As QA engineers, we’re often caught in the middle of development and security teams — balancing feature delivery with risk mitigation. But when it comes to Web Application Firewalls (WAFs), many of us have only a surface-level understanding, often leaving testing gaps and friction with DevSecOps.
This article distills the key takeaways from a session I recently delivered internally, breaking down what WAFs are, how they work, and how QA can actively contribute to WAF integration and testing.
What Is a WAF — And Why Should QA Care?
A Web Application Firewall protects web applications from common threats like SQL Injection (SQLi), Cross-Site Scripting (XSS), and Broken Authentication. Unlike traditional firewalls that operate on lower network layers, a WAF works at Layer 7 (Application Layer) and analyzes HTTP/HTTPS traffic.
🔁 It acts as a reverse proxy — inspecting, filtering, and deciding whether to forward traffic to your web servers.
Think of it as a smart bouncer that not only checks IDs but also reads your body language.
Forward Proxy vs. Reverse Proxy — Quick Refresher
Example of Forward Proxy
In the above diagram, the client’s real IP (192.168.1.10) is hidden. Server sees the proxy’s IP (203.0.113.5) instead of the client’s. It’s used for anonymity, security, and content filtering (e.g., VPN, corporate proxies).
Example of Reverse Proxy
In the above diagram, server’s real IP (10.0.0.1) is hidden. Client only sees the reverse proxy’s IP (203.0.113.5). It’s used for security, load balancing, and caching (e.g., Nginx, Cloudflare, AWS ALB).
Key Differences
Forward Proxy: Client-side (client requests go through the proxy before reaching the internet).
Reverse Proxy: Server-side (client requests are directed to the proxy, which forwards them to backend servers).
Purpose of Web Application Firewall (WAF)
- Protects applications from common web threats like SQL Injection, Cross-Site Scripting (XSS), and Broken Authentication.
- Works at Layer 7 (Application Layer) of the OSI Model.
- Acts as a reverse proxy, shielding backend servers from direct exposure.
How Does a WAF Work?
Traffic Inspection
- Looks into request headers, bodies, cookies, and URLs
- Detects anomalies and known malicious patterns
- Rule-Based Decisions
Signature-based (blocklists)
Behavioral-based (AI/ML models for zero-day threats)
Response Handling
Blocks, challenges (CAPTCHA), or logs the request
Sends alerts for attack attempts
WAF in Action: Threat Examples
🛑 SQL Injection (SQLi)
Payload:
GET /search?query=' OR 1=1 -- HTTP/1.1
The WAF inspects the query string, recognizes the ' OR 1=1 injection pattern, and blocks it before it hits your database.
⚠️ Cross-Site Scripting (XSS)
Payload:
GET /profile?name=
WAFs catch and block scripts embedded in inputs that could steal sessions or deface content.
Where QA Engineers Come In
1. Functional Testing — Preventing False Positives
Ensure the WAF does not break valid user actions:
- Form submissions with special characters (<, {}, etc.)
- OAuth or SSO login flows
- File uploads, multi-step forms
2. Security Testing — Catching False Negatives
Ensure the WAF does block real threats:
- Simulate XSS and SQLi attacks
- Test API rate limiting
- Verify payload filters for headers and cookies
3. Performance Testing — Is the WAF Slowing Us Down?
- Compare API response times before/after WAF
- Run load tests with concurrency and observe latency
4. Regression Testing — After WAF Deployment
- Run automated suites to catch functionality breakage
- Validate all 3rd-party service integrations and APIs
Final Thoughts
A WAF isn’t just a security checkbox — it affects the entire request lifecycle. As QA engineers, it’s on us to ensure it’s properly configured, doesn’t block real users, and effectively filters real threats. And if we want secure applications, we can’t leave that responsibility to just the security team — QA has a huge role to play.
Let me know what kind of WAF testing you’re doing, or drop a comment if you’ve seen false positives break a release. Follow for more QA + DevSecOps insights.
