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html-safe-json

v2.0.3

Published

Secure JSON.stringify for injecting into HTML's `<script>` tag.

Downloads

1,465

Readme

html-safe-json

Secure JSON.stringify for injecting into HTML's <script> tag.

  • 📦 No dependencies - use it anywhere
  • 🚀 Fast - ~36% faster than htmlescape
  • 🛡️ Secure - fully tested and used in production

Links worth reading to understand the issue

What exactly does html-safe-json do:

It wraps JSON.stringify, exposing the same API to you but on the result it does some changes - it encodes 6 strings into unicode representation to prevent possible XSS attacks (or syntax errors in older browsers).

These strings are:

  • <script
  • </script
  • ]]>
  • -->
  • \u2028
  • \u2029

Usage

Documentation

You can find documentation here: https://ezez.dev/docs/html-safe-json/latest

API

Function API is identical to native JSON.stringify. It accepts value, replacer and space arguments and returns stringified result.

Real-world example

Usually you probably will use this library like that:

import { stringify } from "html-safe-json"; // or require

const badData = {
    a: "</script><script>alert(1)</script>",
};

const endpoint = (req, res) => {
    const html = `<html><body>
<script>window.data = ${stringify(badData)};</script>
<!-- init your scripts here -->
</body></html>`;
    res.send(html);
};

Demos

If you want to see a difference between html-safe-json and bare JSON.stringify in action you can clone repository of this project, install dependencies and run yarn start:dev script.

Then open 127.0.0.1:1337 in your browser for demos.

Alert message is expected - it demonstrates that JSON.stringify is unsafe, while html-safe-json is safe.

Bonus knowledge / other solutions

Next.js uses (as of v2) htmlescape library to secure data before embedding into HTML. It's more aggressive and encodes every < and > characters thus automatically resolves most of the issues. html-safe-json saves both output bytes and processing power (it's faster by ~36%) by replacing only what is needed.

Benchmark run steps:

  • install deps
  • yarn start:benchmark

html-safe-json is faster by 277 ms (586ms vs 863ms in total) difference: 38 %

Benchmark currently stringifies a string, this is intentional - comparing converting object to string is useless, because both libs uses JSON.stringify. What matters is how fast escaping is done.

License

MIT