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77974a684a
`action="."` strips query parameters from the URL which is not usually what you want. Copy-paste coding of these examples could lead to difficult to track down bugs or even data loss if the query parameter was meant to alter the scope of a form's POST request.
465 lines
17 KiB
Plaintext
465 lines
17 KiB
Plaintext
=====================================
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Cross Site Request Forgery protection
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=====================================
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.. module:: django.middleware.csrf
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:synopsis: Protects against Cross Site Request Forgeries
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The CSRF middleware and template tag 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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website contains a link, a form button or some JavaScript that is intended to
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perform some action on your website, using the credentials of a logged-in user
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who visits the malicious site in their browser. A related type of attack,
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'login CSRF', where an attacking site tricks a user's browser into logging into
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a site with someone else's credentials, is also covered.
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The first defense against CSRF attacks is to ensure that GET requests (and other
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'safe' methods, as defined by 9.1.1 Safe Methods, HTTP 1.1,
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:rfc:`2616#section-9.1.1`) are side-effect free. Requests via 'unsafe' methods,
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such as POST, PUT and DELETE, can then be protected by following the steps
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below.
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.. _Cross Site Request Forgeries: https://www.squarefree.com/securitytips/web-developers.html#CSRF
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.. _using-csrf:
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How to use it
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=============
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To take advantage of CSRF protection in your views, follow these steps:
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1. The CSRF middleware is activated by default in the
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:setting:`MIDDLEWARE_CLASSES` setting. If you override that setting, remember
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that ``'django.middleware.csrf.CsrfViewMiddleware'`` should come before any
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view middleware that assume that CSRF attacks have been dealt with.
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If you disabled it, which is not recommended, you can use
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:func:`~django.views.decorators.csrf.csrf_protect` on particular views
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you want to protect (see below).
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2. In any template that uses a POST form, use the :ttag:`csrf_token` tag inside
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the ``<form>`` element if the form is for an internal URL, e.g.::
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<form action="" method="post">{% csrf_token %}
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This should not be done for POST forms that target external URLs, since
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that would cause the CSRF token to be leaked, leading to a vulnerability.
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3. In the corresponding view functions, ensure that
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:class:`~django.template.RequestContext` is used to render the response so
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that ``{% csrf_token %}`` will work properly. If you're using the
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:func:`~django.shortcuts.render` function, generic views, or contrib apps,
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you are covered already since these all use ``RequestContext``.
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.. _csrf-ajax:
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AJAX
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----
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While the above method can be used for AJAX POST requests, it has some
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inconveniences: you have to remember to pass the CSRF token in as POST data with
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every POST request. For this reason, there is an alternative method: on each
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XMLHttpRequest, set a custom ``X-CSRFToken`` header to the value of the CSRF
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token. This is often easier, because many JavaScript frameworks provide hooks
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that allow headers to be set on every request.
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As a first step, you must get the CSRF token itself. The recommended source for
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the token is the ``csrftoken`` cookie, which will be set if you've enabled CSRF
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protection for your views as outlined above.
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.. note::
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The CSRF token cookie is named ``csrftoken`` by default, but you can control
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the cookie name via the :setting:`CSRF_COOKIE_NAME` setting.
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The CSRF header name is ``HTTP_X_CSRFTOKEN`` by default, but you can
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customize it using the :setting:`CSRF_HEADER_NAME` setting.
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Acquiring the token is straightforward:
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.. code-block:: javascript
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// using jQuery
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function getCookie(name) {
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var cookieValue = null;
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if (document.cookie && document.cookie != '') {
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var cookies = document.cookie.split(';');
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for (var i = 0; i < cookies.length; i++) {
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var cookie = jQuery.trim(cookies[i]);
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// Does this cookie string begin with the name we want?
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if (cookie.substring(0, name.length + 1) == (name + '=')) {
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cookieValue = decodeURIComponent(cookie.substring(name.length + 1));
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break;
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}
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}
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}
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return cookieValue;
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}
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var csrftoken = getCookie('csrftoken');
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The above code could be simplified by using the `JavaScript Cookie library
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<https://github.com/js-cookie/js-cookie/>`_ to replace ``getCookie``:
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.. code-block:: javascript
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var csrftoken = Cookies.get('csrftoken');
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.. note::
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The CSRF token is also present in the DOM, but only if explicitly included
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using :ttag:`csrf_token` in a template. The cookie contains the canonical
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token; the ``CsrfViewMiddleware`` will prefer the cookie to the token in
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the DOM. Regardless, you're guaranteed to have the cookie if the token is
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present in the DOM, so you should use the cookie!
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.. warning::
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If your view is not rendering a template containing the :ttag:`csrf_token`
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template tag, Django might not set the CSRF token cookie. This is common in
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cases where forms are dynamically added to the page. To address this case,
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Django provides a view decorator which forces setting of the cookie:
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:func:`~django.views.decorators.csrf.ensure_csrf_cookie`.
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Finally, you'll have to actually set the header on your AJAX request, while
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protecting the CSRF token from being sent to other domains using
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`settings.crossDomain <https://api.jquery.com/jQuery.ajax>`_ in jQuery 1.5.1 and
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newer:
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.. code-block:: javascript
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function csrfSafeMethod(method) {
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// these HTTP methods do not require CSRF protection
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return (/^(GET|HEAD|OPTIONS|TRACE)$/.test(method));
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}
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$.ajaxSetup({
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beforeSend: function(xhr, settings) {
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if (!csrfSafeMethod(settings.type) && !this.crossDomain) {
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xhr.setRequestHeader("X-CSRFToken", csrftoken);
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}
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}
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});
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If you're using AngularJS 1.1.3 and newer, it's sufficient to configure the
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``$http`` provider with the cookie and header names:
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.. code-block:: javascript
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$http.defaults.xsrfCookieName = 'csrftoken';
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$http.defaults.xsrfHeaderName = 'X-CSRFToken';
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Other template engines
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----------------------
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When using a different template engine than Django's built-in engine, you can
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set the token in your forms manually after making sure it's available in the
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template context.
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For example, in the Jinja2 template language, your form could contain the
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following:
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.. code-block:: html
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<div style="display:none">
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<input type="hidden" name="csrfmiddlewaretoken" value="{{ csrf_token }}">
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</div>
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You can use JavaScript similar to the :ref:`AJAX code <csrf-ajax>` above to get
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the value of the CSRF token.
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The decorator method
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--------------------
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.. module:: django.views.decorators.csrf
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Rather than adding ``CsrfViewMiddleware`` as a blanket protection, you can use
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the ``csrf_protect`` decorator, which has exactly the same functionality, on
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particular views that need the protection. It must be used **both** on views
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that insert the CSRF token in the output, and on those that accept the POST form
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data. (These are often the same view function, but not always).
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Use of the decorator by itself is **not recommended**, since if you forget to
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use it, you will have a security hole. The 'belt and braces' strategy of using
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both is fine, and will incur minimal overhead.
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.. function:: csrf_protect(view)
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Decorator that provides the protection of ``CsrfViewMiddleware`` to a view.
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Usage::
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from django.views.decorators.csrf import csrf_protect
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from django.shortcuts import render
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@csrf_protect
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def my_view(request):
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c = {}
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# ...
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return render(request, "a_template.html", c)
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If you are using class-based views, you can refer to
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:ref:`Decorating class-based views<decorating-class-based-views>`.
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Rejected requests
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=================
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By default, a '403 Forbidden' response is sent to the user if an incoming
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request fails the checks performed by ``CsrfViewMiddleware``. This should
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usually only be seen when there is a genuine Cross Site Request Forgery, or
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when, due to a programming error, the CSRF token has not been included with a
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POST form.
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The error page, however, is not very friendly, so you may want to provide your
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own view for handling this condition. To do this, simply set the
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:setting:`CSRF_FAILURE_VIEW` setting.
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.. _how-csrf-works:
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How it works
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============
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The CSRF protection is based on the following things:
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1. A CSRF cookie that is set to a random value (a session independent nonce, as
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it is called), which other sites will not have access to.
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This cookie is set by ``CsrfViewMiddleware``. It is meant to be permanent,
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but since there is no way to set a cookie that never expires, it is sent with
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every response that has called ``django.middleware.csrf.get_token()``
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(the function used internally to retrieve the CSRF token).
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2. A hidden form field with the name 'csrfmiddlewaretoken' present in all
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outgoing POST forms. The value of this field is the value of the CSRF
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cookie.
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This part is done by the template tag.
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3. For all incoming requests that are not using HTTP GET, HEAD, OPTIONS or
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TRACE, a CSRF cookie must be present, and the 'csrfmiddlewaretoken' field
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must be present and correct. If it isn't, the user will get a 403 error.
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This check is done by ``CsrfViewMiddleware``.
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4. In addition, for HTTPS requests, strict referer checking is done by
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``CsrfViewMiddleware``. This is necessary to address a Man-In-The-Middle
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attack that is possible under HTTPS when using a session independent nonce,
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due to the fact that HTTP 'Set-Cookie' headers are (unfortunately) accepted
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by clients that are talking to a site under HTTPS. (Referer checking is not
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done for HTTP requests because the presence of the Referer header is not
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reliable enough under HTTP.)
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If the :setting:`CSRF_COOKIE_DOMAIN` setting is set, the referer is compared
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against it. This setting supports subdomains. For example,
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``CSRF_COOKIE_DOMAIN = '.example.com'`` will allow POST requests from
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``www.example.com`` and ``api.example.com``. If the setting is not set, then
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the referer must match the HTTP ``Host`` header.
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Expanding the accepted referers beyond the current host or cookie domain can
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be done with the :setting:`CSRF_TRUSTED_ORIGINS` setting.
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This ensures that only forms that have originated from trusted domains can be
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used to POST data back.
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It deliberately ignores GET requests (and other requests that are defined as
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'safe' by :rfc:`2616`). These requests ought never to have any potentially
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dangerous side effects , and so a CSRF attack with a GET request ought to be
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harmless. :rfc:`2616` defines POST, PUT and DELETE as 'unsafe', and all other
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methods are assumed to be unsafe, for maximum protection.
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.. versionchanged:: 1.9
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Checking against the :setting:`CSRF_COOKIE_DOMAIN` setting was added.
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Caching
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=======
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If the :ttag:`csrf_token` template tag is used by a template (or the
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``get_token`` function is called some other way), ``CsrfViewMiddleware`` will
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add a cookie and a ``Vary: Cookie`` header to the response. This means that the
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middleware will play well with the cache middleware if it is used as instructed
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(``UpdateCacheMiddleware`` goes before all other middleware).
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However, if you use cache decorators on individual views, the CSRF middleware
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will not yet have been able to set the Vary header or the CSRF cookie, and the
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response will be cached without either one. In this case, on any views that
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will require a CSRF token to be inserted you should use the
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:func:`django.views.decorators.csrf.csrf_protect` decorator first::
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from django.views.decorators.cache import cache_page
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from django.views.decorators.csrf import csrf_protect
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@cache_page(60 * 15)
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@csrf_protect
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def my_view(request):
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...
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If you are using class-based views, you can refer to :ref:`Decorating
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class-based views<decorating-class-based-views>`.
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Testing
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=======
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The ``CsrfViewMiddleware`` will usually be a big hindrance to testing view
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functions, due to the need for the CSRF token which must be sent with every POST
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request. For this reason, Django's HTTP client for tests has been modified to
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set a flag on requests which relaxes the middleware and the ``csrf_protect``
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decorator so that they no longer rejects requests. In every other respect
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(e.g. sending cookies etc.), they behave the same.
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If, for some reason, you *want* the test client to perform CSRF
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checks, you can create an instance of the test client that enforces
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CSRF checks::
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>>> from django.test import Client
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>>> csrf_client = Client(enforce_csrf_checks=True)
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.. _csrf-limitations:
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Limitations
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===========
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Subdomains within a site will be able to set cookies on the client for the whole
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domain. By setting the cookie and using a corresponding token, subdomains will
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be able to circumvent the CSRF protection. The only way to avoid this is to
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ensure that subdomains are controlled by trusted users (or, are at least unable
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to set cookies). Note that even without CSRF, there are other vulnerabilities,
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such as session fixation, that make giving subdomains to untrusted parties a bad
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idea, and these vulnerabilities cannot easily be fixed with current browsers.
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Edge cases
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==========
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Certain views can have unusual requirements that mean they don't fit the normal
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pattern envisaged here. A number of utilities can be useful in these
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situations. The scenarios they might be needed in are described in the following
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section.
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Utilities
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---------
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The examples below assume you are using function-based views. If you
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are working with class-based views, you can refer to :ref:`Decorating
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class-based views<decorating-class-based-views>`.
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.. function:: csrf_exempt(view)
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This decorator marks a view as being exempt from the protection ensured by
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the middleware. Example::
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from django.views.decorators.csrf import csrf_exempt
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from django.http import HttpResponse
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@csrf_exempt
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def my_view(request):
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return HttpResponse('Hello world')
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.. function:: requires_csrf_token(view)
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Normally the :ttag:`csrf_token` template tag will not work if
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``CsrfViewMiddleware.process_view`` or an equivalent like ``csrf_protect``
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has not run. The view decorator ``requires_csrf_token`` can be used to
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ensure the template tag does work. This decorator works similarly to
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``csrf_protect``, but never rejects an incoming request.
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Example::
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from django.views.decorators.csrf import requires_csrf_token
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from django.shortcuts import render
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@requires_csrf_token
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def my_view(request):
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c = {}
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# ...
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return render(request, "a_template.html", c)
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.. function:: ensure_csrf_cookie(view)
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This decorator forces a view to send the CSRF cookie.
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Scenarios
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---------
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CSRF protection should be disabled for just a few views
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Most views requires CSRF protection, but a few do not.
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Solution: rather than disabling the middleware and applying ``csrf_protect`` to
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all the views that need it, enable the middleware and use
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:func:`~django.views.decorators.csrf.csrf_exempt`.
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CsrfViewMiddleware.process_view not used
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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There are cases when ``CsrfViewMiddleware.process_view`` may not have run
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before your view is run - 404 and 500 handlers, for example - but you still
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need the CSRF token in a form.
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Solution: use :func:`~django.views.decorators.csrf.requires_csrf_token`
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Unprotected view needs the CSRF token
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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There may be some views that are unprotected and have been exempted by
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``csrf_exempt``, but still need to include the CSRF token.
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Solution: use :func:`~django.views.decorators.csrf.csrf_exempt` followed by
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:func:`~django.views.decorators.csrf.requires_csrf_token`. (i.e. ``requires_csrf_token``
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should be the innermost decorator).
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View needs protection for one path
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A view needs CSRF protection under one set of conditions only, and mustn't have
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it for the rest of the time.
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Solution: use :func:`~django.views.decorators.csrf.csrf_exempt` for the whole
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view function, and :func:`~django.views.decorators.csrf.csrf_protect` for the
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path within it that needs protection. Example::
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from django.views.decorators.csrf import csrf_exempt, csrf_protect
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@csrf_exempt
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def my_view(request):
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@csrf_protect
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def protected_path(request):
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do_something()
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if some_condition():
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return protected_path(request)
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else:
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do_something_else()
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Page uses AJAX without any HTML form
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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A page makes a POST request via AJAX, and the page does not have an HTML form
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with a :ttag:`csrf_token` that would cause the required CSRF cookie to be sent.
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Solution: use :func:`~django.views.decorators.csrf.ensure_csrf_cookie` on the
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view that sends the page.
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Contrib and reusable apps
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=========================
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Because it is possible for the developer to turn off the ``CsrfViewMiddleware``,
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all relevant views in contrib apps use the ``csrf_protect`` decorator to ensure
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the security of these applications against CSRF. It is recommended that the
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developers of other reusable apps that want the same guarantees also use the
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``csrf_protect`` decorator on their views.
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Settings
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========
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A number of settings can be used to control Django's CSRF behavior:
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* :setting:`CSRF_COOKIE_AGE`
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* :setting:`CSRF_COOKIE_DOMAIN`
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* :setting:`CSRF_COOKIE_HTTPONLY`
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* :setting:`CSRF_COOKIE_NAME`
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* :setting:`CSRF_COOKIE_PATH`
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* :setting:`CSRF_COOKIE_SECURE`
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* :setting:`CSRF_FAILURE_VIEW`
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* :setting:`CSRF_HEADER_NAME`
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* :setting:`CSRF_TRUSTED_ORIGINS`
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