PCI DSS 4.0 Requirements: CSP Headers, SRI Checks, and Third-Party JavaScript Security

With the introduction of PCI DSS 4.0, businesses handling payment card data must adhere to stricter security measures to protect sensitive customer information. Among these security enhancements, Content Security Policy (CSP) headers, Subresource Integrity (SRI) checks, and third-party JavaScript validation have gained prominence as critical requirements to prevent data injection attacks, unauthorized content modifications, and ensure the integrity of web resources.

This blog explores these security controls and their role in maintaining PCI DSS 4.0 compliance.

Understanding Content Security Policy (CSP) Headers

What is CSP?

Content Security Policy (CSP) is an additional security layer that helps mitigate Cross-Site Scripting (XSS), Clickjacking, and data injection attacks by restricting the sources from which scripts, styles, and other resources can be loaded.

PCI DSS 4.0 Requirement for CSP Headers

Under PCI DSS 4.0, merchants handling payment card data via web applications must:

Implement a strong CSP header to restrict the execution of untrusted scripts.
Define approved domains from which scripts, styles, and images can be loaded.
Prevent inline script execution unless explicitly authorized via CSP nonce or hash.
Enable reporting mechanisms to monitor and log CSP violations for security audits.

How to Implement CSP Headers?

A secure CSP policy can be configured in the web server settings or within the application itself. Example of a CSP header:

Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted-scripts.com; style-src 'self' 'unsafe-inline'; img-src 'self' data:

This configuration:

Allows scripts only from the same origin and trusted-scripts.com.
Restricts styles to same origin and allows inline styles (with caution).
Permits images from the same origin and data URLs.

To strengthen security, unsafe-inline should be avoided whenever possible, and nonce or hash attributes should be used for inline scripts.

Understanding Subresource Integrity (SRI) Checks

What is SRI?

Subresource Integrity (SRI) is a security feature that ensures externally loaded resources (e.g., JavaScript, CSS) have not been tampered with by verifying their cryptographic hash. This prevents attackers from injecting malicious scripts into web pages through compromised third-party resources.

PCI DSS 4.0 Requirement for SRI Checks

Organizations using externally hosted resources must:

Implement SRI attributes on externally loaded scripts and stylesheets.
Ensure integrity verification of third-party resources before execution.
Enforce HTTPS connections to load third-party scripts securely.
Periodically audit third-party dependencies to identify security risks.

How to Implement SRI Checks?

To use SRI, a cryptographic hash (SHA-256, SHA-384, or SHA-512) of the resource must be included in the script or link tag. Example:

<script src="https://cdn.trusted.com/library.js"
        integrity="sha384-oF2sVXqJl0YvY/U3ZfC+XhPbK0JgY1p4Xx9X5r5uC9U="
        crossorigin="anonymous"></script>

In this example:

The integrity attribute ensures that the script executes only if it matches the expected hash.
The crossorigin="anonymous" attribute prevents credentialed requests from altering the integrity check.

Third-Party JavaScript Security (Effective April 1, 2025)

PCI DSS 4.0 Requirement for Third-Party JavaScript Validation

Starting April 1, 2025, organizations must:

Maintain an inventory of all third-party scripts loaded within the Cardholder Data Environment (CDE).
Implement strict access control to restrict unnecessary third-party script execution.
Ensure periodic validation and monitoring of all third-party scripts for potential security risks.
Use SRI checks and CSP policies to enforce integrity and control script execution.
Deploy automated scanning tools to detect unauthorized or modified third-party scripts in real time.

How to Secure Third-Party JavaScript?

Whitelist trusted domains in CSP to prevent unauthorized script loading.
Use SRI hashes to verify the integrity of third-party scripts.
Enable Content Security Policy violation reports to track unexpected script execution.
Conduct frequent security audits on third-party services and dependencies.
Use sandboxing techniques to restrict permissions of untrusted scripts.

By enforcing strict validation measures, businesses can reduce the risk of Magecart-style attacks, where attackers inject malicious code into payment forms via compromised third-party scripts.

Benefits of CSP, SRI, and Third-Party JavaScript Security for PCI DSS Compliance

By implementing CSP headers, SRI checks, and third-party script validation, organizations benefit from:

Protection Against Code Injection Attacks – Prevents XSS and malware injection attempts.
Reduced Risk from Third-Party Dependencies – Ensures external scripts remain unaltered and trusted.
Improved Compliance with PCI DSS 4.0 – Meets new security control requirements.
Better Security Visibility – CSP reports and monitoring provide insights into potential threats.
Prevention of Supply Chain Attacks – Minimizes risks from third-party services compromising payment pages.

Conclusion

PCI DSS 4.0 introduces more stringent requirements to enhance web security and protect payment card data from modern cyber threats. Implementing Content Security Policy (CSP) headers, Subresource Integrity (SRI) checks, and strict third-party JavaScript validation ensures that organizations safeguard their websites from malicious modifications, unauthorized scripts, and supply chain attacks.

Starting April 1, 2025, businesses must implement additional safeguards to monitor and control third-party scripts, ensuring compliance and reducing risks associated with external dependencies.

Businesses should proactively enforce these security measures to maintain compliance and build a secure online payment environment for their customers.

Is your e-commerce platform PCI DSS 4.0 ready? Start implementing CSP, SRI, and third-party script validation today!