Django2.0手册:Cross Site Request Forgery protection



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 RFC 7231#section-4.2.1) are side effect free.
Requests via ‘unsafe’ methods, such as POST, PUT, and DELETE, can then be
protected by following the steps below.

How to use it¶

To take advantage of CSRF protection in your views, follow these steps:

  1. The CSRF middleware is activated by default in the MIDDLEWARE
    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).

  2. 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 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
    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.

AJAX¶

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.

First, you must get the CSRF token. How to do that depends on whether or not
the CSRF_USE_SESSIONS setting is enabled.

Acquiring the token if CSRF_USE_SESSIONS is False

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().

Acquiring the token if CSRF_USE_SESSIONS is True

If you activate CSRF_USE_SESSIONS, you must include the CSRF token
in your HTML and read the token from the DOM with JavaScript:

{% csrf_token %}
<script type="text/javascript">
// using jQuery
var csrftoken = jQuery("[name=csrfmiddlewaretoken]").val();
</script>

Setting the token on the AJAX request

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';

Using CSRF in Jinja2 templates¶

Django’s Jinja2 template backend
adds {{ csrf_input }} to the context of all templates which is equivalent
to {% csrf_token %} in the Django template language. For example:

<form method="post">{{ csrf_input }}

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).

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.shortcuts import render
from django.views.decorators.csrf import csrf_protect

@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.

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.

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.

CSRF failures are logged as warnings to the django.security.csrf logger.

Changed in Django 1.11:

In older versions, CSRF failures are logged to the django.request
logger.

How it works¶

The CSRF protection is based on the following things:

  1. A CSRF cookie that is based on a random secret value, which other sites
    will not have access to.

    This cookie is set by CsrfViewMiddleware. It is sent with every
    response that has called django.middleware.csrf.get_token() (the
    function used internally to retrieve the CSRF token), if it wasn’t already
    set on the request.

    In order to protect against BREACH attacks, the token is not simply the
    secret; a random salt is prepended to the secret and used to scramble it.

    For security reasons, the value of the secret is changed each time a
    user logs in.

  2. A hidden form field with the name ‘csrfmiddlewaretoken’ present in all
    outgoing POST forms. The value of this field is, again, the value of the
    secret, with a salt which is both added to it and used to scramble it. The
    salt is regenerated on every call to get_token() so that the form field
    value is changed in every such response.

    This part is done by the template tag.

  3. 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.

    When validating the ‘csrfmiddlewaretoken’ field value, only the secret,
    not the full token, is compared with the secret in the cookie value.
    This allows the use of ever-changing tokens. While each request may use its
    own token, the secret remains common to all.

    This check is done by CsrfViewMiddleware.

  4. 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 secret, 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 7231). 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 7231 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).

Removing the Referer header

To avoid disclosing the referrer URL to third-party sites, you might want
to disable the referer on your site’s <a> tags. For example, you
might use the <meta name="referrer" content="no-referrer"> tag or
include the Referrer-Policy: no-referrer header. Due to the CSRF
protection’s strict referer checking on HTTPS requests, those techniques
cause a CSRF failure on requests with ‘unsafe’ methods. Instead, use
alternatives like <a rel="noreferrer" ...>" for links to third-party
sites.

Caching¶

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
.

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.

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)

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.

Edge cases¶

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.

Utilities¶

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.http import HttpResponse
from django.views.decorators.csrf import csrf_exempt

@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.shortcuts import render
from django.views.decorators.csrf import requires_csrf_token

@requires_csrf_token
def my_view(request):
    c = {}
    # ...
    return render(request, "a_template.html", c)

This decorator forces a view to send the CSRF cookie.

Scenarios¶

CSRF protection should be disabled for just a few views

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().

CsrfViewMiddleware.process_view not used

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()

Unprotected view needs the 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).

View needs protection for one path

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()

Page uses AJAX without any HTML form

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.

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.

Frequently Asked Questions¶

Is posting an arbitrary CSRF token pair (cookie and POST data) a vulnerability?¶

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.

Is it a problem that Django’s CSRF protection isn’t linked to a session by default?¶

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.

If you wish to store the CSRF token in the user’s session, use the
CSRF_USE_SESSIONS setting.

Why might a user encounter a CSRF validation failure after logging in?¶

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.