The CSRF middleware and template tag provides easy-to-use protection against Cross Site Request Forgeries. This type of attack occurs when a malicious website contains a link, a form button or some JavaScript that is intended to perform some action on your website, 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 (and other ‘safe’ methods, as defined by 9.1.1 Safe Methods, HTTP 1.1, RFC 2616#section-9.1.1) are side-effect free. Requests via ‘unsafe’ methods, such as POST, PUT and DELETE, can then be protected by following the steps below.
To take advantage of CSRF protection in your views, follow these steps:
The CSRF middleware is activated by default in the
MIDDLEWARE_CLASSES setting. If you override that setting, remember
that 'django.middleware.csrf.CsrfViewMiddleware' should come before any
view middleware that assume that CSRF attacks have been dealt with.
If you disabled it, which is not recommended, you can use
csrf_protect() on particular views
you want to protect (see below).
In any template that uses a POST form, use the 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.
In the corresponding view functions, ensure that
RequestContext is used to render the response so
that {% csrf_token %} will work properly. If you’re using the
render() function, generic views, or contrib apps,
you are covered already since these all use RequestContext.
While the above method can be used for AJAX POST requests, it has some
inconveniences: you have to remember to pass the CSRF token in as POST data with
every POST request. For this reason, there is an alternative method: on each
XMLHttpRequest, set a custom X-CSRFToken header to the value of the CSRF
token. This is often easier, because many JavaScript frameworks provide hooks
that allow headers to be set on every request.
As a first step, you must get the CSRF token itself. The recommended source for
the token is the csrftoken cookie, which will be set if you’ve enabled CSRF
protection for your views as outlined above.
Note
The CSRF token cookie is named csrftoken by default, but you can control
the cookie name via the CSRF_COOKIE_NAME setting.
The CSRF header name is HTTP_X_CSRFTOKEN by default, but you can
customize it using the CSRF_HEADER_NAME setting.
Acquiring the token is straightforward:
// using jQuery
function getCookie(name) {
var cookieValue = null;
if (document.cookie && document.cookie != '') {
var cookies = document.cookie.split(';');
for (var i = 0; i < cookies.length; i++) {
var cookie = jQuery.trim(cookies[i]);
// Does this cookie string begin with the name we want?
if (cookie.substring(0, name.length + 1) == (name + '=')) {
cookieValue = decodeURIComponent(cookie.substring(name.length + 1));
break;
}
}
}
return cookieValue;
}
var csrftoken = getCookie('csrftoken');
The above code could be simplified by using the JavaScript Cookie library to replace getCookie:
var csrftoken = Cookies.get('csrftoken');
Note
The CSRF token is also present in the DOM, but only if explicitly included
using csrf_token in a template. The cookie contains the canonical
token; the CsrfViewMiddleware will prefer the cookie to the token in
the DOM. Regardless, you’re guaranteed to have the cookie if the token is
present in the DOM, so you should use the cookie!
Warning
If your view is not rendering a template containing the csrf_token
template tag, Django might not set the CSRF token cookie. This is common in
cases where forms are dynamically added to the page. To address this case,
Django provides a view decorator which forces setting of the cookie:
ensure_csrf_cookie().
Finally, you’ll have to actually set the header on your AJAX request, while protecting the CSRF token from being sent to other domains using settings.crossDomain in jQuery 1.5.1 and newer:
function csrfSafeMethod(method) {
// these HTTP methods do not require CSRF protection
return (/^(GET|HEAD|OPTIONS|TRACE)$/.test(method));
}
$.ajaxSetup({
beforeSend: function(xhr, settings) {
if (!csrfSafeMethod(settings.type) && !this.crossDomain) {
xhr.setRequestHeader("X-CSRFToken", csrftoken);
}
}
});
If you’re using AngularJS 1.1.3 and newer, it’s sufficient to configure the
$http provider with the cookie and header names:
$httpProvider.defaults.xsrfCookieName = 'csrftoken';
$httpProvider.defaults.xsrfHeaderName = 'X-CSRFToken';
When using a different template engine than Django’s built-in engine, you can set the token in your forms manually after making sure it’s available in the template context.
For example, in the Jinja2 template language, your form could contain the following:
<div style="display:none">
<input type="hidden" name="csrfmiddlewaretoken" value="{{ csrf_token }}">
</div>
You can use JavaScript similar to the AJAX code above to get the value of the CSRF token.
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).
Use of the decorator by itself is not recommended, 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.
csrf_protect(view)¶Decorator that provides the protection of CsrfViewMiddleware to a view.
Usage:
from django.views.decorators.csrf import csrf_protect
from django.shortcuts import render
@csrf_protect
def my_view(request):
c = {}
# ...
return render(request, "a_template.html", c)
If you are using class-based views, you can refer to Decorating class-based views.
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.
The error page, however, is not very friendly, so you may want to provide your
own view for handling this condition. To do this, simply set the
CSRF_FAILURE_VIEW setting.
The CSRF protection is based on the following things:
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).
For security reasons, the value of the CSRF cookie is changed each time a user logs in.
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.
For all incoming requests that are not using HTTP GET, HEAD, OPTIONS or TRACE, 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.
In addition, for HTTPS requests, strict referer checking is done by
CsrfViewMiddleware. This means that even if a subdomain can set or
modify cookies on your domain, it can’t force a user to post to your
application since that request won’t come from your own exact domain.
This also addresses a man-in-the-middle attack that’s possible under HTTPS
when using a session independent nonce, due to the fact that HTTP
Set-Cookie headers are (unfortunately) accepted by clients even when
they are talking to a site under HTTPS. (Referer checking is not done for
HTTP requests because the presence of the Referer header isn’t reliable
enough under HTTP.)
If the CSRF_COOKIE_DOMAIN setting is set, the referer is compared
against it. This setting supports subdomains. For example,
CSRF_COOKIE_DOMAIN = '.example.com' will allow POST requests from
www.example.com and api.example.com. If the setting is not set, then
the referer must match the HTTP Host header.
Expanding the accepted referers beyond the current host or cookie domain can
be done with the CSRF_TRUSTED_ORIGINS setting.
This ensures that only forms that have originated from trusted domains can be used to POST data back.
It deliberately ignores GET requests (and other requests that are defined as ‘safe’ by RFC 2616). These requests ought never to have any potentially dangerous side effects , and so a CSRF attack with a GET request ought to be harmless. RFC 2616 defines POST, PUT and DELETE as ‘unsafe’, and all other methods are also assumed to be unsafe, for maximum protection.
The CSRF protection cannot protect against man-in-the-middle attacks, so use HTTPS with HTTP Strict Transport Security. It also assumes validation of the HOST header and that there aren’t any cross-site scripting vulnerabilities on your site (because XSS vulnerabilities already let an attacker do anything a CSRF vulnerability allows and much worse).
Checking against the CSRF_COOKIE_DOMAIN setting was added.
If the 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. This means that the
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 or the CSRF cookie, and the
response will be cached without either one. In this case, on any views that
will require a CSRF token to be inserted you should use the
django.views.decorators.csrf.csrf_protect() decorator first:
from django.views.decorators.cache import cache_page
from django.views.decorators.csrf import csrf_protect
@cache_page(60 * 15)
@csrf_protect
def my_view(request):
...
If you are using class-based views, you can refer to Decorating class-based views.
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.
If, for some reason, you want the test client to perform CSRF checks, you can create an instance of the test client that enforces CSRF checks:
>>> from django.test import Client
>>> csrf_client = Client(enforce_csrf_checks=True)
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.
Certain views can have unusual requirements that mean they don’t fit the normal pattern envisaged here. A number of utilities can be useful in these situations. The scenarios they might be needed in are described in the following section.
The examples below assume you are using function-based views. If you are working with class-based views, you can refer to Decorating class-based views.
csrf_exempt(view)[source]¶This decorator marks a view as being exempt from the protection ensured by the middleware. Example:
from django.views.decorators.csrf import csrf_exempt
from django.http import HttpResponse
@csrf_exempt
def my_view(request):
return HttpResponse('Hello world')
requires_csrf_token(view)¶Normally the csrf_token template tag will not work if
CsrfViewMiddleware.process_view or an equivalent like csrf_protect
has not run. The view decorator requires_csrf_token can be used to
ensure the template tag does work. This decorator works similarly to
csrf_protect, but never rejects an incoming request.
Example:
from django.views.decorators.csrf import requires_csrf_token
from django.shortcuts import render
@requires_csrf_token
def my_view(request):
c = {}
# ...
return render(request, "a_template.html", c)
Most views requires CSRF protection, but a few do not.
Solution: rather than disabling the middleware and applying csrf_protect to
all the views that need it, enable the middleware and use
csrf_exempt().
There are cases when CsrfViewMiddleware.process_view may not have run
before your view is run - 404 and 500 handlers, for example - but you still
need the CSRF token in a form.
Solution: use requires_csrf_token()
There may be some views that are unprotected and have been exempted by
csrf_exempt, but still need to include the CSRF token.
Solution: use csrf_exempt() followed by
requires_csrf_token(). (i.e. requires_csrf_token
should be the innermost decorator).
A view needs CSRF protection under one set of conditions only, and mustn’t have it for the rest of the time.
Solution: use csrf_exempt() for the whole
view function, and csrf_protect() for the
path within it that needs protection. Example:
from django.views.decorators.csrf import csrf_exempt, csrf_protect
@csrf_exempt
def my_view(request):
@csrf_protect
def protected_path(request):
do_something()
if some_condition():
return protected_path(request)
else:
do_something_else()
A page makes a POST request via AJAX, and the page does not have an HTML form
with a csrf_token that would cause the required CSRF cookie to be sent.
Solution: use ensure_csrf_cookie() on the
view that sends the page.
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.
A number of settings can be used to control Django’s CSRF behavior:
No, this is by design. Without a man-in-the-middle attack, there is no way for an attacker to send a CSRF token cookie to a victim’s browser, so a successful attack would need to obtain the victim’s browser’s cookie via XSS or similar, in which case an attacker usually doesn’t need CSRF attacks.
Some security audit tools flag this as a problem but as mentioned before, an attacker cannot steal a user’s browser’s CSRF cookie. “Stealing” or modifying your own token using Firebug, Chrome dev tools, etc. isn’t a vulnerability.
No, this is by design. Not linking CSRF protection to a session allows using the protection on sites such as a pastebin that allow submissions from anonymous users which don’t have a session.
Generating a new token for each request is problematic from a UI perspective because it invalidates all previous forms. Most users would be very unhappy to find that opening a new tab on your site has invalidated the form they had just spent time filling out in another tab or that a form they accessed via the back button could not be filled out.
For security reasons, CSRF tokens are rotated each time a user logs in. Any page with a form generated before a login will have an old, invalid CSRF token and need to be reloaded. This might happen if a user uses the back button after a login or if they log in in a different browser tab.
May 02, 2016