Subresource Integrity (SRI)

Subresource Integrity (SRI) is a browser feature that checks a fetched script or stylesheet against a hash you provide. If the downloaded file does not match, the browser blocks it.

Why it matters

Many pages load libraries from a CDN or other third party. If that host is compromised, every site using the file would run the attacker's version. SRI means a tampered file is rejected instead of executed, which limits supply-chain risk. It works best with versioned, immutable files, since any legitimate change alters the hash.

Example

<script src="https://cdn.example.com/lib.min.js"
  integrity="sha384-BASE64HASHHERE"
  crossorigin="anonymous"></script>

The integrity attribute holds the algorithm and base64 digest. The crossorigin attribute is needed so the browser can check cross-origin resources.

Common mistakes

Adding SRI to a URL whose content changes, such as a "latest" file, which then breaks the page.
Forgetting crossorigin, which makes the check fail.
Believing SRI protects scripts that load further scripts dynamically.
Copying a hash from an untrusted source instead of computing it from a known-good file.

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