Using Content Delivery Networks (CDNs) to host files such as scripts and stylesheets that are shared among multiple sites can improve site performance and conserve bandwidth. However, using CDNs also comes with a risk, in that if an attacker gains control of a CDN, the attacker can inject arbitrary malicious content into files on the CDN (or replace the files completely) and thus can also potentially attack all sites that fetch files from that CDN.
The Subresource Integrity feature enables you to mitigate the risk of attacks such as this, by ensuring that the files your Web application or Web document fetches (from a CDN or anywhere) have been delivered without a third-party having injected any additional content into those files — and without any other changes of any kind at all having been made to those files.
Using Subresource IntegrityEDIT
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 any <script> or <link> element.
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.
An integrity value may contain multiple hashes separated by whitespace. A resource will be loaded if it matches one of those hashes.
Example integrity string with base64-encoded sha384 hash:
sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC
An integrity value’s “hash” part is, strictly speaking, a cryptographic digest formed by applying a particular hash function to some input (for example, a script or stylesheet file). But it’s common to use the shorthand hash to mean cryptographic digest, so that’s what’s used in this article.
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