django/docs/topics/security.txt

==================
Security in Django
==================

This document is an overview of Django's security features. It includes advice
on securing a Django-powered site.

.. _cross-site-scripting:

Cross site scripting (XSS) protection
=====================================

.. highlightlang:: html+django

XSS attacks allow a user to inject client side scripts into the browsers of
other users. This is usually achieved by storing the malicious scripts in the
database where it will be retrieved and displayed to other users, or by getting
users to click a link which will cause the attacker's JavaScript to be executed
by the user's browser. However, XSS attacks can originate from any untrusted
source of data, such as cookies or Web services, whenever the data is not
sufficiently sanitized before including in a page.

Using Django templates protects you against the majority of XSS attacks.
However, it is important to understand what protections it provides
and its limitations.

Django templates :ref:`escape specific characters <automatic-html-escaping>`
which are particularly dangerous to HTML. While this protects users from most
malicious input, it is not entirely foolproof. For example, it will not
protect the following:

.. code-block:: html+django

    <style class="{{ var }}">...</style>

If ``var`` is set to ``'class1 onmouseover=javascript:func()'``, this can result
in unauthorized JavaScript execution, depending on how the browser renders
imperfect HTML.

It is also important to be particularly careful when using ``is_safe`` with
custom template tags, the :tfilter:`safe` template tag, :mod:`mark_safe
<django.utils.safestring>`, and when autoescape is turned off.

In addition, if you are using the template system to output something other
than HTML, there may be entirely separate characters and words which require
escaping.

You should also be very careful when storing HTML in the database, especially
when that HTML is retrieved and displayed.


Cross site request forgery (CSRF) protection
============================================

CSRF attacks allow a malicious user to execute actions using the credentials
of another user without that user's knowledge or consent.

Django has built-in protection against most types of CSRF attacks, providing you
have :ref:`enabled and used it <using-csrf>` where appropriate. However, as with
any mitigation technique, there are limitations. For example, it is possible to
disable the CSRF module globally or for particular views. You should only do
this if you know what you are doing. There are other :ref:`limitations
<csrf-limitations>` if your site has subdomains that are outside of your
control.

:ref:`CSRF protection works <how-csrf-works>` by checking for a nonce in each
POST request. This ensures that a malicious user cannot simply "replay" a form
POST to your Web site and have another logged in user unwittingly submit that
form. The malicious user would have to know the nonce, which is user specific
(using a cookie).

When deployed with :ref:`HTTPS <security-recommendation-ssl>`,
``CsrfViewMiddleware`` will check that the HTTP referer header is set to a
URL on the same origin (including subdomain and port). Because HTTPS
provides additional security, it is imperative to ensure connections use HTTPS
where it is available by forwarding insecure connection requests and using
HSTS for supported browsers.

Be very careful with marking views with the ``csrf_exempt`` decorator unless
it is absolutely necessary.

.. _sql-injection-protection:

SQL injection protection
========================

SQL injection is a type of attack where a malicious user is able to execute
arbitrary SQL code on a database. This can result in records
being deleted or data leakage.

By using Django's querysets, the resulting SQL will be properly escaped by
the underlying database driver. However, Django also gives developers power to
write :ref:`raw queries <executing-raw-queries>` or execute
:ref:`custom sql <executing-custom-sql>`. These capabilities should be used
sparingly and you should always be careful to properly escape any parameters
that the user can control. In addition, you should exercise caution when using
:meth:`extra() <django.db.models.query.QuerySet.extra>`.

Clickjacking protection
=======================

Clickjacking is a type of attack where a malicious site wraps another site
in a frame. This attack can result in an unsuspecting user being tricked
into performing unintended actions on the target site.

Django contains :ref:`clickjacking protection <clickjacking-prevention>` in
the form of the
:mod:`X-Frame-Options middleware <django.middleware.clickjacking.XFrameOptionsMiddleware>`
which in a supporting browser can prevent a site from being rendered inside
a frame. It is possible to disable the protection on a per view basis
or to configure the exact header value sent.

The middleware is strongly recommended for any site that does not need to have
its pages wrapped in a frame by third party sites, or only needs to allow that
for a small section of the site.

.. _security-recommendation-ssl:

SSL/HTTPS
=========

It is always better for security, though not always practical in all cases, to
deploy your site behind HTTPS. Without this, it is possible for malicious
network users to sniff authentication credentials or any other information
transferred between client and server, and in some cases -- **active** network
attackers -- to alter data that is sent in either direction.

If you want the protection that HTTPS provides, and have enabled it on your
server, there are some additional steps you may need:

* If necessary, set :setting:`SECURE_PROXY_SSL_HEADER`, ensuring that you have
  understood the warnings there thoroughly. Failure to do this can result
  in CSRF vulnerabilities, and failure to do it correctly can also be
  dangerous!

* Set up redirection so that requests over HTTP are redirected to HTTPS.

  This could be done using a custom middleware. Please note the caveats under
  :setting:`SECURE_PROXY_SSL_HEADER`. For the case of a reverse proxy, it may be
  easier or more secure to configure the main Web server to do the redirect to
  HTTPS.

* Use 'secure' cookies.

  If a browser connects initially via HTTP, which is the default for most
  browsers, it is possible for existing cookies to be leaked. For this reason,
  you should set your :setting:`SESSION_COOKIE_SECURE` and
  :setting:`CSRF_COOKIE_SECURE` settings to ``True``. This instructs the browser
  to only send these cookies over HTTPS connections. Note that this will mean
  that sessions will not work over HTTP, and the CSRF protection will prevent
  any POST data being accepted over HTTP (which will be fine if you are
  redirecting all HTTP traffic to HTTPS).

* Use HTTP Strict Transport Security (HSTS)

  HSTS is an HTTP header that informs a browser that all future connections
  to a particular site should always use HTTPS. Combined with redirecting
  requests over HTTP to HTTPS, this will ensure that connections always enjoy
  the added security of SSL provided one successful connection has occurred.
  HSTS is usually configured on the web server.

.. _host-headers-virtual-hosting:

Host header validation
======================

Django uses the ``Host`` header provided by the client to construct URLs in
certain cases. While these values are sanitized to prevent Cross Site Scripting
attacks, a fake ``Host`` value can be used for Cross-Site Request Forgery,
cache poisoning attacks, and poisoning links in emails.

Because even seemingly-secure web server configurations are susceptible to fake
``Host`` headers, Django validates ``Host`` headers against the
:setting:`ALLOWED_HOSTS` setting in the
:meth:`django.http.HttpRequest.get_host()` method.

This validation only applies via :meth:`~django.http.HttpRequest.get_host()`;
if your code accesses the ``Host`` header directly from ``request.META`` you
are bypassing this security protection.

For more details see the full :setting:`ALLOWED_HOSTS` documentation.

.. warning::

   Previous versions of this document recommended configuring your web server to
   ensure it validates incoming HTTP ``Host`` headers. While this is still
   recommended, in many common web servers a configuration that seems to
   validate the ``Host`` header may not in fact do so. For instance, even if
   Apache is configured such that your Django site is served from a non-default
   virtual host with the ``ServerName`` set, it is still possible for an HTTP
   request to match this virtual host and supply a fake ``Host`` header. Thus,
   Django now requires that you set :setting:`ALLOWED_HOSTS` explicitly rather
   than relying on web server configuration.

Additionally, as of 1.3.1, Django requires you to explicitly enable support for
the ``X-Forwarded-Host`` header (via the :setting:`USE_X_FORWARDED_HOST`
setting) if your configuration requires it.

Session security
================

Similar to the :ref:`CSRF limitations <csrf-limitations>` requiring a site to
be deployed such that untrusted users don't have access to any subdomains,
:mod:`django.contrib.sessions` also has limitations. See :ref:`the session
topic guide section on security <topics-session-security>` for details.

.. _user-uploaded-content-security:

User-uploaded content
=====================

.. note::
    Consider :ref:`serving static files from a cloud service or CDN
    <staticfiles-from-cdn>` to avoid some of these issues.

* If your site accepts file uploads, it is strongly advised that you limit
  these uploads in your Web server configuration to a reasonable
  size in order to prevent denial of service (DOS) attacks. In Apache, this
  can be easily set using the LimitRequestBody_ directive.

* If you are serving your own static files, be sure that handlers like Apache's
  ``mod_php``, which would execute static files as code, are disabled. You don't
  want users to be able to execute arbitrary code by uploading and requesting a
  specially crafted file.

* Django's media upload handling poses some vulnerabilities when that media is
  served in ways that do not follow security best practices. Specifically, an
  HTML file can be uploaded as an image if that file contains a valid PNG
  header followed by malicious HTML. This file will pass verification of the
  library that Django uses for :class:`~django.db.models.ImageField` image
  processing (Pillow). When this file is subsequently displayed to a
  user, it may be displayed as HTML depending on the type and configuration of
  your web server.

  No bulletproof technical solution exists at the framework level to safely
  validate all user uploaded file content, however, there are some other steps
  you can take to mitigate these attacks:

  1. One class of attacks can be prevented by always serving user uploaded
     content from a distinct top-level or second-level domain. This prevents
     any exploit blocked by `same-origin policy`_ protections such as cross
     site scripting. For example, if your site runs on ``example.com``, you
     would want to serve uploaded content (the :setting:`MEDIA_URL` setting)
     from something like ``usercontent-example.com``. It's *not* sufficient to
     serve content from a subdomain like ``usercontent.example.com``.

  2. Beyond this, applications may choose to define a whitelist of allowable
     file extensions for user uploaded files and configure the web server
     to only serve such files.

.. _same-origin policy: http://en.wikipedia.org/wiki/Same-origin_policy

.. _additional-security-topics:

Additional security topics
==========================

While Django provides good security protection out of the box, it is still
important to properly deploy your application and take advantage of the
security protection of the Web server, operating system and other components.

* Make sure that your Python code is outside of the Web server's root. This
  will ensure that your Python code is not accidentally served as plain text
  (or accidentally executed).
* Take care with any :ref:`user uploaded files <file-upload-security>`.
* Django does not throttle requests to authenticate users. To protect against
  brute-force attacks against the authentication system, you may consider
  deploying a Django plugin or Web server module to throttle these requests.
* Keep your :setting:`SECRET_KEY` a secret.
* It is a good idea to limit the accessibility of your caching system and
  database using a firewall.

.. _LimitRequestBody: http://httpd.apache.org/docs/2.2/mod/core.html#limitrequestbody