Introduction
Web Application Firewalls (WAFs) act as critical sentries for your application infrastructure, filtering malicious traffic before it reaches your servers. But with a growing variety of open-source WAFs available—each with its own philosophy, architecture, and trade-offs—it can be difficult to determine which one fits your specific use case. Whether you're running a high-performance NGINX stack, an Apache server, or a Kubernetes-based application mesh, there’s likely a WAF designed with your needs in mind. Here's a quick overview of the most prominent open-source WAFs, where they shine, and where they might fall short
Naxsi (NGINX Anti-XSS & SQL Injection)
Naxsi is an open-source, high-performance WAF module originally built for NGINX, though it now has limited support for Apache via proxy configurations. It uses a scoring-based model instead of traditional signature matching, making it efficient for detecting XSS and SQL injection attempts with low overhead. The original project has been archived, but the project continues to be maintained and has found a new home at this repository. Ideal for environments prioritizing performance and simplicity, such as microservices or edge proxy layers.
CrowdSec
CrowdSec is an open-source, behavior-based security engine that analyzes logs to detect malicious behavior and block attackers in real time. While not a traditional inline WAF, it works well as a complementary solution for both network and application layers. It supports agents across NGINX, Apache, SSH, and more. CrowdSec includes a collaborative threat intelligence network and offers a generous free tier for individual and small-scale usage. Best for dynamic or cloud-native architectures needing scalable, intelligence-driven protection.
ModSecurity (OWASP CRS)
ModSecurity is one of the most mature and widely adopted open-source WAFs. It supports real-time application monitoring, logging, and access control. It’s often used with the OWASP Core Rule Set (CRS), a powerful community-maintained security ruleset. Originally built for Apache, ModSecurity 3.x now supports NGINX and IIS, though the NGINX version lacks full parity with Apache in some features. Best suited for users needing comprehensive rule-based protection and deep inspection of HTTP traffic.
SafeLine
SafeLine is a high-performance, reverse‑proxy WAF incorporating intelligent semantic analysis to block SQLi, XSS, HTTP flood, and more. Its community edition brings enterprise-grade protection to your own servers, with Docker/Kubernetes support and real-time dashboards. Ideal for teams seeking robust, AI-driven proxy‑based security without vendor lock-in.
BunkerWeb
BunkerWeb is an open-source, full‑featured WAF server built for production readiness. Written in Python and pre‑bundled for Linux, Docker, Swarm, and Kubernetes, it aims for “secure by default,” and includes ModSecurity & OWASP CRS under the hood. It suits teams that want a turnkey WAF solution with flexible deployment.
Coraza WAF
Coraza is a modern, Go‑based WAF library fully compatible with ModSecurity’s ruleset (OWASP CRS v4+). Designed for cloud-native applications, it offers high performance, extensibility, and observability. Great for teams migrating off ModSecurity toward Go ecosystems.
open-appsec
Next, we have an emerging ML-first WAF, open-appsec blocks zero-day exploits like Log4Shell without signatures, using supervised and unsupervised models. Integrations include NGINX, proxy managers, Envoy, and Istio. Still in beta, but promising for proactive, adaptive defenses.
Comparison of WAFs
A thorough comparison of each WAF can be summarized in a tabular format for easy understanding. The following table has been generated with the help of for, and giving credit where it is due is just good practice.
The symbols used in the table are explained below.
✅ = Supported / Built-in
⚠️ = Partially / Requires Customization
❌ = Not Supported
| Feature | NAXSI | CrowdSec | ModSecurity | SafeLine | BunkerWeb | Coraza WAF | open-appsec |
|---|---|---|---|---|---|---|---|
| Signature-based Detection | ⚠️ Basic scoring | ❌ | ✅ OWASP CRS / custom | ✅ | ✅ | ✅ | ⚠️ ML + optional rules |
| Machine Learning / Behavior Analysis | ❌ | ✅ (log-based) | ❌ | ⚠️ Pattern + context | ❌ | ❌ | ✅ (core feature) |
| OWASP CRS Compatibility | ❌ | ❌ | ✅ | ⚠️ Partial | ✅ (via ModSec) | ✅ | ⚠️ No, ML-focused |
| Custom Rule Writing | ✅ (scoring rules) | ✅ (scenarios) | ✅ | ✅ | ✅ | ✅ | ✅ (policies) |
| Rate Limiting / DoS Protection | ❌ | ✅ | ⚠️ Requires extension | ✅ | ✅ | ⚠️ Custom logic | ✅ |
| IP Reputation / Threat Feeds | ❌ | ✅ (community blocklists) | ❌ | ✅ | ✅ | ⚠️ Possible via plugin | ✅ |
| XSS / SQLi Detection | ✅ (score-based) | ✅ (based on logs) | ✅ (rules) | ✅ | ✅ | ✅ | ✅ |
| Geo-blocking / ASN Filtering | ❌ | ✅ | ❌ | ✅ | ✅ | ❌ | ✅ |
| TLS Termination Support | ❌ (depends on NGINX) | ❌ | ❌ | ✅ | ✅ | ❌ | ✅ |
| HTTP/2 and WebSocket Support | ⚠️ (via NGINX) | ❌ | ✅ | ✅ | ✅ | ✅ | ✅ |
| Kubernetes / Cloud-native Support | ⚠️ Manual setup | ✅ | ⚠️ Needs effort | ✅ | ✅ | ✅ | ✅ |
| Built-in Dashboard / UI | ❌ | ✅ | ❌ | ✅ | ✅ | ⚠️ 3rd-party | ✅ |
| Logging & Analytics | ⚠️ (NGINX error log) | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| Docker Deployment Support | ⚠️ Community-built | ✅ | ⚠️ | ✅ | ✅ | ✅ | ✅ |
| Reverse Proxy Capable | ✅ | ❌ | ❌ | ✅ | ✅ | ❌ (library only) | ✅ |
| Language / Stack | C (NGINX module) | Go | C++ | Go | Go + Lua | Go | Python + Go |
| License | GPLv3 | MIT | Apache 2.0 | GPLv3 | AGPLv3 | Apache 2.0 | Apache 2.0 |
Conclusion
Usage of WAF highly depends on the infrastructure you are using, and certainly your requirements for the WAF. No matter which WAF you select, it is important to note that using a WAF certainly improves the security of your tech infrastructure. In the upcoming days, a comprehensive installation tutorial shall be written for some of the popular WAFs mentioned above.
