Demystifying WAF Rules: Signature-based vs. Anomaly Detection

Demystifying WAF Rules: Signature-based vs. Anomaly Detection

May 23, 2024
Waf rules

Web application firewalls (WAFs) are critical security tools for protecting websites and web applications from various threats, including SQL injection, cross-site scripting (XSS), and file injection attacks. WAFs work by inspecting incoming traffic and blocking any requests that match known attack patterns, which are defined in WAF rules.

There are two main types of WAF rules: signature-based and anomaly detection.

Signature-based rules

Signature-based rules are based on known attack patterns. Security vendors constantly update these rules to keep up with the latest threats. Signature-based rules are very effective at blocking known attacks but can be slow to detect new attacks.

Functionalities of Signature-based Rules

Signature-based WAF rules match incoming traffic against a database of known attack signatures. These signatures are typically text strings or regular expressions representing malicious patterns. When a match is found, the WAF rule blocks the request.

Signature-based rules are very effective at blocking known attacks. They are also relatively easy to understand and maintain. However, signature-based rules can be slow to detect new attacks.

Advantages of Signature-based Detection

  • Precision and Efficiency: Signature-based detection is effective because it targets specific attack patterns, resulting in low false positives due to its surgical precision and straightforwardness.

  • Ease of Signature Creation and Updates: Security professionals can quickly create and share new signatures to update the WAF against new attack methods. This allows the WAF to stay alert and adapt to evolving threats.

Limitations of Signature-based Detection

  • Reactive Nature and Inability to Detect Unknown Threats: Signature-based detection is good at spotting known threats but needs help with new attack patterns or zero-day exploits. The Web Application Firewall is at risk from novel threats until new signatures are created and added to the ruleset.
  • Resource Constraints and Performance Implications: Maintaining an extensive signature database can strain server resources and impact application performance, while a small database risks missing important attack patterns and compromising the effectiveness of WAFs.

Anomaly detection rules

Anomaly detection rules look for unusual patterns in traffic. These rules can be more effective at detecting new attacks but also generate false positives.

Anomaly based detection

Functionalities of Anomaly detection rules

Anomaly detection WAF rules look for unusual patterns in traffic. These patterns can include sudden spikes in traffic, requests from distinctive locations, or requests with extraordinary characters. When an anomaly is detected, the WAF rule can block or log the request for further investigation.

Anomaly detection rules can more effectively detect new attacks than signature-based ones because they are not limited to known attack patterns. However, anomaly detection rules can also generate false positives. 

Advantages of Anomaly detection rules

  • Proactive Defense Against Unknown Threats: Identify unusual behavior from any source, making it effective against new and unknown threats. This proactive approach is crucial in a constantly changing threat environment.
  • Reduced Maintenance Overhead: Anomaly-based detection adapts to the application environment, reducing the need for frequent updates and manual interventions in maintaining the WAF.
  • Comprehensive Protection: It monitors unusual behavior to detect threats that may evade signature-based detection, enhancing security by addressing more potential risks and mitigating the chance of overlooking complex attack methods.

Limitations of Anomaly detection rules

  • Increased False Positive Rate: Anomaly-based detection is good at spotting unusual activity but can result in more false alarms. Normal behavior that strays from the baseline may be mistakenly seen as threats, requiring more scrutiny.
  • Complexity and Resource Requirements: These systems need powerful computing and advanced algorithms to detect anomalies, which can increase hardware and software demands and impact performance and scalability accurately.

Combining Both WAF Rule Types

The best way to protect your website or web application is to use a combination of signature-based and anomaly-detection WAF rules. Signature-based rules will block known attacks, while anomaly detection rules will help to detect new attacks.

When configuring your WAF, you can adjust the sensitivity of the anomaly detection rules. This will help to reduce the number of false positives. You can also create custom anomaly detection rules to look for specific patterns in traffic.


WAF rules are an essential part of any web security strategy. By understanding the different types of WAF rules and their functionalities, you can choose the proper rules to protect your website or web application. 

Managing and configuring WAF rules can be a complex task. For a user-friendly and comprehensive WAF solution, consider Modshield SB. Modshield SB WAF provides a robust set of pre-configured rules based on the OWASP Top 10 and allows for customization and easy integration with your existing infrastructure. Also, Modshield SB WAF offers features like built-in load balancing and data loss prevention, making it a powerful one-stop shop for web application security.

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