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Escape JSON strings for safe execution as literal javascript and inclusion in HTML <script> environments

Project description

==============================================
escapejson function and django template filter
==============================================

**JSON is not javascript.** Many developers erroneously think that they can
just place the output of ``json.dumps(obj)`` inside ``<script>`` tags and be
good to go -- but this is dangerously vulnerable to cross-site scripting
attacks from 2 important edge cases for how JSON differs from javscript: (1)
the handling of a literal ``</script>`` within script blocks, and (2) the
behavior of two pesky unicode whitespace characters.

This very simple library provides a function ``escapejson``, and a Django
template filter of the same name. The output of ``escapejson`` should be safe
for inclusion in HTML ``<script>`` tags, and interpretation directly as
javascript.

NOTE: this escaping is only "safe" if the input is a syntactically valid JSON
string. The output is NOT safe if you pass it invalid JSON, whether from
untrusted JSON input or from a broken encoder. This library does not validate
the correctness of the JSON it is fed. Always use a conformant JSON encoder
(e.g. ``json.dumps``) to ensure that the JSON is valid to start with.

Installation
============

::

pip install escapejson

Supports python 2.7 and 3.3+. Supports Django 1.7, 1.8, 1.9, 1.10. (Django not required).

Usage
=====

Example API usage (with or without Django)
-----------------------------------------
::

import json
from escapejson import escapejson

my_obj = {'message': '</script><script>alert("oh no!")</script>'}
my_str = json.dumps(myobj)
my_safe_str = escapejson(my_str)

Example Django templates usage
------------------------------

First, add ``"escapejson"`` to ``INSTALLED_APPS`` in your project's ``settings.py``.::

# settings.py
INSTALLED_APPS = [
...,
"escapejson",
...,
]


Then, use the ``escapejson`` library and filter::

{% load escapejson %}

<script>
var my_obj = {{obj_or_str|escapejson}};
</script>

This filter will attempt to JSON-encode any non-string object that is passed to it before
escaping, or just escape any string that is passed to it.


What it protects against
========================

</script> attacks
-----------------

Any string containing a literal ``</script>`` inside javascript within HTML
script tags will be interpreted by modern browsers as closing the script tag,
resulting at best in broken scripts and syntax errors, and at worst in
full-blown XSS. By escaping all ``/`` characters as ``\/`` (a valid optional
escape in the JSON spec), this is mitigated.

U+2028 and U+2029
-----------------

Two funky unicode whitespace characters count as valid JSON, but cause syntax
errors in javascript. This is mitigated by replacing the literal characters
with the strings ``\u2028`` and ``\u2029``.
[`reference <http://timelessrepo.com/json-isnt-a-javascript-subset/>`_]

Project details


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