Web Application Firewall (WAF): A Practical Introduction

Modern web applications are constantly exposed to malicious traffic, SQL injections, cross-site scripting attacks, automated bots, and exploit attempts against known vulnerabilities. A Web Application Firewall (WAF) acts as a critical security layer that helps mitigate these threats before they reach your application.

A WAF is a piece of software designed to protect web applications from known attack patterns and cyber threat vectors. As the name suggests, it operates specifically at the web application layer and inspects HTTP/HTTPS traffic.

A WAF can protect:

• Backend services (REST APIs, GraphQL, microservices)
• Frontend applications
• Monolithic and distributed architectures

Important Note

A WAF can only inspect HTTP/HTTPS traffic.
It is not suitable for:

• SSH connections
• Custom TCP/UDP protocols
• Database-native or internal service traffic

Using a WAF for non-HTTP traffic provides no real protection.

Where Does a WAF Sit in the System Stack?

A WAF is almost always the first entry point into your system.

In a typical architecture:

1. The client sends a request
2. The request hits the WAF
3. The WAF acts as a reverse proxy
4. Allowed traffic is forwarded to the backend services

By sitting at the edge, a WAF ensures that malicious traffic is filtered out early, reducing load and risk on downstream systems.

What Does a WAF Need to Do?

For a WAF to be effective, it must handle several responsibilities:

1. TLS Decryption
   Since modern traffic is encrypted, the WAF must terminate TLS to inspect requests.

2. Request Inspection
   Analyze headers, query parameters, request bodies, and cookies for:

   • SQL injection
   • Cross-site scripting (XSS)
   • Known framework or runtime vulnerabilities
   • Exploit payloads targeting server-side rendering or evaluation flaws

3. Traffic Blocking
   If a request matches a known malicious pattern, it must be blocked immediately.

4. Traffic Allowance
   Legitimate traffic should pass through without unnecessary friction.

5. Rate Limiting
   Protects against brute-force attacks, bot abuse, and denial-of-service attempts.

6. High Performance (Critical Requirement)
   A WAF sits directly in the request path.
   If it is slow, your entire application becomes slow.
   A production-grade WAF must be:

   • Low latency
   • Highly optimized
   • Capable of handling high throughput

Common Web Attacks & How a WAF Helps

A WAF primarily protects against known, pattern-based web attacks. Below are common examples.

SQL Injection (SQLi)

username=admin' OR '1'='1

How it happens
Attackers inject malicious SQL through user input fields.

How a WAF helps
Detects SQL patterns and blocks malicious queries before they reach the database.

Cross-Site Scripting (XSS)

<script>
  alert("XSS");
</script>

How it happens
Malicious JavaScript is injected into inputs and executed in the browser.

How a WAF helps
Blocks script-based payloads and encoded JavaScript patterns.

Remote Code Execution (RCE)

${{7*7}}

How it happens
Attackers exploit known framework or runtime vulnerabilities.

How a WAF helps
Blocks known exploit signatures and malicious expressions.

Path Traversal

../../../../etc/passwd

How it happens
Attackers manipulate file paths to access restricted files.

How a WAF helps
Detects traversal patterns and blocks unauthorized access.

Brute Force & Bot Attacks

POST /login (repeated thousands of times)

How it happens
Automated requests are used to guess credentials or abuse APIs.

How a WAF helps
Applies rate limits and blocks abnormal traffic patterns.

Why Is a WAF Important?

A WAF is not a silver bullet and does not provide 100% protection.

However, it:

• Significantly reduces the attack surface
• Stops automated and low-effort attacks
• Buys valuable time during active incidents
• Saves engineering effort and operational cost

In practice, a WAF acts as a strong first line of defense, allowing teams to focus on deeper security improvements.

Ways to Deploy a WAF

Cloud-Managed (Edge) WAF

A WAF operated by a third-party provider and deployed at the network edge, before traffic reaches your infrastructure.

Examples

• AWS WAF
• Cloudflare WAF

Pros

• Quick setup with minimal maintenance
• Automatic scaling
• Effective against bots and common web attacks

Cons

• Limited fine-grained control
• Vendor lock-in
• Costs scale with traffic

Self-Hosted (Reverse Proxy) WAF

A WAF deployed within your own infrastructure, typically running as a reverse proxy in front of the application.

Pros

• Full control over rules and behavior
• Deep customization
• Better suited for strict compliance needs

Cons

• Operational overhead
• Requires security expertise

Self-hosted WAFs are ideal when control and customization are more important than operational simplicity.

One Real-World Deployment Example

In production, a common setup looks like this:

Cloudflare WAF → Load Balancer → Application → Database

In this architecture, the WAF filters malicious traffic at the edge, the load balancer distributes clean requests, the application handles business logic, and the database remains isolated from direct internet exposure.

Final Thoughts

A WAF should never replace secure coding, proper authentication, or regular patching.
However, when deployed correctly, it dramatically improves your security posture by stopping common attacks before they reach your application.

A well-configured WAF is not just a security tool, it is a force multiplier for your engineering team.