webpack-subresource-integrity

Webpack plugin for enabling Subresource Integrity.

Subresource Integrity (SRI) is a security feature that enables browsers to verify that files they fetch (for example, from a CDN) are delivered without unexpected manipulation.

Features

• Optional integration with html-webpack-plugin.
• Automatic support for dynamic imports (also known as code splitting.)
• Compatible with all major Webpack versions, up to and including Webpack 5.

Installation

npm install webpack-subresource-integrity --save-dev

yarn add --dev webpack-subresource-integrity

Webpack Configuration Example

import SriPlugin from 'webpack-subresource-integrity';

const compiler = webpack({
    output: {
        crossOriginLoading: 'anonymous',
    },
    plugins: [
        new SriPlugin({
            hashFuncNames: ['sha256', 'sha384'],
            enabled: process.env.NODE_ENV === 'production',
        }),
    ],
});

Setting the integrity attribute for top-level assets

For the plugin to take effect it is essential that you set the integrity attribute for top-level assets (i.e. assets loaded by your HTML pages.)

With HtmlWebpackPlugin

When html-webpack-plugin is injecting assets into the template (the default), the integrity attribute will be set automatically. The crossorigin attribute will be set as well, to the value of output.crossOriginLoading webpack option. There is nothing else to be done.

With HtmlWebpackPlugin({ inject: false })

When you use html-webpack-plugin with inject: false, you are required to set the integrity and crossorigin attributes in your template as follows:

<% for (var index in htmlWebpackPlugin.files.js) { %>
  <script
     src="<%= htmlWebpackPlugin.files.js[index] %>"
     integrity="<%= htmlWebpackPlugin.files.jsIntegrity[index] %>"
     crossorigin="<%= webpackConfig.output.crossOriginLoading %>"
  ></script>
<% } %>

<% for (var index in htmlWebpackPlugin.files.css) { %>
  <link
     rel="stylesheet"
     href="<%= htmlWebpackPlugin.files.css[index] %>"
     integrity="<%= htmlWebpackPlugin.files.cssIntegrity[index] %>"
     crossorigin="<%= webpackConfig.output.crossOriginLoading %>"
  />
<% } %>

Without HtmlWebpackPlugin

The correct value for the integrity attribute can be retrieved from the integrity property of Webpack assets.

Note that with Webpack versions before 5, that property is not copied over by Webpack's stats module so you'll have to access the "original" asset on the compilation object. For example:

compiler.plugin("done", stats => {
    const mainAssetName = stats.toJson().assetsByChunkName.main;
    const integrity = stats.compilation.assets[mainAssetName].integrity;
});

Note that you're also required to set the crossorigin attribute. It is recommended to set this attribute to the same value as the webpack output.crossOriginLoading configuration option.

Web Server Configuration

If your page can be loaded through plain HTTP (as opposed to HTTPS), you must set the Cache-Control: no-transform response header or your page will break when assets are loaded through a transforming proxy. See below for more information.

Content Security Policy

Consider adding the following rule to your CSP file:

Content-Security-Policy: require-sri-for script style;

It ensures that if, for some reason, this plugin fails to add integrity attributes to all your assets, your page will fail to load rather than load with unverified assets.

require-sri-for has never officially shipped in Chrome or Firefox, and both appear to be leaning towards removing their implementations.

Options

hashFuncNames

Required option, no default value.

An array of strings, each specifying the name of a hash function to be used for calculating integrity hash values. For example, ['sha256', 'sha512'].

See SRI: Cryptographic hash functions

enabled

Default value: true

When this value is falsy, the plugin doesn't run and no integrity values are calculated. It is recommended to disable the plugin in development mode.

Exporting integrity values

You might want to export generated integrity hashes, perhaps for use with SSR. We recommend webpack-assets-manifest for this purpose. When configured with option integrity: true it will include the hashes generated by this plugin in the manifest (requires webpack-assets-manifest version >= 3 which in turn requires Webpack >= 4).

Example usage with webpack-assets-manifest.

Caveats

Preloading

This plugin adds the integrity attribute to <link rel="preload"> tags, but preloading with SRI doesn't work as expected in current Chrome versions. The resource will be loaded twice, defeating the purpose of preloading. This problem doesn't appear to exist in Firefox or Safari. See issue #111 for more information.

Proxies

By its very nature, SRI can cause your page to break when assets are modified by a proxy. This is because SRI doesn't distinguish between malicious and benevolent modifications: any modification will prevent an asset from being loaded.

Notably, this issue can arise when your page is loaded through Chrome Data Saver.

This is only a problem when your page can be loaded with plain HTTP, since proxies are incapable of modifying encrypted HTTPS responses.

Presumably, you're looking to use SRI because you're concerned about security and thus your page is only served through HTTPS anyway. However, if you really need to use SRI and HTTP together, you should set the Cache-Control: no-transform response header. This will instruct all well-behaved proxies (including Chrome Data Saver) to refrain from modifying the assets.

Browser support

Browser support for SRI is widely implemented. Your page will still work on browsers without support for SRI, but subresources won't be protected from tampering.

See Can I use Subresource Integrity?

Hot Reloading

This plugin can interfere with hot reloading and therefore should be disabled when using tools such as webpack-dev-server. This shouldn't be a problem because hot reloading is usually used only in development mode where SRI is not normally needed.

For testing SRI without setting up a full-blown web server, consider using a tool such as http-server.

Safari and Assets that Require Cookies

As detailed in Webpack Issue #6972, the crossOrigin attribute can break loading of assets in certain edge cases due to a bug in Safari. Since SRI requires the crossOrigin attribute to be set, you may run into this case even when source URL is same-origin with respect to the asset.

Further Reading

• MDN: Subresource Integrity

License

Copyright (c) 2015-present Waysact Pty Ltd

MIT (see LICENSE)