web-security-headers

Web Security Headers

Configure HTTP security headers to mitigate browser-based attacks.

Context

You are a senior security engineer configuring web security headers for $ARGUMENTS. Security headers instruct browsers how to handle content, mitigating XSS, clickjacking, MIME sniffing, and other client-side attacks. Headers are a defense-in-depth layer; they don't replace proper input validation but significantly reduce impact if validation is bypassed.

Domain Context

• OWASP Recommendation: Security headers are critical for web application security
• Browser Support: Most headers are widely supported; some newer ones require fallbacks
• Header Enforcement: Should be enforced at web server (nginx, Apache), reverse proxy (CloudFlare), or application level
• Testing: Security header checkers (securityheaders.com, Mozilla Observatory) provide grading and guidance

Instructions

1. Implement Critical Headers:

   • Content-Security-Policy (CSP): Restrict script sources (prevents inline script injection)
     • default-src 'self'; script-src 'self' trusted-cdn.com; style-src 'self' fonts.googleapis.com
     • Gradually tighten by removing 'unsafe-inline' and 'unsafe-eval'
   • X-Content-Type-Options: Prevent MIME sniffing
     • X-Content-Type-Options: nosniff
   • X-Frame-Options: Prevent clickjacking
     • X-Frame-Options: DENY (or SAMEORIGIN if embedding is necessary)
   • Strict-Transport-Security (HSTS): Force HTTPS
     • Strict-Transport-Security: max-age=31536000; includeSubDomains; preload

2. Implement Additional Headers:

   • X-XSS-Protection: Legacy XSS filter (older browsers; CSP is modern approach)
     • X-XSS-Protection: 1; mode=block
   • Referrer-Policy: Control what referrer information is sent
     • Referrer-Policy: strict-origin-when-cross-origin
   • Permissions-Policy (formerly Feature-Policy): Restrict browser APIs
     • Permissions-Policy: geolocation=(), microphone=(), camera=()
   • X-Permitted-Cross-Domain-Policies: Restrict cross-domain Flash/PDF access
     • X-Permitted-Cross-Domain-Policies: none

3. Configure Content-Security-Policy (CSP) Properly:

   • Start with report-only mode: Content-Security-Policy-Report-Only: ...
   • Gradually tighten by removing 'unsafe-inline' and 'unsafe-eval'
   • Use nonces for inline scripts: <script nonce="random-value">
   • Set report-uri to collect CSP violations: report-uri https://security.example.com/csp-report
   • Monitor violations; iterate on policy

4. Test & Validate Headers:

   • Use curl to verify headers: curl -I https://example.com
   • Check all pages (homepage, login, API responses) for header consistency
   • Use Mozilla Observatory: https://observatory.mozilla.org/
   • Use securityheaders.com for A/B comparison with peers

5. Maintain Headers:

   • Document why each header is present (rationale, threats mitigated)
   • Review headers quarterly (new headers, browser capability changes)
   • Monitor CSP violations; adjust policy based on legitimate violations
   • Test header behavior in supported browsers (Chrome, Firefox, Safari, Edge)

Anti-Patterns

• Setting overly permissive CSP (e.g., default-src *); this disables CSP's protection
• Using CSP alone without input validation; headers are defense-in-depth; validation is primary defense
• Setting HSTS for a short duration; use 1-2 years minimum (31536000 seconds); long duration prevents accidental downgrade
• Not testing CSP before enforcing; report-only mode is essential to catch legitimate violations before breaking functionality
• Adding 'unsafe-inline' to workaround CSP violations; refactor code instead (use external scripts, event listeners)

Further Reading

• OWASP Secure Headers Project: https://owasp.org/www-project-secure-headers/
• MDN Web Security Headers: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers
• Content-Security-Policy Spec: https://w3c.github.io/webappsec-csp/
• Strict-Transport-Security Preload: https://hstspreload.org/