🛡️ Building a Web Application Firewall (WAF) Microservice in Java Spring Boot

 In today’s cloud-native microservice architectures, security is no longer just about firewalls or SSL certificates. Attackers exploit vulnerabilities like SQL Injection, Cross-Site Scripting (XSS), and Command Injection directly through application endpoints.

This is where a Web Application Firewall (WAF) comes in — acting as the first line of defense for your microservices.

In this post, we’ll build a lightweight, extensible WAF microservice using Java Spring Boot that inspects incoming HTTP traffic and blocks malicious requests before they reach your core application logic.

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🚀 What We'll Build

We’ll create a WAF microservice that:

• Intercepts all incoming requests.

• Scans parameters, headers, and body content for malicious patterns (SQLi, XSS, etc).

• Logs and blocks suspicious requests with a 403 Forbidden.

• Can be deployed in front of any backend microservice (as an API Gateway or Spring Cloud Gateway filter).

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🏗️ Architecture Overview

[Client Request]
        ↓
     [WAF Service]
  - Input Inspection
  - Threat Detection
  - Logging & Decision
        ↓
 [Target Microservice]
 (e.g., Blog API, Auth, Product Service)

Key Components:

• WafFilter — a Spring Boot filter that analyzes incoming requests.

• ThreatPatternRegistry — holds regex-based threat detection rules.

• RequestAnalyzer — performs scanning and risk scoring.

• WafController — optional endpoint to check health, metrics, or add dynamic rules.

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🧩 Project Setup

Create a new Spring Boot project.

🔍 Step 1: Define Threat Patterns

Create a class to register known malicious signatures

🧠 Step 2: Request Analyzer

Create a service that scans the request for these patterns

🚦 Step 3: WAF Filter

Add a Spring filter that blocks malicious requests before they reach controllers.

⚙️ Step 4: Application Entry Point

🧾 Step 5: Configuration

Add this to application.yml

🧪 Step 6: Test Your WAF

Run your WAF service.

☁️ Step 7: Deploying with Other Services

In a microservice ecosystem:

• Deploy waf-microservice as a reverse proxy or API gateway filter in front of others (e.g., Auth, Blog, Payment).

• Configure Kubernetes ingress or Docker Compose to route requests through WAF first.

Example:

[Client] → [WAF:9090] → [Auth:8081]
                    → [Blog:8083]
                    → [User:8085]

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🧩 Optional Enhancements

• Add JWT validation to allow/block by role.

• Integrate with Redis to track repeated attackers (IP blocking).

• Add Prometheus metrics for blocked requests.

• Add an admin UI for live rule management.

• Integrate with Kafka to publish threat logs to SIEM systems.

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✅ Conclusion

With just a few classes, we built a microservice WAF capable of catching basic OWASP Top 10 threats and protecting your backend APIs.
While not a replacement for enterprise-grade firewalls, this pattern is perfect for internal microservice environments, staging pipelines, or API-level protection.

Security should always be layered — combine this WAF with:

• Proper input validation and sanitization.

• HTTPS/TLS encryption.

• SAST and DAST tools (SonarQube, OWASP ZAP, Fortify).

• Cloud WAF (e.g., AWS WAF, Cloudflare) for production-grade traffic.

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🔗 Related Reading

• OWASP Top 10 Web Application Security Risks

• Building Secure APIs with Spring Boot and JWT

• Cloud-Native Microservices with Zero Trust Architecture