What are the integrity and crossorigin attributes?

Bootstrapcdn recently changed their links. It now looks like this:

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" 
rel="stylesheet" 
integrity="sha256-MfvZlkHCEqatNoGiOXveE8FIwMzZg4W85qfrfIFBfYc= sha512-dTfge/zgoMYpP7QbHy4gWMEGsbsdZeCXz7irItjcC3sPUFtf0kuFbDz/ixG7ArTxmDjLXDmezHubeNikyKGVyQ==" 
crossorigin="anonymous">

What do the integrity and crossorigin attributes mean? How do they affect the loading of the stylesheet?

jim.taylor.1974

Both attributes have been added to Bootstrap CDN to implement Subresource Integrity.

Subresource Integrity defines a mechanism by which user agents may verify that a fetched resource has been delivered without unexpected manipulation Reference

Integrity attribute is to allow the browser to check the file source to ensure that the code is never loaded if the source has been manipulated.

Crossorigin attribute is present when a request is loaded using 'CORS' which is now a requirement of SRI checking when not loaded from the 'same-origin'. More info on crossorigin

More detail on Bootstrap CDNs implementation

Witold Kaczurba

integrity - defines the hash value of a resource (like a checksum) that has to be matched to make browser execute it. The hash ensures that file was unmodified and contains expected data. This way browser will not load different (e.g. malicious) resource. Imagine situation in which your JavaScript files were hacked on the CDN, and there was no way of knowing it. Integrity attribute prevents loading content that does not match.

Invalid SRI will be blocked (Chrome developer-tools), regardless of cross-origin. Below NON-CORS case when integrity attribute does not match:

Integrity can be calculated using: https://www.srihash.org/ Or typing into console (link):

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

crossorigin - defines options used when the resource is loaded from a server on different origin. (See CORS here: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS). It effectively changes HTTP request sent by browser. If “crossorigin” attribute is added - it will result in adding origin: <ORIGIN> key-value pair into HTTP request as shown below.

crossorigin can be set to either: “anonymous” or “use-credentials”. Both will result in adding origin: into the request. The latter however will ensure that credentials are checked. No crossorigin attribute in the tag will result in sending request without origin: key-value pair.

Here is a case when requesting “use-credentials” from CDN:

<script 
        src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/js/bootstrap.min.js"
        integrity="sha384-vBWWzlZJ8ea9aCX4pEW3rVHjgjt7zpkNpZk+02D9phzyeVkE+jo0ieGizqPLForn" 
        crossorigin="use-credentials"></script>

A browser can cancel the request if crossorigin incorrectly set.