Webpack plugin for enabling Subresource Integrity
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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.
- 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.)
``shell`
yarn add --dev webpack-subresource-integrity
`shell`
npm install webpack-subresource-integrity --save-dev
`javascript
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()],
});
`
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. Thecrossorigin attribute will be set as well, to the value ofoutput.crossOriginLoading webpack option. There is nothing else to
be done.
#### With HtmlWebpackPlugin({ inject: false })
When you use html-webpack-plugin with inject: false, you areintegrity
required to set the and crossorigin attributes in your
template as follows:
`ejs
<% for (let index in htmlWebpackPlugin.files.js) { %>
integrity="<%= htmlWebpackPlugin.files.jsIntegrity[index] %>"
crossorigin="<%= webpackConfig.output.crossOriginLoading %>"
>
<% } %>
<% for (let index in htmlWebpackPlugin.files.css) { %>
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 fromintegrity
the property of Webpack assets. For example:
`javascript`
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 andscript tags, you're also required to set the crossorigin
attribute. It is recommended to set this attribute to the same value
as the webpack configuration option.
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.
#### 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 Webpackproduction
mode is ornone 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.
valuesYou 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.
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] (withrealContentHash, which is enabled by default in production mode). Using[contenthash] with realContentHash disabled, or using a different type of[chunkhash]
hash placeholder (such as ) provides weaker guarantees, which is
why this plugin will output a warning in these cases. See issue
#162
for more information.
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.
This plugin adds the integrity attribute to
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 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?
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.
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
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
Copyright (c) 2015-present Waysact Pty Ltd
MIT (see LICENSE)
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