
|
|
|
.. _9.1.1 Safe Methods, HTTP 1.1, RFC 2616: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html
|
|
|
|
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. Similarly,
|
|
``CsrfResponseMiddleware`` will send the ``Vary: Cookie`` header if it inserted
|
|
a token. This means that these 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. In this case, on any views
|
|
that will require a CSRF token to be inserted you should use the
|
|
:func:`django.views.decorators.vary.vary_on_cookie` decorator first::
|
|
|
|
from django.views.decorators.cache import cache_page
|
|
from django.views.decorators.vary import vary_on_cookie
|
|
|
|
@cache_page(60 * 15)
|
|
@vary_on_cookie
|
|
def my_view(request):
|
|
# ...
|
|
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
If you are using ``CsrfResponseMiddleware`` and 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. You should
|
|
use the template tag or :meth:`django.middleware.csrf.get_token` to get
|
|
the CSRF token and ensure it is included when your form is submitted.
|
|
|
|
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.
|