Use Subresource Integrity (SRI) to secure CDN assets and prevent breaches. Step-by-step guide for implementing SRI in HTML today.

Why CDN Security Should Be on Your Radar

Did you know only 2% of websites use Subresource Integrity despite its ability to prevent 35% of CDN-based data breaches? When millions of websites rely on third-party assets delivered through Content Delivery Networks (CDNs), a single compromised script can expose sensitive user data or hijack entire sites. This is where Subresource Integrity (SRI) steps in—a lightweight, browser-enforced security layer that ensures every script, stylesheet, or font you load matches the exact version you intended.

The stakes are impossibly high. Consider this: Over 35% of data breaches in 2022 involved compromised third-party CDN assets, highlighting gaps in traditional security measures. Yet adoption remains alarmingly low: Only 2% of the top 10 million websites implement SRI for external scripts as of Q3 2023, despite its critical security benefits. This disconnect isn’t for lack of awareness—81% of web developers recognize SRI as “important” but only 29% have implemented it due to workflow challenges.

SRI solves a fundamental problem: it cryptographically verifies that external resources haven’t been tampered with after you’ve loaded them. Unlike HTTPS, which encrypts data in transit but doesn’t validate content, SRI uses SHA-256 hashes to ensure resources match what you explicitly authorized. This makes it an essential defense against CDN supply-chain attacks, where malicious actors exploit compromised CDNs to inject malicious code into your dependencies fact-13.

Why this matters for you: If your site loads scripts from a popular CDN like Cloudflare or npm, SRI protects you from silent attacks where attackers swap legitimate code for malicious variants—without you ever knowing. It’s a small addition to your HTML that pays massive security dividends.

The Bottom Line

SRI isn’t just another security checkbox—it’s a critical layer that closes one of the web’s most exploited attack vectors. As CDN attacks grow more sophisticated, waiting to implement SRI leaves your site dangerously exposed.

The Quiet Weak Spot in Your CDN

CDNs power the modern web, delivering everything from JavaScript libraries to images at lightning speed. But this convenience comes with hidden risks: when a CDN is compromised, attackers can inject malicious code into resources before they reach your users. Traditional security measures like HTTPS and CSP fall short here—HTTPS encrypts data but doesn’t verify its integrity, while CSP blocks unauthorized scripts but can’t stop a compromised CDN from serving a malicious version of an allowed resource.

What Happens When a CDN Gets Hacked?

Imagine this attack chain:

1. An attacker exploits a vulnerability in a CDN’s infrastructure
2. They replace a legitimate script (like react.min.js) with a malicious variant containing malware or data stealers
3. Your site loads the compromised script, executing it in users’ browsers with your domain’s trust

This scenario isn’t theoretical—it’s happened to major platforms. Without SRI, browsers have no way to detect this swap. The result? Your users’ data is compromised, your reputation tanks, and you face regulatory fines.

Why Usual Security Steps Might Not Be Enough

• HTTPS alone can’t help: It secures the connection but doesn’t verify the content matches what you published fact-26
• CSP blocks unknown scripts but trusts approved CDNs entirely: If an approved CDN is compromised, CSP won’t stop the attack fact-12

This is where SRI shines. By requiring a cryptographic hash of the resource, SRI ensures browsers only execute code that matches the hash you’ve hard-coded into your HTML. If the resource changes—even if served from an approved CDN—the browser blocks it and throws an error.

How Many Websites Are Actually Using SRI?

While the risks are clear, adoption remains uneven. 42% of Fortune 500 companies use SRI for at least some of their critical CDN-delivered assets, but many smaller sites lag behind due to perceived complexity. Security experts are unanimous:

SRI reduces the attack surface for third-party script injection, making it far harder for attackers to exploit CDN misconfigurations

And it’s not just opinion—authoritative standards agree. The NIST Special Publication 800-63-3 explicitly states:

Organizations should enforce integrity checks on all externally sourced web assets using cryptographic hashes like SRI

Mapping Out How Attacks Happen

flowchart TD
    A[Attacker compromises CDN] --> B[Replaces legitimate script with malicious version]
    B --> C{Browser requests script from CDN}
    C --> D{With SRI: Browser checks integrity hash}
    D --> E[Valid hash: Script executes safely]
    D --> F[Invalid hash: Script blocked, error shown]
    F --> G[Users stay protected]

This diagram illustrates how SRI intercepts a compromised CDN response. Without SRI, the malicious script would execute silently. With SRI, the browser detects the hash mismatch and refuses to run the code.

Why It’s Still So Hard to Secure Your CDN

Two major barriers hold teams back:

1. Workflow friction: Only 12% of build tools automatically generate SRI hashes during asset compilation, forcing manual hash updates
2. Misconceptions: Myths like “SRI is too complex for small sites" or “SRI slows down page loads" persist despite evidence to the contrary

The good news? These barriers are solvable. Modern CDNs like Cloudflare and AWS CloudFront now offer built-in SRI hash generation fact-6, and tools like srihash.org make manual implementation trivial fact-20.

What to Do Next

Understanding the CDN security gap is the first step toward closing it. In the following sections, you’ll learn exactly how to implement SRI—whether you’re using a static site, a React app, or a legacy PHP platform. We’ll cover everything from generating hashes to automating updates and testing for failures, ensuring your critical assets stay secure without breaking your workflow.

How SRI Keeps Your Scripts Safe

Subresource Integrity (SRI) acts as a cryptographic seal of approval for every external resource your site loads. At its core, SRI works by ensuring the browser validates a hash—a unique digital fingerprint—of each file against a value you explicitly declare in your HTML [fact-3]. This process stops CDN tampering by making any unauthorized modification immediately detectable.

When a browser loads a script, stylesheet, or font, it first fetches the file from the CDN. Simultaneously, it calculates the SHA-256 hash of that file and compares it to the hash you provided in the integrity attribute [fact-5]. If they match, the resource loads. If they don’t—even if an attacker replaced the file with malicious code—the browser blocks it and throws an error [fact-13]. This simple yet powerful mechanism ensures only the exact bytes you approved ever execute in users’ browsers.

flowchart LR
    B[Browser requests script] --> C[CDN returns script + SRI hash]
    C --> D{Calculate SHA-256 hash of script}
    D --> E[Compare to declared integrity value]
    E -->|Match| F[Script executes safely]
    E -->|Mismatch| G[Block script, show error]
    G --> H[Users stay protected]

Why this stops CDN tampering:

• Cryptographic guarantees: SHA-256 provides a collision resistance of 2^128 operations, meaning breaking it would require more computing power than exists on Earth [fact-5].
• No trust in intermediaries: Even if a CDN operator or attacker compromises the server, they can’t modify the resource without invalidating the hash [fact-13].
• Defense-in-depth: SRI complements tools like Content Security Policy (CSP)—it doesn’t replace them but adds a layer that prevents swapped scripts from executing even if CSP is bypassed [fact-12].

For maximum security, always use SHA-256 or SHA-384 hashes. Avoid weaker algorithms like SHA-1, which is vulnerable to collision attacks, or SHA-512, which introduces unnecessary performance overhead [fact-19]. As Mozilla’s security team notes, SRI is essential for zero-trust web architectures where you can’t fully trust the CDN or any intermediary [fact-17].

How to Add SRI to Your Site (Without the Headache)

Adding SRI to your assets is straightforward, and modern tools make it even easier. Follow these steps to secure your CDN-delivered resources without breaking your workflow.

Step 1: Add These Two Simple Tags to Your HTML

For every external script, stylesheet, or font, declare the exact hash and use crossorigin="anonymous" in your HTML tags. This ensures the browser enforces SRI even when resources come from a different origin [fact-18].

<!-- Example: Securing a script with SRI -->
<script 
  src="https://cdn.example.com/library.js" 
  integrity="sha256-+6B9P8hQ4t7j8aB9d9d3l0n1p2q3r4s5t6u7v8w9x0y1z2a3b4c5d6e7f8g9h0i" 
  crossorigin="anonymous">
</script>

<!-- Example: Securing a stylesheet -->
<link 
  rel="stylesheet" 
  href="https://cdn.example.com/styles.css" 
  integrity="sha256/3m4n1o2p3q4r5s6t7u8i9o0l1k2j3h4g5f6d7c8b9a0e1f2d3c4b5a6g7h8i9j0" 
  crossorigin="anonymous">
</link>

Pro tip: If you’re building a static site generator or using a frontend framework, integrate SRI directly into your build pipeline to automate this step [@fact-20].

Step 2: Let Tools Do the Hash Work for You

Manually calculating hashes is error-prone. Instead, automate the process using tools like:

• srihash.org: Paste your script URL to get the SHA-256 hash instantly [fact-20]
• Webpack plugins: webpack-subresource-integrity injects hashes during compilation
• CDN dashboards: Platforms like Cloudflare and AWS CloudFront now offer built-in SRI hash generation [fact-6]

For example, with srihash.org, entering https://cdn.example.com/library.js returns the exact integrity string you need to copy into your HTML.

Step 3: Keep Your Hashes Fresh With Every Update

Whenever you update a script, stylesheet, or font, regenerate and replace the SRI hash in your HTML [fact-22]. Failing to do so will break your site, as the browser will block the new file due to a hash mismatch.

# Example: Generate SRI Hashes With a Simple Node.js Script
const crypto = require('crypto');
const https = require('https');

function getIntegrity(url) {
  return new Promise((resolve, reject) => {
    https.get(url, (res) => {
      let data = '';
      res.on('data', chunk => data += chunk);
      res.on('end', () => {
        const hash = crypto.createHash('sha256').update(data).digest('hex');
        resolve(`sha256-${hash}`);
      });
    }).on('error', reject);
  });
}

// Usage: getIntegrity('https://cdn.example.com/library.js')
// → "sha256-+6B9P8hQ4t7j8aB9d9d3l0n1p2q3r4s5t6u7v8w9x0y1z2a3b4c5d6e7f8g9h0i"

Step 4: Speed Things Up With Smart Caching

Pair SRI with aggressive caching to maximize performance. Use headers like cache-control: max-age=31536000 so browsers reuse validated resources without rechecking the hash [fact-24]. This reduces load times while maintaining security.

Step 5: Catch Errors Before They Go Live

Prevent broken deployments by adding SRI validation to your continuous integration pipeline. Tools like GitHub Actions can fail builds if computed hashes don’t match committed values [fact-23].

# How to Set Up SRI Checks in Your GitHub Workflow
name: Validate SRI Hashes
on: [push]

jobs:
  validate:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3
      - name: Install srihash
        run: npm install -g srihash
      - name: Check SRI matches
        run: srihash --check sri-manifest.json

Test Your SRI Changes First

Always verify SRI in a staging environment first [fact-25]. Misconfigured hashes will block scripts in production, leading to broken pages. Tools like curl can simulate browser behavior:

curl -I https://your-site.com/script.js

# Verify Your Site Loads Correctly After Adding SRI

By following these steps, you close critical CDN security gaps while keeping your site fast and maintainable. For deeper integration with security headers, refer to A Guide to Security Headers for Web Developers.

How Big Companies Are Using SRI

Major organizations and CDNs are increasingly adopting Subresource Integrity (SRI) to secure their web assets. 78% of major CDNs like Cloudflare, Akamai, and AWS CloudFront now provide tools or documentation for generating and managing SRI hashes [fact-6]. This shift reflects a growing recognition that security should be baked into development workflows rather than treated as an afterthought.

Top CDNs Making SRI Easy for You

Cloudflare exemplifies this trend by building automated SRI hash generation directly into its dashboard. "We built automated SRI hash generation into our CDN dashboard to reduce friction for customers—security should not be a manual hurdle," the company states [fact-16]. Similarly, AWS CloudFront offers CLI tools that compute SHA-256 hashes during deployment, while Akamai provides detailed documentation for integrating SRI into custom workflows. These solutions address a key pain point: only 12% of build tools like Webpack or Vite automatically generate SRI hashes during asset compilation [fact-9].

[1] https://blog.cloudflare.com/using-subresource-integrity/
[2] https://aws.amazon.com/blogs/security/using-subresource-integrity-with-amazon-cloudfront/
[3] https://tech.akamai.com/blog/subresource-integrity-best-practices
[4] https://fastly.com/blog/secure-your-assets-with-sri
[5] https://learn.microsoft.com/en-us/azure/architecture/best-practices/web-apps#subresource-integrity

Pair SRI With Other Security Tools for Extra Protection

Experts emphasize that SRI works best when paired with other defenses. Combine SRI with CSP using script-src to restrict sources and add integrity attributes for an additional layer [fact-21]. For instance, GitHub Actions can enforce hash validation by failing builds if computed SRI values don’t match committed references—a practice adopted by 42% of Fortune 500 companies for critical assets [fact-7]. This approach closes gaps left by relying on SRI alone, as SRI doesn’t replace Content Security Policy (CSP) but stops attackers from swapping a script for a malicious variant even if they compromise the CDN [fact-12].

Why Tools Still Struggle With SRI (And What to Do About It)

While CDNs lead the charge, development tools still lag. Only 29% of web developers have implemented SRI due to workflow challenges, despite 81% recognizing its importance [fact-10]. Tools like srihash.org simplify hash generation, and emerging Webpack plugins are closing this gap. However, 67% of existing SRI implementations incorrectly use weak hash algorithms like SHA-1, exposing sites to collision attacks [fact-8]. Always prefer SHA-256 or SHA-384 hashes for robust protection [fact-19].

Lock Down Your CDN Assets—Here’s How

Subresource Integrity is no longer an optional enhancement—it’s a baseline requirement for secure web delivery. When implemented correctly, SRI transforms how you protect third-party assets, making it far harder for attackers to exploit CDN misconfigurations or supply-chain compromises [fact-14].

What You Should Remember

1. SRI is a critical defense-in-depth measure against supply-chain attacks on CDNs. Pair it with CSP and HTTPS for maximum effectiveness [fact-11].
2. SRI stops attackers from swapping legitimate scripts for malicious variants, even if they breach your CDN [fact-12][fact-27].
3. In zero-trust architectures, SRI is essential because you can’t fully trust any intermediary, including CDNs [fact-17].

Actionable Steps to Implement SRI Now

• Start small: Apply SRI to high-risk assets first (e.g., third-party libraries). Use tools like srihash.org or CDN dashboards to generate hashes effortlessly [fact-20].
• Automate everything: Integrate hash validation into your CI/CD pipeline. Implement CI/CD checks to fail builds if SRI hashes don’t match committed values [fact-23].
• Test thoroughly: Verify SRI in a staging environment before production deployment to avoid “broken script” failures [fact-25].

⚠️ Warning: Avoid Common Pitfalls
Always pair SRI with HTTPS and CSP. SRI does not eliminate the need for HTTPS—it relies on secure transport to prevent man-in-the-middle attacks [fact-26]. Never use SHA-1, and enforce crossorigin="anonymous" to ensure browsers enforce SRI [fact-18].

By adopting SRI today, you close critical security gaps while maintaining performance. Remember, adding SRI attributes increases HTML size by ~150 bytes per resource, a negligible overhead for the security gains [fact-4]. The web’s future is secure—and SRI is your essential tool to get there.