lib/django-1.4/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 :ttag:`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.

Markup library
--------------

If you use :mod:`django.contrib.markup`, you need to ensure that the filters are
only used on trusted input, or that you have correctly configured them to ensure
they do not allow raw HTML output. See the documentation of that module for more
information.

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).

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

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.

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).

.. _additional-security-topics:

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 webserver 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 webserver to
   ensure it validates incoming HTTP ``Host`` headers. While this is still
   recommended, in many common webservers 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 webserver 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.



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.
* 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.
* 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