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.

Upgrading from version 1.x? Read the migration guide.

Features

• Optional integration with html-webpack-plugin.
• Automatic support for dynamic imports (also known as code splitting.)
• Compatible with Webpack 5 (for Webpack versions 1-4 see 1.x branch.)

Installation

yarn add --dev webpack-subresource-integrity

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

Recommended Webpack Configuration

import { SubresourceIntegrityPlugin } from "webpack-subresource-integrity";
// or: const { SubresourceIntegrityPlugin } = require('webpack-subresource-integrity');

const compiler = webpack({
  output: {
    // the following setting is required for SRI to work:
    crossOriginLoading: "anonymous",
  },
  plugins: [new SubresourceIntegrityPlugin()],
});

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

<% for (let 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. For example:

compiler.plugin("done", (stats) => {
  const integrityValues = stats
    .toJson()
    .assets.map((asset) => [asset.name, asset.integrity]);
});

Note that when you add the integrity attribute on your link and script tags, 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.

When using caching, stale assets will fail to load since they will not pass integrity checks. It is vital that you configure caching correctly in your web server. See below for more information.

Options

hashFuncNames

Default value: ["sha384"]

An array of strings, each specifying the name of a hash function to be used for calculating integrity hash values.

See SRI: Cryptographic hash functions

The default is chosen based on the current suggestion by the W3C which reads:

At the time of writing, SHA-384 is a good baseline.

See here for additional information on why SHA-384 was chosen by the W3C over their initial suggestion, SHA-256.

As one of the commenters in that discussion points out, "SRI hashes are likely delivered over SSL" which today (2021) is often using SHA-256 so that there is probably little harm in downgrading this to sha256 instead.

By using SHA-256 you will save 21 bytes per chunk and perhaps a few CPU cycles, although SHA-384 is actually faster to compute on some hardware. Not that it matters, as the difference is dwarfed by all the other work a browser has to do in order to download and parse a JS or CSS asset.

You probably want to use sha512 instead of the default only if you're paranoid. It will cost you an additional 21 bytes per chunk; the CPU overhead is virtually nil because SHA-512 is the same as SHA-384, just not truncated.

Although you can specify multiple hash functions here, doing so is pointless as long as all mainstream browsers only support the SHA-2 family, which is the case today. Worse, it's detrimental since it adds unnecessary overhead.

The reason is that as per the spec, only the strongest hash function is used and so eg. ['sha256', 'sha512'] is equivalent to ['sha512'] unless SHA-512 was one day deemed weaker than SHA-256 by user agents, which is an unlikely scenario. As one of the authors of the W3C spec puts it:

The support for multiple hashes is in the spec for backward-compatibility once we introduce new hash algorithms (e.g. SHA3).

enabled

Default value: "auto"

One of "auto", true, or false.

true means to enable the plugin and false means to disable it.

auto is the default and means to enable the plugin when the Webpack mode is production or none and disable it when it is development.

hashLoading

Default value: "eager"

One of "eager" or "lazy"

"eager" means that integrity hashes for all assets will be defined in the entry chunk.

"lazy" means that integrity hashes for any given asset will be defined in its direct parents in the chunk graph. This can lead to duplication of hashes across assets, but can significantly reduce the size of your entry chunk(s) if you have a large number of async chunks.

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.

Example usage with webpack-assets-manifest.

Caveats

Caching

Using SRI presents a potential risk to the availability of your website when HTTP response caching is setup incorrectly. Stale asset versions are always problematic but SRI exacerbates the risk.

Without SRI, inconsequential changes (such as whitespace-only changes) don't matter, and your website might still look OK when a stale CSS asset is used. Even with a stale JS asset there's a chance your website will still be more or less working as expected.

With SRI, however, a stale asset will fail hard. This is because the browser won't tell the difference between a version of your asset that has been tampered with and one that is simply outdated: both will fail the integrity check and refuse to load.

It's therefore imperative that, if you do use caching, you use a robust setup: one where any change in content, no matter how miniscule or inconsequential, will cause the cache to be invalidated.

With Webpack and long-term caching this means using [contenthash] (with realContentHash, which is enabled by default in production mode). Using [contenthash] with realContentHash disabled, or using a different type of hash placeholder (such as [chunkhash]) provides weaker guarantees, which is why this plugin will output a warning in these cases. See issue #162 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.

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.

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 13 (and earlier versions) and Assets that Require Cookies

As detailed in Webpack Issue #6972, the crossOrigin attribute can break loading of assets in Safari versions prior to 14 in certain edge cases due to a browser bug. 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. The fix for this issue landed in Safari Technology Preview 106 and was released with Safari 14

Further Reading

• MDN: Subresource Integrity

License

Copyright (c) 2015-present Waysact Pty Ltd

MIT (see LICENSE)