Web Application Firewalls (WAF) Explained
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How WAFs protect web applications from OWASP top 10 attacks and when to deploy them.
What Is a WAF?
A Web Application Firewall (WAF) inspects HTTP/HTTPS traffic between clients and web applications. Unlike traditional network firewalls that operate at layers 3-4, WAFs understand application-layer (layer 7) protocols and can detect attacks hidden in legitimate-looking requests.
WAFs protect against the OWASP Top 10 vulnerabilities including SQL injection, cross-site scripting (XSS), cross-site request forgery (CSRF), and more.
How WAFs Work
WAFs use several detection methods:
- Signature-based — Matches requests against known attack patterns (like antivirus signatures)
- Anomaly-based — Builds a baseline of normal traffic and flags deviations
- Machine learning — Trained models detect sophisticated attacks that bypass static rules
- Positive security model — Only allows requests matching a defined schema, blocking everything else
Deployment Models
| Model | Example | Pros | Cons |
|---|---|---|---|
| Cloud WAF | Cloudflare, AWS WAF | Easy setup, DDoS protection | Latency, vendor lock-in |
| Reverse proxy | ModSecurity + Nginx | Full control, no data sharing | Maintenance burden |
| Inline appliance | F5, Imperva | High throughput | Expensive, complex |
When to Use a WAF
Every public-facing web application benefits from a WAF. However, a WAF is not a substitute for secure coding practices. It is a defense-in-depth layer that catches attacks that slip past input validation, parameterized queries, and other application-level defenses.
Start with a cloud WAF in detection-only mode, review the logs, and gradually enable blocking rules to avoid false positives.