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Web Application Firewall (WAF)

A Web Application Firewall (WAF) is a security control that monitors, filters, and blocks malicious HTTP traffic targeting web applications.

What is WAF?How it typically works?Common techniquesImpactFurther reading

What is WAF?

Web applications are a prime target for attackers seeking to exploit vulnerabilities such as SQL injection, cross-site scripting (XSS), and remote file inclusion. A WAF provides a protective layer by inspecting incoming traffic and blocking malicious requests before they reach the application. For SecOps, WAFs are essential to protect publicly accessible applications, reduce incident response workload, and ensure compliance with standards such as PCI DSS.

How it typically works?

  1. Traffic inspection: incoming HTTP and HTTPS requests are analyzed.
  2. Rule evaluation: requests are compared against predefined or custom security rules.
  3. Decision: malicious requests are blocked, sanitized, or flagged.
  4. Logging: all requests and blocks are logged for analysis.
  5. Integration: WAFs often integrate with SIEM and other SOC tools.

Modern WAFs also use machine learning to adapt rules dynamically, improving accuracy while reducing false positives.

Common techniques

  • Rule-based WAF: relies on signature or pattern matching to detect known attacks.
  • Behavioral WAF: uses anomaly detection to identify unusual request patterns.
  • Cloud-based WAF: delivered as a managed service for scalability.
  • Reverse proxy WAF: deployed in front of web servers to inspect all traffic.
  • Inline vs out-of-band: inline blocks requests; out-of-band only monitors.
  • Integration with CDNs: WAF functionality is often bundled with content delivery services.

Impact

WAFs block many of the most common web application attacks, providing a strong defense for exposed assets. They help organizations achieve compliance and reduce the burden on development teams by catching malicious traffic at the edge.

However, WAFs are not foolproof. Skilled attackers may bypass them with obfuscation or novel payloads, and poorly configured rules can block legitimate users. For SecOps, WAFs are a valuable but complementary layer, not a substitute for secure coding and testing.

Further reading

Related topics

DASTRASPSecure Coding Practices