A Web Application Firewall (WAF) inspects HTTP traffic between the internet and your web application, filtering out malicious requests before they reach your servers. It protects against the most common web attacks — SQL injection, cross-site scripting (XSS), file inclusion, and other OWASP Top 10 threats — without requiring changes to your application code.
While a WAF is not a substitute for secure coding practices, it provides essential defense-in-depth. Applications have bugs, and even well-coded applications may have undiscovered vulnerabilities. A properly configured WAF catches exploitation attempts for known vulnerability patterns and buys you time to patch your application.
How Web Application Firewalls Work
A WAF operates at Layer 7 of the OSI model — the application layer. Unlike traditional network firewalls that filter traffic based on IP addresses and ports, a WAF understands HTTP and can inspect the contents of requests: URL paths, query parameters, POST body data, HTTP headers, and cookies.
When a request arrives, the WAF evaluates it against a set of rules. Signature-based rules look for known attack patterns — specific strings or patterns in the request that indicate an SQL injection attempt, an XSS payload, or a known exploit. Behavioral rules analyze request patterns — too many requests from one IP, unusual request sizes, or request patterns that match automated scanning tools. Anomaly-based rules establish a baseline of normal traffic and flag requests that deviate significantly.
Based on the rule evaluation, the WAF takes an action: allow the request to pass through to your application, block the request and return an error page, challenge the request with a CAPTCHA or JavaScript challenge, or log the request for later analysis without blocking it.
Types of WAF Deployment
Cloud-based WAFs (Cloudflare, AWS WAF, Akamai) operate as a reverse proxy — all traffic passes through the WAF provider's network before reaching your servers. This is the easiest deployment option: you change your DNS records to point to the WAF provider, and all traffic is filtered automatically. Cloud WAFs also provide DDoS protection and CDN capabilities as part of the service.
Host-based WAFs (ModSecurity, Coraza) run on your web server alongside your application. They provide the same protection without routing traffic through a third party, which is important for organizations with data sovereignty requirements. The tradeoff is higher operational overhead — you manage the WAF software, rules, and updates yourself.
Appliance-based WAFs (F5, Imperva) are hardware or virtual appliances that sit in front of your web servers on your network. They offer the highest performance and most advanced features but are the most expensive option and primarily suited for large enterprise deployments.
Essential WAF Rules
Start with the OWASP Core Rule Set (CRS) — a comprehensive, well-maintained set of rules that protect against the most common web attacks. The CRS covers SQL injection detection with hundreds of patterns, XSS prevention for reflected and stored attacks, remote file inclusion and local file inclusion, command injection prevention, scanner and bot detection, and protocol enforcement for HTTP compliance.
Beyond the CRS, configure custom rules for your specific application. Block access to administrative paths from non-whitelisted IPs. Rate limit login endpoints to prevent brute-force attacks. Block requests with unusually large payloads that could indicate buffer overflow attempts. And restrict HTTP methods to only those your application uses — if your application only handles GET and POST, block PUT, DELETE, PATCH, and OPTIONS.
Avoiding False Positives
The biggest challenge with WAF deployment is false positives — legitimate requests that are incorrectly blocked. A user submitting a blog comment that contains SQL syntax, a developer pasting code into a support form, or an API receiving JSON payloads that resemble attack patterns can all trigger WAF rules.
Deploy your WAF in monitoring mode first — log potential blocks without actually blocking traffic. Analyze the logs for a week to identify false positives. Create exceptions for legitimate traffic patterns before switching to blocking mode. Continue monitoring after enabling blocking and adjust rules as new false positives are identified.
For API endpoints that receive structured data (JSON, XML), configure the WAF to parse the content type correctly and apply rules to individual fields rather than the raw request body. This dramatically reduces false positives for API traffic.
WAF Performance Considerations
A WAF adds latency to every request — typically 1 to 5 milliseconds for cloud-based WAFs and less than 1 millisecond for host-based WAFs. For most applications, this is negligible. However, if you have strict latency requirements, test the impact with your specific traffic patterns and rule configuration.
Cloud-based WAFs can actually improve performance for geographically distributed users by caching static content at edge locations and serving it from the closest point of presence. The combined CDN and WAF functionality means you get both security and performance benefits.
Monitoring and Incident Response
Monitor your WAF logs continuously. Track blocked request rates, the most commonly triggered rules, source IP addresses of malicious traffic, and trends over time. A sudden spike in blocked requests may indicate a targeted attack. A new rule triggering frequently on legitimate traffic indicates a false positive that needs adjustment.
Integrate WAF logs with your SIEM (Security Information and Event Management) system for correlation with other security data. When a WAF blocks an attack attempt, cross-reference the source IP with authentication logs, access logs, and other security signals to determine if the attacker gained access through another vector.
ZeonEdge configures and manages web application firewalls for businesses that need enterprise-grade protection. Learn more about our security services.
Sarah Chen
Senior Cybersecurity Engineer with 12+ years of experience in penetration testing and security architecture.