Blog | G5 Cyber Security

PCI DSS & User Interface Security

G5 Cyber Security

TL;DR

Yes, PCI DSS (and similar standards) absolutely cover user interface security for cardholders. While not explicitly detailing *how* to build UIs, they require you protect cardholder data throughout the entire system – and that includes what users see and interact with. Focus on preventing skimming, manipulation, and ensuring secure input/output of payment details.

Understanding the Requirements

PCI DSS doesn’t say ‘use this specific JavaScript library’. It focuses on outcomes. Here’s how it applies to user interfaces:

  1. Requirement 6: Develop and Maintain Secure Systems and Applications – This is the big one. It covers everything from coding practices to regular vulnerability scanning. Your UI is part of your application, so it *must* be secure.
    • 6.3.1: Regularly test applications for vulnerabilities. Use automated scanners (like OWASP ZAP or Burp Suite) and manual penetration testing.
    • 6.3.2: Implement strong coding practices. This means avoiding common web application flaws like Cross-Site Scripting (XSS), SQL Injection, and Cross-Site Request Forgery (CSRF).
  2. Requirement 4: Protect Cardholder Data in Transit – If your UI transmits card data (even encrypted), it needs to be done securely.
    • Use HTTPS/TLS with strong ciphers. Ensure certificates are valid and properly configured.
    • Avoid transmitting sensitive data unnecessarily.
  3. Requirement 3: Protect Stored Cardholder Data – While less direct, if your UI displays card details (even masked), it’s part of the overall protection scheme.
    • Ensure any stored data is encrypted and access controlled.
    • Consider tokenisation to avoid storing actual card numbers on your systems at all.

Practical Steps to Improve UI Security

Here’s a breakdown of what you can do, from simple checks to more advanced measures:

  1. Input Validation: Never trust user input. Validate *everything* on both the client-side (for usability) and, crucially, the server-side.
    • Client-Side: Use JavaScript to check basic format (e.g., card number length, expiry date). Don’t rely on this for security – it’s easily bypassed.
    • Server-Side: Implement robust validation rules in your backend code. This is where the real protection happens. 
      // Example Python (Flask) input validation
      from wtforms import Form, StringField, validators

class PaymentForm(Form):
    card_number = StringField('Card Number', [validators.DataRequired(), validators.Length(min=16, max=19)])
    expiry_date = StringField('Expiry Date', [validators.DataRequired()])
  2. Output Encoding: Protect against XSS by encoding any user-supplied data before displaying it on the page.
    • Use appropriate escaping functions provided by your framework (e.g., in PHP, use htmlspecialchars()).
  3. HTTPS/TLS: Ensure all pages handling cardholder data are served over HTTPS.
    • Check your certificate validity using online tools like SSL Labs Server Test (https://www.ssllabs.com/ssltest/).
    • Enforce HTTP Strict Transport Security (HSTS) to prevent downgrade attacks. 
      // Example HSTS header in Apache configuration
      Header always set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"
  4. Content Security Policy (CSP): A powerful mechanism to control the resources your browser is allowed to load, mitigating XSS attacks.
    • Configure CSP headers carefully. Start with a restrictive policy and gradually allow necessary resources. 
      // Example CSP header
      Content-Security-Policy: default-src 'self'; script-src 'self' https://trusted-cdn.example.com; style-src 'self' https://trusted-cdn.example.com; img-src 'self' data:; font-src 'self'
  5. Regular Security Scanning: Use automated tools to identify vulnerabilities.
    • OWASP ZAP (free and open source)
    • Burp Suite (commercial, but powerful)
    • Qualys Web Application Scanning (commercial)
  6. Penetration Testing: Hire a qualified cyber security firm to perform regular penetration tests.

Important Considerations

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