Subresource Integrity is a security feature enabling browsers to verify that resources they fetch are delivered without unexpected manipulation. It works by providing a cryptographic hash that a fetched resource must match. This is particularly important for content delivered via CDNs or other third-party hosts.
Subresource Integrity (SRI) is a browser feature that lets you pin a script or stylesheet to a cryptographic hash. You add an integrity attribute containing the expected hash to the script or link tag; the browser downloads the resource, hashes it, and refuses to run it if the hash doesn't match. It guarantees that a file loaded from a CDN or third party is byte-for-byte the version you approved.
SRI is excellent against a tampered static file: if an attacker modifies a hosted library, the hash breaks and the browser blocks it. But it only works for resources that don't change. Many third-party scripts are designed to update frequently or are generated per request, and you cannot pin a hash to a moving target. SRI also does nothing once a script legitimately loads and then behaves maliciously at runtime.
SRI is a strong first layer for the static dependencies you can hash, and PCI DSS references it as one acceptable technique. For the dynamic scripts SRI can't cover, cside adds runtime inspection, analysing what each script does on the page and blocking malicious behaviour, so both your pinned and your unpinnable dependencies are accounted for.
Because SRI requires a fixed hash, and many third-party scripts change on every load or update without warning. Pinning a hash to them would break the script the moment the vendor ships a new version. SRI fits static, versioned files; it doesn't fit dynamic ones.
SRI can satisfy the integrity part of the requirement for static scripts, but PCI DSS 6.4.3 and 11.6.1 also expect you to inventory scripts and monitor for unauthorised changes across all of them, including the dynamic ones SRI cannot pin. It is one tool in the set, not the whole answer.
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