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Best Practices

This page lists several best practices for writing secure web applications with playdoh.

|safe considered harmful

Using something like mystring|safe in a template will prevent Jinja2 from auto-escaping it. Sadly, this requires us to be really sure that mystring is not raw, user-entered data. Otherwise we introduce an XSS vulnerability.

We have therefore eliminated the need for |safe for localized strings. This works:

{{ _('Hello <strong>world</strong>!') }}

String interpolation

When you interpolate data into such a string, however, the resulting output will lose its "safeness" and be escaped again. To mark the localized part of an interpolated string as safe, do not use |f(...)|safe. The data could be unsafe. Instead, use the helper |fe(...). It will escape all its arguments before doing string interpolation, then return HTML that's safe to use:

{{ _('Welcome back, <strong>{username}</strong>!')|fe(username=user.display_name) }}

|f(...)|safe is to be considered unsafe and should not pass code review.

If you interpolate into a base string that does not contain HTML, you may keep on using |f(...) without |safe, of course, as the auto-escaping won't harm anything:

{{ _('Author name: {author}')|f(author=user.display_name) }}

Form fields

Jinja2, unlike Django templates, by default does not consider Django forms "safe" to display. Thus, you'd use something like {{ form.myfield|safe }}.

In order to minimize the use of |safe (and thus possible unsafe uses of it), playdoh monkey-patches the Django forms framework so that form fields' HTML representations are considered safe by Jinja2 as well. Therefore, the following works as expected:

{{ form.myfield }}

Mmmmh, Cookies

Django's default way of setting a cookie is set_cookie on the HTTP response. Unfortunately, both secure cookies (i.e., HTTPS-only) and httponly (i.e., cookies not readable by JavaScript, if the browser supports it) are disabled by default.

To be secure by default, we use commonware's cookies app. It makes secure and httponly cookies the default, unless specifically requested otherwise.

To disable either of these patches, set COOKIES_SECURE = False or COOKIES_HTTPONLY = False in settings.py.

You can exempt any cookie by passing secure=False or httponly=False to the set_cookie call, respectively:

response.set_cookie('hello', value='world', secure=False, httponly=False)

Content Security Policy (CSP) compliance

Content Security Policy is a web-security-related proposal put forward by Mozilla Security. It is currently a W3C working draft.

Its primary goal is to mitigate cross-site scripting (XSS) risk by enforcing certain policies on a web site.

While the proposal is still subject to change there are certain best practices that should be implemented today, because they prepare web applications for CSP compliance and should already be considered standard practices, even if a project does not enforce CSP yet:

Avoid inline CSS and JS

Avoid the use of inline CSS and JavaScript inside your HTML documents. This includes:

Replace the following methods of writing CSS with a separate file:

  • <style> elements
  • style attributes on HTML elements

Replace the following methods of writing JavaScript with a separate file:

  • <script> elements that wrap inline code
  • javascript: URIs
  • event-handling HTML attributes (like onclick)

Do not create code from strings

In JavaScript, never create code from strings, including calls to:

  • eval()
  • new Function() constructor
  • setTimeout() called with a non-callable argument
  • setInterval() called with a non-callable argument

CSRF-protect your forms

Django comes with a built-in, cookie-based CSRF protection facility. Sadly, the integrity of cookies can be compromised under certain circumstances (through Flash, or across subdomains on the same domain), so we replaced the CSRF method with a session-based method (as is common across web frameworks).

To CSRF-protect a form for logged-in users, just add this to your template, inside the <form> tag:

{{ csrf() }}

To make this work for anonymous users, with a light-weight session stored in Django's cache, decorate a view with @anonymous_csrf:

from session_csrf import anonymous_csrf

@anonymous_csrf
def login(request):
    ...

If a form is supposed to be CSRF-protected for logged-in users, but not for anonymous users, use the @anonymous_csrf_exempt decorator:

from session_csrf import anonymous_csrf_exempt

@anonymous_csrf_exempt
def protected_in_another_way(request):
    ...

Finally, to disable CSRF protection on a form altogether (if you know what you're doing!), Django's csrf_exempt decorator still works as expected.

To learn more about this method, refer to the django-session-csrf README.

CEF (Common Event Format) logging

Playdoh is set up for Common Event Format (CEF) logging. CEF is a unified logging format for security-relevant events and can be used by ArcSight and similar applications.

For example, to log a user resetting their password, you would do something like this:

import logging
from funfactory.log import log_cef

def pw_reset(request, user):
    log_cef('Password Reset', logging.INFO, request, username=user.username,
            signature='PASSWORDRESET', msg='User requested password reset')

For more information about logging and suggestions on what kinds of events to log, refer to the Mozilla Security Wiki.

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