Websites hosted on the Internet are continuously inundated with unwanted traffic coming from bots (robots, and automated programs that trawl the internet for vulnerable websites), hacktivists, and focused attack groups. The intent is simple – exploit vulnerabilities to:
- Deface a website
- Steal sensitive information (usernames and passwords, sensitive private information, etc.)
- Compromise the web server and add it to their bot network. The bot can then be used to target other vulnerable websites and networks.
Thousands of websites get hacked every day due to vulnerabilities in code or misconfigurations. One of the best ways to protect your web application from online threats is to use a Web Application Firewall (WAF) or Web Application and API Protection (WAAP) solution.
A WAF / WAAP solution helps protect websites and APIs by inspecting and filtering traffic between the web servers and the Internet. The solution can help defend web applications from attacks such as SQL Injection, File Inclusion, Cross-Site-Scripting, Remote Code Execution, etc.
Many Commercial WAF / WAAP solutions exist but over the years, many open source web application firewall and freemium WAF solutions have also come to the aid of website owners to help secure their web applications. Below are some of the top open-source/freemium solutions:
List of Top free WAF (Web Application Firewall)
1. Haltdos WAF Community Edition
Type: Freemium
Haltdos WAF CE is a free version by Haltdos. It is a high-performance Web Application and API Protection solution designed to safeguard Web Applications and APIs. Unlike many other open-source WAF solutions in the list, Haltdos uses a new HTTP request processing engine designed for handling performance, creating complex rules, and mitigating sophisticated attacks. Supports complex attack mitigation techniques such as captcha, rate limiting, anomaly detection, redirection, request termination, and connection termination.
Pros of Haltdos WAF CE:
- Support for protection against OWASP Top 10 attacks including SQL Injection, XSS, RFI, LFI, RCE, etc.
- Built-in 1000+ rules with daily threat intel from Haltdos
- GUI managed WAF
- Support for Anti-Bot and API security
- Built-in load balancing and server monitoring
- Easy False Positive management
- Good documentation
Link: Free Haltdos Community WAF
2. SafeLine WAF
Description: High-performance reverse proxy with built-in intelligent WAF, ideal for penetration testing, security experimentation, and production use.
Key Features:
- Blocks SQLi, XSS, HTTP Floods via semantic analysis
- Real-time dashboard, Docker/Kubernetes support
- Self-hosted with full data control
GitHub: SafeLine GitHub
Best For: Red teams, bug bounty hunters, and developers.
3. ModSecurity + OWASP CRS
Description: Industry-standard WAF engine with OWASP Core Rule Set (CRS). Note: ModSecurity reached End-of-Life (EOL) in 2024, but CRS remains updated.
Key Features:
- Signature-based threat detection (SQLi, XSS)
- Highly customizable rules
- Integrates with Apache, Nginx, IIS
GitHub: ModSecurity | OWASP CRS
Best For: Legacy environments needing proven, customizable protection
4. Coraza WAF
Description: Modern ModSecurity replacement written in Go, optimized for cloud-native apps.
Key Features:
- Supports OWASP CRS v4.x
- High performance with low latency
- Integrates with OpenTelemetry for observability
GitHub: Coraza
Best For: Teams migrating from ModSecurity to future-proof solutions
5. NAXSI
Description: Lightweight WAF for Nginx, using scoring-based blocking.
Key Features:
- Whitelist-centric security model
- Minimal resource consumption
- Blocks XSS, SQLi via pattern analysis
GitHub: NAXSI
Best For: Nginx users seeking simplicity and performance
6. open-appsec
Description: Machine-learning-powered WAF blocking zero-day threats without signatures.
Key Features:
- Stops OWASP Top 10 threats proactively
- Auto-tuning to reduce false positives
- Kubernetes-native deployment
GitHub: open-appsec
Best For: AI-driven prevention against emerging attacks
7. Shadow Daemon
Description: Language-specific WAF for PHP, Python, and Perl apps.
Key Features:
- Analyzes requests using multiple algorithms
- Modular architecture with web UI
- Deployable as standalone or module
GitHub: Shadow Daemon
Best For: Developers securing PHP/Python/Perl applications
8. Lua-resty-WAF
Description: WAF framework for OpenResty (Nginx + Lua), enabling custom security logic.
Key Features:
- Extensible via Lua scripting
- Supports OWASP CRS
- High-performance traffic inspection
GitHub: Lua-resty-WAF
Best For: Advanced users building behavior-based rules
9. WebKnight
Description: Customizable WAF for IIS servers.
Key Features:
- Real-time traffic analysis
- Anomaly detection
- GUI-based configuration
Download: WebKnight
Best For: Windows-centric environments
10. IronBee
Description: Qualys-developed open-source WAF with real-time monitoring.
Key Features:
- Flexible deployment (cloud/on-prem)
- Protocol-agnostic inspection
- Community-driven rules
GitHub: IronBee
Best For: Experimental use and research
11. New Addition: FreeWAFF
Description: Emerging community-driven WAF with automated rule generation (released 2024).
Key Features:
- AI-assisted policy creation
- Integrated IP reputation database
- Slack/Telegram alerting
GitHub: FreeWAFF
Best For: Startups needing automated, low-maintenance securit
Key Trends in 2025:
- ML-Powered Protection: Solutions like Haltdos, open-appsec and SafeLine use AI to reduce false positives 110.
- API-First Security: Haltdos, Coraza and Shadow Daemon prioritize API threat blocking 813.
- Cloud-Native Focus: Kubernetes support in SafeLine/open-appsec simplifies scaling 46.
