Subresource Integrity

Websites sometimes choose to rely on a third party such as a Content Delivery Network (CDN) to host some of their resources, rather than self-host all their resources. For example, a document served from https://example.com might include a resource from another location:

<script src="https://not-example.com/script.js"></script>

This comes with a risk, in that if an attacker gains control of the third-party host, the attacker can inject arbitrary malicious content into its files (or replace the files completely) and thus can also potentially attack sites that fetch files from it.

Subresource Integrity enables you to mitigate some risks of attacks such as this, by ensuring that the files your web application or web document fetches have been delivered without an attacker having injected any additional content into those files — and without any other changes of any kind at all having been made to those files.

You use the Subresource Integrity feature by specifying a base64-encoded cryptographic hash of a resource (file) you're telling the browser to fetch, in the value of the integrity attribute of a <script> element or a <link> element with rel="stylesheet", rel="preload", or rel="modulepreload".

An integrity value begins with at least one string, with each string including a prefix indicating a particular hash algorithm (currently the allowed prefixes are sha256, sha384, and sha512), followed by a dash, and ending with the actual base64-encoded hash.

Example integrity string with base64-encoded sha384 hash:

sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC

So oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC is the "hash" part, and the prefix sha384 indicates that it's a sha384 hash.

SRI Hash Generator

The SRI Hash Generator is an online tool you can use to generate SRI hashes.

Using OpenSSL

You can generate SRI hashes from the command-line using OpenSSL with a command invocation such as:

cat FILENAME.js | openssl dgst -sha384 -binary | openssl base64 -A

In a Windows environment, you can create a tool for generating SRI hashes with the following code:

@echo off
set bits=384
openssl dgst -sha%bits% -binary %1% | openssl base64 -A > tmp
set /p a= < tmp
del tmp
echo sha%bits%-%a%
pause

To use that code:

1. Save that code in a file named sri-hash.bat in the Windows SendTo folder in your environment (for example, C:\Users\USER\AppData\Roaming\Microsoft\Windows\SendTo).
2. Right-click a file in the File Explorer, select Send to…, and then select sri-hash. You will see the integrity value in a command box.
3. Select the integrity value and right-click to copy it to the Clipboard.
4. Press any key to dismiss the command box.

Using shasum

You can generate SRI hashes using shasum with a command invocation such as:

shasum -b -a 384 FILENAME.js | awk '{ print $1 }' | xxd -r -p | base64

• The pipe-through xxd step takes the hexadecimal output from shasum and converts it to binary.
• The pipe-through awk step is necessary because shasum will pass the hashed filename in its output to xxd. That can have disastrous consequences if the filename happens to have valid hex characters in it — because xxd will also decode that and pass it to base64.

For subresource-integrity verification of a resource served from an origin other than the document in which it's embedded, browsers additionally check the resource using Cross-Origin Resource Sharing (CORS), to ensure the origin serving the resource allows it to be shared with the requesting origin. Therefore, the resource must be served with an Access-Control-Allow-Origin header that allows the resource to be shared with the requesting origin; for example:

Access-Control-Allow-Origin: *

In the following examples, assume that oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC is already known to be the expected SHA-384 hash (digest) of a particular script example-framework.js, and there's a copy of the script hosted at https://example.com/example-framework.js.

You can use the following <script> element to tell a browser that before executing the https://example.com/example-framework.js script, the browser must first compare the script to the expected hash, and verify that there's a match.

<script
  src="https://example.com/example-framework.js"
  integrity="sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC"
  crossorigin="anonymous"></script>

Browsers handle SRI by doing the following:

1. When a browser encounters a <script> or <link> element with an integrity attribute, before executing the script or before applying any stylesheet specified by the <link> element, the browser must first compare the script or stylesheet to the expected hash given in the integrity value.

   For subresource-integrity verification of a resource served from an origin other than the document in which it's embedded, browsers additionally check the resource using Cross-Origin Resource Sharing (CORS), to ensure the origin serving the resource allows it to be shared with the requesting origin.

2. If the script or stylesheet doesn't match its associated integrity value, the browser must refuse to execute the script or apply the stylesheet, and must instead return a network error indicating that fetching of that script or stylesheet failed.

• Content Security Policy
• The Content-Security-Policy HTTP header.
• A CDN that can not XSS you: Using Subresource Integrity
• SRI Hash Generator