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parcel-optimizer-csp

A Parcel 2.0+ plugin for generating and inserting a CSP based on a super simple JSON config.

It automatically calculates a hash for your inline scripts and appends that to the script-src in the CSP so that browsers will run it.

Why I made this:

We have an inline script for Xchart.com that registers global error handlers. We do this as inline script, because if there's an error with loading a JS file required to run the app we want to know about it.

Another common reason would be inline scripts inserted for analytics.

BUT we're using a Content Security Policy (CSP) to prevent malicious scripts from being injected in any way.

CSP's script-src is one of the main reasons to use a CSP. It specifies valid sources for JavaScript and so, can completely prevent XSS (Cross-site scripting).

But, one of the tricky things people quickly notice is that if you have an inline snippet in the HTML that you pass to Parcel, like this:

<script>
  console.log('I might be an analytics snippet or something')
</script>

This code will not run when you've specified a script-src unless you've listed: 'unsafe-inline' which... kind of negates the whole point of setting a script-src to begin with.

If you're using CSP to prevent injected scripts, you can either specify a nonce or a hash of all the allowed scripts. A nonce is supposed to be unique from the server for each render, that doesn't work so well if you are building a PWA or static site (JAM Stack) when you may not have a server rendering on each request.

The other option is to calculate and specify a hash of each inline script that is allows to run: That's the main reason this plugin exists.

Why is it an Optimizer plugin?

It's very important that this happens last because if there's anything else that modifies the contents between the opening and closing <script> the hash would be different. So using the "optimize" stage made sense.

How to use it

Create a .csprc config at the root of your directory. It should contain something like this:

script-src is the only one that gets processed at all. Any other keys you specify are simply used to generate the final CSP string.

{
  "script-src": ["'self'", "polyfill.io"],
  "object-src": "'none'",
  "base-uri": "'none'"
}

Then, you must tell parcel to use the plugin. So you create / edit your .parcelrc as follows:

{
  "extends": "@parcel/config-default",
  "optimizers": {
    "*.html": ["...", "parcel-optimizer-csp"]
  }
}

Note: the "..." is important it tells parcel to do all the other stuff it would normally to do optimize. This just tacks on our new plugin at the end.

If with that .csprc and .parcelrc when you use Parcel to build your project for production this plugin will inject prepend the following CSP into the <head> your final HTML:

<meta
  http-equiv="Content-Security-Policy"
  content="script-src 'self' polyfill.io 'sha256-uhzHJtYe3+WE2TVTxNYzdpz1n2axTAdgGXDltcHRQvc='; object-src 'none'; base-uri 'none';"
/>

Please note: certain CSP values need to be in single quotes. For example: 'self' and 'none'. This plugin does not attempt to quote them for you. It simply inserts them the way you've specified them.

Running tests

npm test

install

npm install parcel-optimizer-csp

Change log

  • 1.0.0: First public release.

credits

If you like this follow @HenrikJoreteg on twitter.

license

MIT