Glossary

WAF (Web Application Firewall)

A WAF (Web Application Firewall) is an application-layer (L7) firewall that analyses HTTP traffic and blocks malicious requests before they reach the application. Protects against SQL injection, XSS, CSRF, HTTP-level DDoS, and other OWASP Top 10 attacks.

Operating modes

Detection mode — logs suspicious requests but does not block them. For initial tuning
Prevention mode — actively blocks attacks

Types

Network-based — hardware appliance in front of servers
Host-based — a module on the server (ModSecurity for Nginx/Apache)
Cloud-based — Cloudflare WAF, AWS WAF, Fastly. Easiest to set up

Limitations

A WAF is not a replacement for secure code — it is an additional layer of protection. False positives (blocking legitimate requests) require careful rule tuning.