Introduction
In today’s security climate, where supply chain attacks have become common and third-party code is everywhere, trust is no longer a default — it must be verified. That’s why modern security strategies, including PCI DSS v4.0 and Zero Trust Architecture, increasingly rely on mechanisms like Subresource Integrity (SRI).
SRI is a simple but powerful browser feature that protects users from tampered or compromised scripts — and yet, many websites still don’t use it.
In this blog, we’ll explain what Subresource Integrity is, how it works, why it’s critical in a Zero Trust model, and how Breachfin helps enforce it.
What Is Subresource Integrity (SRI)?
Subresource Integrity is a browser-side security feature that allows web developers to ensure that files (like scripts or stylesheets) loaded from a CDN or third-party domain haven’t been tampered with.
It works by attaching a cryptographic hash to the resource, like this:
<script src="https://cdn.example.com/library.js"
integrity="sha384-abc123..."
crossorigin="anonymous"></script>
The browser:
- Downloads the file
- Computes its hash
- Compares it to the provided
integrityvalue - Blocks the file if the hashes don’t match
Why It Matters in a Zero-Trust Environment
Zero Trust is a security model that assumes no component is inherently trustworthy, whether it’s a user, device, or third-party CDN.
That means:
- Even a popular CDN can be hijacked or misconfigured
- Even a trusted vendor can be compromised
- Even a script you didn’t write can break your security
If you’re loading JavaScript from anywhere other than your own domain — analytics, payment providers, chat widgets — you’re trusting external code to execute in your users’ browsers.
Without SRI, you’re blind to whether that code is what you expected.
Real-World Exploit Scenarios
- CDN Tampering: An attacker compromises a public CDN and injects a credit card skimmer into
library.min.js. Every site that references that file now unknowingly spreads malware. - DNS Hijacking: A man-in-the-middle attack swaps the returned file from a third-party source with malicious code.
- Supply Chain Attack: A vendor pushes a software update that includes an injected backdoor, and your site delivers it to all visitors instantly.
With SRI in place, the browser would block any file that doesn’t exactly match the expected hash.
Why So Many Sites Don’t Use It
Despite its power, SRI adoption remains limited because:
- Developers aren’t aware of it
- CDNs don’t always provide integrity hashes
- Maintaining hashes for changing files requires CI/CD automation
- Frameworks (like React or Vue) don’t enforce SRI by default
That’s where tools like Breachfin become critical.
How Breachfin Helps Enforce SRI Usage
Breachfin automatically scans your site for:
- External scripts and stylesheets
- Missing or malformed
integrityattributes - Mismatches between declared and actual script hashes
- Use of high-risk third-party domains without integrity enforcement
When SRI is missing or invalid, Breachfin flags it in the dashboard and can send webhook alerts to your security team or SIEM tool.
This not only hardens your client-side posture but also supports PCI DSS 6.4.3 and 11.6.1 compliance by validating script integrity at runtime.
Best Practices for SRI Implementation
- Always use SRI for third-party JS/CSS
- Use exact versioned URLs — avoid dynamically changing file paths
- Automate hash generation in your CI/CD pipeline
- Set
crossorigin="anonymous"to avoid CORS-related failures - Scan production sites regularly to detect broken or missing SRI entries
Final Thoughts
In a Zero-Trust world, verifying what you load is just as important as verifying who you trust.
SRI is not optional. It’s a low-cost, high-impact control that protects your users, your brand, and your compliance posture from silent supply chain attacks.
With Breachfin, you don’t have to guess whether SRI is being used correctly — you’ll know.
Scan your site today at breachfin.com and uncover missing or broken SRI protection before attackers do.
