A Web Application Firewall (WAF) is a security system that monitors, filters, and blocks HTTP/HTTPS traffic to and from a web application. It protects against common web attacks like SQL injection, cross-site scripting (XSS), and other OWASP Top 10 vulnerabilities.
How WAFs Work
WAFs inspect incoming requests and outgoing responses:
- Request comes in
- WAF analyzes against rules and patterns
- Malicious requests: blocked
- Legitimate requests: passed through
What WAFs Protect Against
Injection Attacks
- SQL Injection: Malicious database queries in input fields
- Command Injection: Executing system commands
- LDAP Injection: Manipulating directory services
Cross-Site Scripting (XSS)
Injecting malicious scripts that execute in users' browsers.
Cross-Site Request Forgery (CSRF)
Tricking users into performing unwanted actions.
DDoS Attacks
Overwhelming servers with traffic. Basic protection only; dedicated DDoS protection is more robust.
Bot Traffic
Blocking scrapers, credential stuffers, spam bots.
Zero-Day Exploits
Virtual patching for vulnerabilities before code is fixed.
WAF Deployment Models
Network-Based (Hardware)
Physical appliances in your data center.
- Lowest latency
- Highest cost
- Requires maintenance
Host-Based (Software)
Installed on your servers.
- Full customization
- Consumes server resources
- Complex management
Cloud-Based
WAF as a service.
- Easy to implement
- Always updated
- Pay-as-you-go
- Most popular option today
Popular WAF Services
Cloud WAFs
- Cloudflare: Integrated with CDN, generous free tier
- AWS WAF: Native for AWS infrastructure
- Azure WAF: Native for Azure
- Sucuri: WordPress-focused
Enterprise
- Akamai: Large-scale protection
- Imperva: Advanced features
- F5: Hardware and cloud options
WAF Modes
Detection Mode
Logs threats but doesn't block. Good for initial deployment.
Prevention Mode
Actively blocks detected threats.
Learning Mode
Observes traffic to understand normal patterns before enforcing rules.
WAF Best Practices
- Start in detection mode, review logs, then enable prevention
- Keep rules updated
- Monitor for false positives (legitimate traffic blocked)
- Don't rely solely on WAF. It's one layer of defense
- Use with other security measures (secure code, HTTPS, authentication)