Compute sha256 of CSP <script/> in NodeJS

Automatically generate the SHA-256 hash of the script tags for the Content Security Policy

Sep 17, 2022

#javascript #nodejs #security #webdev


Kim Davies

I have been using the same strategy to compute automatically SHA-256 hash for my <script /> tags since a couple of years and as I recently applied it again at work and in Papyrs (I am currently migrating the kit that generate the blogspaces from vanilla JavaScript to Astro), I thought it could be an interesting subject for a new short blog post.

Getting started

While sha256 can be set manually, I personally rather like to generate these automatically once the build script is over - as a post-processing job. This means that I rely on the fact that what is built is trustable and that I compute the values once the scripts have been generated.

To do so, I create a dedicated script - e.g. ./build-csp.mjs - that I append to my build script:

// package.json { "scripts": { "build:csp": "node build-csp.mjs", "build": "astro build && npm run build:csp" } }

Search and update

As I aim to inject the shas at the end of the process, I add a placeholder within the Content Security Policy (CSP) to search and update the computed values. e.g. in following CSP meta tag I used the placeholder {{EXTRA\_SHAS}}:

<meta http-equiv="Content-Security-Policy" content="default-src 'none'; script-src 'self' {{EXTRA_SHAS}}; />

Because such a placeholder is not a valid policy, the browser would throw an error while evaluating the rules. What can notably happen when I develop since I post process only the production build.

That is why I often just avoid the use of CSP for my local environment. e.g. in my Astro project I skip the rendering of the meta tag if not a PROD build.

--- const csp = import.meta.env.PROD; --- { csp && ( <meta http-equiv="Content-Security-Policy" content="default-src 'none'; script-src 'self' {{EXTRA_SHAS}}; /> ) }

List all HTML files

To compute the sha256 sha for the content of the <script /> that are used in the HTML files I first need to, well, list the HTML files that have been built.

For such purpose, I use a recursive exploration function that I most probably found on Stackoverflow and keep using since then.

import { readdirSync, lstatSync } from "fs"; import { join } from "path"; export const findEntryPoints = (dir, files) => { readdirSync(dir).forEach((file) => { const fullPath = join(dir, file); if (lstatSync(fullPath).isDirectory()) { findEntryPoints(fullPath, files); } else { files.push(fullPath); } }); }; const entryPoints = []; findEntryPoints("dist", entryPoints);

Once I got all the files, I filter those that are .html.

import { extname } from "path"; const htmlEntryPoints = entryPoints.filter((entry) => [".html"].includes(extname(entry)) );

Process and update CSP

Once I got all the target files, I batch update these by reading their content, generating the related shas and ultimately update the rules.

import { join } from "path"; import { readFile } from "fs/promises"; const updateCSP = async (entry) => { const indexHtml = await readFile(join(process.cwd(), entry), "utf-8"); const scriptHashes = await computeHashes(indexHtml); await writeCSP({ scriptHashes, indexHtml, entry }); }; const promises =; await Promise.all(promises);

Compute sha256

To create the hashes for the content of the <script /> tags, I use a regex to find those tags and use the Crypto API of NodeJS to effectively compute the values.

import { createHash } from "crypto"; const computeHashes = (indexHtml) => { const sw = /<script[\s\S]*?>([\s\S]*?)<\/script>/gm; const scriptHashes = []; let m; while ((m = sw.exec(indexHtml))) { const content = m[1]; scriptHashes.push( `'sha256-${createHash("sha256") .update(content).digest("base64")}'` ); } return scriptHashes; };

Write results

Finally, I replace the placeholder I declared earlier with these effective shas.

import { writeFile } from "fs/promises"; const writeCSP = async ({ scriptHashes, indexHtml, entry }) => writeFile( entry, indexHtml.replace( "{{EXTRA_SHAS}}", => sha256).join(" ") ), "utf-8" );

And voilà 🥳


In life it is sometimes the small things that make you happy and developing this little helper script is one of those.

To infinity and beyond