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

Compatibility

• v0.x: supports python 2.7 and 3.3, and Django < 3.0.
• v1.x: supports python 3.6+, and Django 1.11+.

Django is not required for use.

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]