What Is a WAF?
Web Application Firewall (WAF) is a specialized security system that analyzes HTTP/HTTPS traffic to protect web applications against application-layer (OSI Layer 7) attacks. While traditional firewalls operate at the network and transport layers, a WAF provides a security layer that understands web application logic and detects application-level threats.
The primary goal of a WAF: block malicious requests before they reach the web server.
What Attacks Does a WAF Block?
The leading threats on the OWASP (Open Web Application Security Project) Top 10 list are addressed by a WAF:
| Attack Type | Description |
|---|---|
| SQL Injection | Injecting malicious code into database queries |
| Cross-Site Scripting (XSS) | Executing malicious JavaScript in the browser |
| Command Injection | Running operating system commands on the server |
| Path Traversal | Accessing unauthorized directories or files |
| Remote File Inclusion | Including malicious files from a remote server |
| HTTP Flood / DDoS | Application-layer denial-of-service attack |
| Session Hijacking | Stealing or manipulating session cookies |
| Bot attacks | Brute force, scraping, credential stuffing |
WAF Operating Modes
Whitelist (Positive Model)
Only defined legitimate requests are permitted; all others are blocked. The most secure mode, though defining comprehensive rule sets takes time. Preferred for critical banking or payment applications.
Blacklist (Negative Model)
Known attack signatures and malicious patterns are blocked; undefined traffic is passed through. Easier to manage, but provides limited protection against zero-day attacks.
Hybrid Model
Combines both approaches to provide signature-based protection against known threats and anomaly-based protection against deviations from normal behavior. Modern WAF products generally operate in hybrid mode.
WAF Types
Network-Based WAF (Hardware WAF)
Deployed as a physical appliance in the data center. Low latency, high performance. Initial deployment cost is high. Suitable for large enterprises and data centers.
Host-Based WAF (Software WAF)
A software module installed on the web server (such as ModSecurity). Low cost, high customization potential. Consumes server resources and requires maintenance.
Cloud-Based WAF
Delivered as a service over CDN infrastructure. Cloudflare, AWS WAF, and Azure Application Gateway are examples. Can be deployed within minutes via DNS redirect, with high DDoS protection capacity.
Difference Between WAF and Traditional Firewall
| Feature | Traditional Firewall | WAF |
|---|---|---|
| OSI Layer of operation | 3–4 (Network/Transport) | 7 (Application) |
| HTTP content analysis | No | Yes |
| SQL Injection detection | No | Yes |
| Session management | No | Yes |
| SSL/TLS inspection | Limited | Full |
| Web traffic optimization | No | Yes (some products) |
A WAF and a traditional NGFW are not competitors — they are complementary. Both should be used together for a secure web infrastructure.
Enterprise WAF Scenarios
E-Commerce and Payment Systems
A WAF is mandatory for PCI-DSS compliance. Skimming attacks targeting card data theft and injection attempts are blocked by the WAF.
Enterprise Web Portals and APIs
HR portals, customer panels, and REST APIs benefit from WAF protection. Specialized WAF rules can be created for API security.
E-Government and Healthcare Applications
In applications that handle personal data, web application security under the scope of KVKK is now becoming a mandatory requirement.
WAF Management and Maintenance
Installing a WAF is not enough; regular maintenance is critical:
- Rule updates: The signature base must be updated against new attack vectors
- False positive management: Legitimate requests must not be blocked; rule fine-tuning is required
- Logging and monitoring: WAF logs must be fed into the SIEM
- Alignment with application changes: WAF rules must be reviewed with every new feature or API update
Conclusion
For any organization that processes customer data, accepts payments, or conducts business processes over the web, a WAF has now become a fundamental security requirement. Cloud-based WAF solutions offer high-level protection at low cost for small and medium-sized organizations. As NRC Sistem, we provide consulting services to your organization on web application security assessment, WAF selection, and configuration.