PCI DSS 4.0 Requirement 4: How to Protect Cardholder Data in Transit

Table of Contents

1. Introduction to Requirement 4
2. Why Protecting Data in Transit Matters
3. Key Elements of Requirement 4
4. Approaches to Secure Data Transmission
5. EOL Software Considerations
6. Real-World Examples of Transmission-Related Breaches
7. Ongoing Monitoring and Validation
8. How HeroDevs Supports Secure Data in Transit
9. Key Takeaways and Next Steps
10. Frequently Asked Questions (FAQs)

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1. Introduction to Requirement 4

Under PCI DSS 3.2.1, Requirement 4 emphasized “Encrypt transmission of cardholder data across open, public networks.” In PCI DSS 4.0, the focus expands to ensure organizations use strong cryptography wherever cardholder data (CHD) may traverse untrusted routes—including internal segments that could be compromised.

Who Does Requirement 4 Affect?

• Merchants handling online or in-store payments (e.g., sending CHD to payment gateways).
• Service providers transmitting cardholder data on behalf of multiple clients.
• Any organization connecting POS devices, web apps, or internal systems over potentially unsafe networks.

Goal of Requirement 4: Ensure that attackers cannot steal cardholder data while it’s “on the move,” by intercepting or tampering with communications.

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2. Why Protecting Data in Transit Matters

Even if you encrypt data at rest (PCI DSS Requirement 3), data can still be exposed whenever it travels across networks. Attackers often exploit:

• Man-in-the-Middle (MitM) Attacks: Intercepting traffic on public Wi-Fi, corporate LANs, or hijacked routers.
• Eavesdropping & Packet Sniffing: Capturing plaintext transmissions over unsecured protocols (e.g., HTTP, FTP).
• Compromised Internal Networks: Malware or insider threats that sniff unencrypted internal traffic.
• Downgrade Attacks: Forcing outdated protocols or ciphers if they’re still enabled.

Strong in-transit encryption ensures that even if attackers capture the raw data stream, they can’t read or misuse it—significantly reducing breach impact.

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3. Key Elements of Requirement 4

PCI DSS 4.0 Requirement 4 can be summarized into these critical objectives:

1. Use Strong Cryptography: Employ modern protocols (TLS 1.2 or higher) and disallow legacy/weak encryption.
2. Document Roles and Processes: Define who is responsible for key management, certificate renewals, and encryption policies.
3. Maintain Trusted Certificates: Certificates must be valid, not expired or self-signed in production. Keep an inventory of keys/certs.
4. Protect All Channels: This applies to web sessions (HTTPS), file transfers (SFTP/FTPS), APIs, and any other data-exchange mechanism carrying cardholder data.
5. Wireless Security: If wireless networks handle CHD, they must use strong encryption (WPA2/WPA3), segmentation, and regular monitoring.

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4. Approaches to Secure Data Transmission

4.1 Modern Encryption Protocols

• TLS 1.2 or 1.3: PCI DSS explicitly forbids outdated protocols (SSL, TLS 1.0/1.1).
• Robust Cipher Suites: Use AES (128-bit or higher), ECC key exchanges (ECDHE), and SHA-256 or stronger. Avoid RC4, MD5, or 3DES.
• Certificate Management: Obtain certs from reputable Certificate Authorities (CAs), track expiration dates, and renew proactively.

Pro Tip: Regularly scan your public endpoints with SSL Labs or similar tools to check for weak ciphers, outdated protocols, or configuration errors.

4.2 Secure VPNs and Tunnels

• IPsec for Site-to-Site: Encrypts all IP traffic between networks (e.g., retail stores and HQ).
• TLS/SSL VPNs for Remote Access: Ensures user connections to internal systems are secure, especially for admin or support tasks.
• Least Privilege: Grant VPN or tunnel access only to essential services, minimizing exposure if credentials are compromised.

4.3 Secure APIs and HTTPS

• HTTPS Everywhere: All public-facing web pages and APIs handling CHD should enforce HTTPS with valid certs.
• HSTS (HTTP Strict Transport Security): Prevents accidental or malicious downgrade to HTTP.
• Secure Configuration: Avoid misconfigurations (e.g., default credentials on cloud load balancers, not validating server certificates in code).

4.4 Wireless and Mobile Payment Security

• WPA2/WPA3 Encryption: Never use WEP or WPA with TKIP—these are easily cracked.
• Network Segmentation: Isolate wireless networks carrying CHD from guest or internal corporate networks.
• Secure Mobile Transactions: Use vendor-approved hardware encrypting readers or tokens to protect card data from capture in the mobile device’s OS.

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5. EOL Software Considerations

End-of-life (EOL) software can sabotage your in-transit encryption by forcing legacy protocols or lacking security patches. For instance:

• Outdated Web Servers: Might only support SSLv3/TLS 1.0.
• Deprecated SSL Libraries: Vulnerabilities like Heartbleed (OpenSSL) remain unpatched in EOL systems.
• Legacy Firmware: Firewalls or routers may stop receiving updates, leaving known exploits wide open.

Mitigation Strategies

1. Inventory & Prioritize: Identify all systems and libraries used for encryption.
2. Upgrade or Replace: Plan migrations away from EOL OSes, routers, or payment terminals.
3. Temporary Compensating Controls: Tunnel outdated devices inside a secure VPN segment if immediate replacement isn’t possible—though this is a short-term fix.

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6. Real-World Examples of Transmission-Related Breaches

Example 1: Weak Wireless Encryption

A major retailer used WEP for store Wi-Fi. Attackers quickly cracked it, intercepted card data transmissions, and stole millions of card numbers.
Lesson: Outdated wireless encryption is a prime entry point for sniffing traffic in transit.

Example 2: Internal LAN Snooping

A supermarket chain didn’t encrypt transaction data on internal networks. Malware installed on back-office servers captured data en route to the payment processor.
Lesson: Treat internal traffic as potentially at-risk if an attacker gains a foothold. Consider encrypting or segmenting internal CHD flows.

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7. Ongoing Monitoring and Validation

Achieving Requirement 4 compliance is not just a one-time event:

1. Regular Configuration Checks: Ensure servers, load balancers, and APIs don’t re-enable insecure protocols.
2. Automated Scans: Tools like Qualys or Tenable can detect open ports, weak ciphers, and SSL certificate issues.
3. Certificate Lifecycle Management: Keep track of cert expiration dates and renew them promptly.
4. Alerting: If a device or service starts offering TLS 1.0, it should trigger an immediate incident response.

Pro Tip: Incorporate these checks into your CI/CD pipeline or DevSecOps practices, so new deployments automatically comply with strong encryption standards.

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8. How HeroDevs Supports Secure Data in Transit

HeroDevs specializes in modernizing legacy environments that threaten PCI compliance—especially in the realm of EOL systems or outdated encryption. By engaging HeroDevs, you can:

1. Assess and Migrate: Identify systems still running deprecated TLS or older SSL libraries and develop an upgrade roadmap.
2. Implement Secure Tunnels and APIs: Deploy modern VPN solutions or enforce HTTPS/TLS across all microservices and external endpoints.
3. Maintain Certificate Inventories: Set up automated tracking/renewal to avoid expired or self-signed certificates in production.
4. Develop a Proactive Security Culture: Train teams to avoid unencrypted transfers, vet new integrations, and maintain continuous compliance.

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9. Key Takeaways and Next Steps

1. Encrypt All Sensitive Traffic: From POS terminals to backend APIs, ensure no plaintext card data crosses untrusted channels.
2. Retire Old Protocols: SSLv3, TLS 1.0/1.1, and WEP are no-gos. PCI DSS mandates TLS 1.2+ (and ideally TLS 1.3).
3. Manage Certs & Keys: Keep an up-to-date inventory, renew before expiry, and use only trusted Certificate Authorities.
4. Monitor & Audit Continuously: Regular scans, SIEM alerts, and real-time detection of unauthorized protocols.
5. Plan for EOL: Don’t let outdated systems force you to use insecure crypto. Upgrade or isolate them ASAP.

Looking Ahead: By rigorously securing data in transit, you’ll not only comply with Requirement 4 but also protect customers from the pervasive threat of intercepted transmissions. This forms a strong foundation for the rest of your PCI DSS 4.0 security posture.

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10. Frequently Asked Questions (FAQs)

Q1. Does Requirement 4 apply to internal traffic too?

Primarily it covers open, public networks, but any internal network that’s not fully secure can be considered at risk. Many breaches occur from insider threats or lateral movement, so encrypting all CHD traffic is a best practice—even internally.

Q2. Can we use self-signed certificates for internal APIs?

PCI DSS 4.0 encourages trusted certificates so that connections aren’t easily spoofed or ignored. While you can use an internal CA for internal APIs, ensure your processes are robust—self-signed certs in production can lead to misconfigurations and potential MitM attacks.

Q3. Are VPNs mandatory if we use HTTPS for everything?

Not necessarily. If all cardholder data transmissions are already fully protected by TLS (HTTPS, SFTP, etc.), you may not need an additional VPN. VPNs are typically used for remote admin or site-to-site encryption. Evaluate your architecture and risk level.

Q4. What if an old POS terminal only supports TLS 1.0?

That’s an EOL scenario. You’ll need to replace or upgrade it. As an interim step, you could wrap it in a secure tunnel, but an assessor may push for a full replacement because TLS 1.0 no longer meets PCI’s definition of “strong cryptography.”

Q5. Can we send PAN via email if we encrypt the file attachment?

Technically yes, but only if the data (including attachments) is strongly encrypted before sending, and keys/passwords are shared securely. Most organizations avoid emailing PAN altogether to prevent user mistakes. A secure file transfer or portal is a safer approach.

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Conclusion

PCI DSS 4.0 Requirement 4 mandates strong cryptography whenever cardholder data traverses networks outside fully trusted zones. By implementing the strategies outlined—modern TLS, secure VPNs, robust certificate management, and continuous monitoring—you dramatically reduce your exposure to interception attacks.

Remember that encryption in transit is just one piece of a holistic PCI DSS strategy. Combine it with secure configurations (Req. 2), encrypted storage (Req. 3), and ongoing vulnerability management (Req. 11) to build a resilient environment. If EOL systems are holding you back, HeroDevs can help you upgrade, retire, or segment them so that every packet carrying CHD remains safe from prying eyes.