Django2.0手册: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 (XSS) protection¶

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 escape specific characters
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:

<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. (Quoting the attribute value would fix this case.)

It is also important to be particularly careful when using is_safe with
custom template tags, the safe template tag, mark_safe, 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 enabled and used it 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 limitations if your site has subdomains that are outside of your
control.

CSRF protection works by checking for a secret in each
POST request. This ensures that a malicious user cannot simply “replay” a form
POST to your website and have another logged in user unwittingly submit that
form. The malicious user would have to know the secret, which is user specific
(using a cookie).

When deployed with HTTPS,
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 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.

Django’s querysets are protected from SQL injection since their queries are
constructed using query parameterization. A query’s SQL code is defined
separately from the query’s parameters. Since parameters may be user-provided
and therefore unsafe, they are escaped by the underlying database driver.

Django also gives developers power to write raw queries or execute 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 extra()
and RawSQL.

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 clickjacking protection in
the form of the
X-Frame-Options middleware
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 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 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 SECURE_SSL_REDIRECT to True, so that requests over HTTP
    are redirected to HTTPS.

    Please note the caveats under 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 SESSION_COOKIE_SECURE and
    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 may either be configured with SECURE_HSTS_SECONDS,
    SECURE_HSTS_INCLUDE_SUBDOMAINS, and SECURE_HSTS_PRELOAD,
    or on the Web server.

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
ALLOWED_HOSTS setting in the
django.http.HttpRequest.get_host() method.

This validation only applies via 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 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 ALLOWED_HOSTS explicitly rather
than relying on web server configuration.

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

Session security¶

Similar to the CSRF limitations requiring a site to
be deployed such that untrusted users don’t have access to any subdomains,
django.contrib.sessions also has limitations. See the session
topic guide section on security
for details.

User-uploaded content¶

Note

Consider serving static files from a cloud service or 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 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 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.

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 user uploaded files.
  • 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 SECRET_KEY a secret.
  • It is a good idea to limit the accessibility of your caching system and
    database using a firewall.
  • Take a look at the Open Web Application Security Project (OWASP) Top 10
    list
    which identifies some common vulnerabilities in web applications. While
    Django has tools to address some of the issues, other issues must be
    accounted for in the design of your project.