What is the difference between Content Security Policy and Subresource Integrity in JavaScript security?

Why This Matters for Security Teams

Content Security Policy, or CSP, and subresource integrity, or SRI, solve different failure modes in JavaScript delivery. CSP limits where the browser may load and execute scripts from, while SRI verifies that a fetched asset matches an expected cryptographic hash. That distinction matters because modern applications often depend on CDNs, tag managers, and third-party libraries that expand the browser trust boundary far beyond the origin server.

Security teams often assume one control can substitute for the other, but that is rarely true. A strict CSP can block unexpected sources, yet it does not prove that an allowed file was not tampered with. SRI can detect alteration, yet it does not stop a page from loading a malicious script from an approved but compromised source. NIST Cybersecurity Framework 2.0 reinforces the need to manage preventive and detective controls together rather than as isolated checks, and NHIMG’s Top 10 NHI Issues shows how hidden third-party dependencies often create security blind spots that teams do not see until incident response begins.

In practice, many security teams encounter script trust failures only after a supply-chain compromise or unsafe front-end dependency has already reached production.

How It Works in Practice

CSP works at the policy layer. The browser evaluates a page’s declared rules and decides whether a script may load or execute based on source, nonce, hash, and related directives. This makes CSP especially useful for reducing injection risk, including many cross-site scripting scenarios. SRI works at the content layer. The browser fetches a file, computes its hash, and executes it only if the file still matches the published integrity value. That makes SRI useful for protecting against tampering in transit or at the source.

In real deployments, these controls are complementary. CSP is the broader gatekeeper, while SRI is the file-level authenticity check. Current guidance from the NIST Cybersecurity Framework 2.0 supports layered controls that reduce exposure across the application lifecycle rather than relying on one browser feature. For external libraries, teams often pin versions, host critical assets themselves, and add SRI to immutable files. For dynamic script patterns, CSP nonces or hashes can be more practical than broad allowlists. NHIMG’s Ultimate Guide to NHIs and lifecycle processes is a useful reminder that unmanaged dependencies become governance problems, not just technical ones.

• Use CSP to restrict script origins, inline execution, and unsafe browser behavior.
• Use SRI for third-party or CDN-hosted static assets that should not change silently.
• Prefer nonces or hashes over broad source allowlists when application design allows it.
• Combine both for higher assurance: CSP reduces where code can come from, SRI verifies what was delivered.

These controls tend to break down in highly dynamic front ends, because frequently changing bundles, runtime script injection, and vendor-managed tag managers make stable hashes and tight policies difficult to maintain.

Common Variations and Edge Cases

Tighter browser controls often increase operational overhead, requiring organisations to balance protection against deployment complexity and breakage risk. That tradeoff is real in applications that use A/B testing, analytics tags, micro-frontends, or rotating build artifacts. There is no universal standard for when SRI should be mandatory versus advisory, but best practice is evolving toward requiring it for stable third-party scripts and high-value dependencies.

Some teams also overestimate what CSP can do. A permissive policy that includes broad wildcards, unsafe-inline, or unreviewed domains may provide only limited protection. Likewise, SRI does not help if the compromised file is published with the expected hash after a build pipeline is poisoned, or if the application loads scripts dynamically in ways that bypass the intended integrity check. For supply-chain exposure, NHIMG’s Shai Hulud npm malware campaign and ASP.NET machine keys RCE attack illustrate how trusted tooling and hidden dependencies can become the attack path. The practical answer is to set clear policy per asset class, document exceptions, and review them continuously instead of treating either control as a one-time header change.

Related resources from NHI Mgmt Group

• What is the difference between provenance and integrity in container security?
• What is the difference between content filtering and intent security for AI agents?
• What is the difference between provenance and Subresource Integrity?
• What is the difference between having a security policy and enforcing a security control?