Security Headers

Security headers are HTTP response headers that configure browser-side security policies. They form a critical layer of defense by instructing browsers to enforce restrictions on how content is loaded, rendered, and interacted with. While they don't fix server-side vulnerabilities, they significantly reduce the exploitability of client-side issues and add defense-in-depth against common attack vectors.

The essential security headers include: Content-Security-Policy (controls resource loading), Strict-Transport-Security (enforces HTTPS), X-Content-Type-Options: nosniff (prevents MIME type sniffing), X-Frame-Options (prevents clickjacking), Referrer-Policy (controls referrer information leakage), Permissions-Policy (restricts browser feature access like camera, microphone, geolocation), and Cross-Origin-Opener-Policy / Cross-Origin-Embedder-Policy (provide process isolation). Each header addresses a specific category of browser-based attacks.

Implementing security headers should be done at the infrastructure level (reverse proxy, CDN, or middleware) to ensure consistent application across all routes and responses. Tools like securityheaders.com and the Mozilla Observatory provide automated scanning and grading of security header configurations. Headers should be tested in staging before production deployment, especially Content-Security-Policy, which can break functionality if misconfigured. Regular audits should verify headers haven't been accidentally removed during infrastructure changes.