Use subresource integrity (sri)

sri warns about requesting scripts or stylesheets without using
subresource integrity.

Why is this important?

A common practice in modern web development is to use third party
resources from CDNs or different services (analytics, ads, etc.).
However, doing so can increase the attack surface of your web site/app.

While there are techniques to verify the agent is talking with the
right server (TLS, HSTS, etc.), an attacker (or administrator) with
access to the server can manipulate the content with impunity.

> If you want to load a crypto miner on 1,000+ websites you don't
attack 1,000+ websites, you attack the 1 website that they all load
content from.
([Scott Helme][weak link])

Subresource integrity [is a standard][sri spec] that mitigates this
by ensuring that an exact representation of a resource, and only that
representation, loads and executes.

What does the hint check?

This hint checks that a website correctly uses SRI, more specifically:

* All the downloaded resources by an
`

Cross-origin resource with invalid crossorigin attribute:

`html

`

Cross-origin resource loaded over HTTP:

`html

`

Same-origin resource with no integrity and originCriteria set to all:

`html

`

$3

Cross-origin resource with multiple hashes and sha384 is one of them:

`html

`

Cross-origin resource with valid crossorigin attribute:

`html

`

Can the hint be configured?

Yes, you can define the baseline algorithm and the origin of the resources
to analyze.

$3

By default the baseline algorithm is sha384 but you can
change it to sha256, or sha512 by specifying that in the
[.hintrc][hintrc] file:

`json
{
"connector": {...},
"formatters": [...],
"hints": {
"sri": ["warning", {
"baseline": "sha512"
}],
...
},
...
}
`

The above will validate that the integrity of all scripts and
styles use sha512.

$3

By default, this hint will analyze only resources with a different origin.
To change this behavior you will have to set the originCriteria property
to one of the following:

* all: All resources will be analyzed
* crossOrigin: Only cross-origin resources will be analyzed

The following will analyze all the resources:

`json
{
"connector": {...},
"formatters": [...],
"hints": {
"sri": ["warning", {
"originCriteria": "all"
}],
...
},
...
}
`

How to use this hint?

This package is installed automatically by webhint:

`bash
npm install hint --save-dev
`

To use it, activate it via the [.hintrc][hintrc] configuration file:

`json
{
"connector": {...},
"formatters": [...],
"hints": {
"sri": "error",
...
},
"parsers": [...],
...
}
`

Note: The recommended way of running webhint is as a devDependency` of
your project.

Further Reading

* [Using Subresource Integrity - by Frederik Braun][using sri]
* [Subresource Integrity specification][sri spec]
* [Protect your site from Cryptojacking with CSP + SRI - by Scott Helme][prevent cryptojacking]
* [SRI Hash Generator][srihash generator]

[collisions]: https://w3c.github.io/webappsec-subresource-integrity/#hash-collision-attacks
[crossorigin]: https://w3c.github.io/webappsec-subresource-integrity/#is-response-eligible
[prevent cryptojacking]: https://scotthelme.co.uk/protect-site-from-cryptojacking-csp-sri/
[secure context]: https://w3c.github.io/webappsec-subresource-integrity/#non-secure-contexts
[hintrc]: https://webhint.io/docs/user-guide/configuring-webhint/summary/
[sri format]: https://w3c.github.io/webappsec-subresource-integrity/#resource-integrity
[sri spec]: https://w3c.github.io/webappsec-subresource-integrity/
[srihash generator]: https://www.srihash.org/
[using sri]: https://frederik-braun.com/using-subresource-integrity.html
[weak link]: https://scotthelme.co.uk/protect-site-from-cryptojacking-csp-sri/#theweaklink