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Top 10 Open-Source WAFs for OWASP-Top-10 and Zero-Day Protection

In today's digital age, web applications are the backbone of most businesses and organizations. They offer an effective way to reach a wider audience and provide a better user experience. However, they are frequently attacked by malicious users, leading to data breaches, financial losses, and reputational damage. To mitigate these risks, Web Application Firewalls (WAFs) are used to protect web applications from various types of attacks.

Keeping this in view, this article will discuss popular open-source web application firewalls and their key features, including the benefits of going open-source to protect your web app. By the end of this article, you will better understand the best open-source WAFs available and be able to choose the right one for your web application security needs.

Let’s get started.

Why Should You Use an Open-Source WAF?

Open-source WAFs offer several benefits over proprietary, closed-source solutions. Firstly, open-source WAFs are community-driven, meaning developers worldwide can contribute to the codebase, suggest new features, and identify and patch vulnerabilities.

This collaborative approach leads to frequent updates and a more comprehensive set of security rules, which can help keep your web applications safe from the latest threats.

Secondly, open-source WAFs are often more flexible and customizable than their proprietary counterparts. Access to the source code allows you to modify and tailor the WAF to suit your specific security needs without being locked into a vendor's solution. This can be particularly useful for complex web applications requiring specific security rules or customizations.

Thirdly, open-source WAFs are generally more cost-effective than proprietary solutions as they are often free to use and don't require expensive licensing fees or hardware. This can be especially beneficial for small or medium-sized businesses with limited budgets that still need robust web application security.

Overall, open-source WAFs offer a powerful, flexible, and cost-effective solution for securing your web applications, with the added benefit of a large and collaborative developer community.

Best WAF Solutions of All Time

open-appsec Web Application Firewall

Are you looking for a way to block web attacks on your web apps before they happen? Look no further, as open-appsec uses two machine learning algorithms to continuously detect and preemptively block threats before they can do any damage. Not only has open-appsec code been published on GitHub, but the effectiveness of its WAF has also been successfully proven in numerous tests by third parties.

The first algorithm is offline and supervised. It analyzes incoming requests against a database of known malicious threat indicators. This algorithm assigns threat scores to each request based on the level of risk. If a request is deemed legitimate, it is allowed access to the web application, but if it's flagged as malicious, it is pushed to the second machine learning algorithm.

The second algorithm is online and unsupervised; it analyzes the malicious request against the structure and behavior of your web application. It uses several criteria to evaluate the request. A few of them include the following:

  • User's Reputation Score

  • Payload Score

  • URL Score

  • Parameter Score

Based on this evaluation, the algorithm will block or permit the request to access the web application. The primary purpose of this unsupervised online ML algorithm is to reduce the chances of false positives, ensuring that legitimate requests are not blocked or prevented from accessing the web application.

In general, these two machine learning algorithms work together to provide comprehensive and effective protection against known and unknown threats while minimizing the risk of false positives. Simply, it protects web applications against numerous attacks, including the OWASP Top 10 attacks and zero-day attacks such as Log4Shell, Text4Shell, and Spring4Shell.

Furthermore, the WAF integrates easily with popular technologies such as Nginx, Kubernetes, and Envoy, making it a flexible choice for businesses that use these technologies. It is also easy to set up and is managed with a cloud-native deployment process that uses declarative APIs and infrastructure-as-code. This is all connected through a secure and encrypted port 443 connection, and through this, an open-appsec agent connects to the Fog that provides the following services to tighten your app’s security:

  • Registration

  • Policy Update

  • Configuration Update

  • Software Updates

  • Logging and Learning Data Synchronization

Most traditional WAFs use signatures to protect against known vulnerabilities, but these signatures are less effective in guarding against unknown vulnerabilities. While some WAFs attempt to overcome this limitation by increasing the scope of their signature database, this can increase false positives, which can be frustrating for users.

To address these issues, the open-appsec WAF uses machine learning to provide more robust protection against known and unknown attacks. Hence, by using machine learning, the WAF can detect patterns and anomalies in the traffic that may indicate an attack, even if it has not been seen before. Try open-appsec in the Playground today.


  • It is an open-source solution, making it accessible to a wide range of users.

  • It provides preemptive protection against attacks, ensuring that web applications are protected from known and unknown vulnerabilities.

  • It has very few false positives due to its machine-learning app security approach.

  • The absence of threat signatures, rules, and exception handling simplifies system maintenance.

  • It provides fast technical support, ensuring users can quickly resolve any issues.

  • It has multiple integrations with popular technologies such as Nginx, Kubernetes, and Envoy.

  • It does not increase web latency, ensuring web applications maintain optimal performance.

  • It discovers and protects APIs.

  • The use of the Snort protection engine allows simple rule creation.


  • It may not have the same level of community support and resources as more established WAFs – although this will improve with time.

Key Features of open-appsec WAF

  • Intrusion Prevention (uses Snort Engine)

  • Machine Learning-Based Threat Prevention

  • SaaS Security Management

  • API Discovery and Security

  • Anti-Bot

  • Infrastructure-As-Code


The open-appsec WAF is available in three pricing plans to meet the needs of different users.

The first plan is the Community Edition, which is free of charge. This version includes the following:

  • ML-based WAF

  • API Security

  • Basic WAF Features

The second plan is the Premium Edition, which offers a pay-as-you-go plan based on the number of HTTP requests monitored and analyzed. This plan includes the following:

  • Automatic IPS Updates

  • Standard 24/7 Support

  • Additional Features to Enhance Security

The third plan is the Enterprise Edition, an annual payment plan based on 100 million HTTP requests. This version includes the following features:

  • A Gateway Virtual Machine

  • ThreatCloud Anonymizer Blocks

  • Enterprise 24/7 Support

  • Other Advanced Features

Overall, the open-appsec WAF pricing plans are designed to provide flexibility and scalability for users, allowing them to choose the best plan that suits their needs and budget while ensuring maximum protection for their web application.

NAXSI by Nginx

Nginx is a multifunctional web server that works as a load balancer, reverse proxy, and HTTP cache service. Combined with the NAXSI module, it is an open-source WAF that protects web applications against SQL injections and cross-site scripting (XSS) attacks.

Nginx Anti XSS and SQL injection (NAXSI) works by monitoring the activities of GET, POST, and PUT HTTP requests. It checks these requests against a default rule set in the naxsi_core.rules files and blocks all suspicious-looking or malicious requests. Unlike most other WAF and antivirus software, NAXSI does not use signature-based detection, which, in turn, prevents 'unknown' signatures from accessing your server.

These rules are comprehensive and cover all attack indicators for malicious attacks and all possible variants. It essentially works as a drop-by-default firewall. Because this WAF approach tightens your app's security surface and increases the chances of false positives, Nginx NAXSI offers a whitelisting feature. This allows you to permit any legitimate request types that might remotely resemble the attack indicators and their variants in the core rules.

Key Features of NAXSI WAF

  • NAXSI Rules

  • XSS and SQLi Protection

  • WAF Learning and Live Mode

  • Whitelist and Exclusion List

ModSecurity Web Application Firewall by ModSecurity

ModSecurity (ModSec) is an open-source WAF that filters incoming requests against core and commercial rulesets to detect and prevent typical web application attacks like XSS and SQL injection. It ensures web app security by monitoring incoming requests and comparing them against a combination of core and commercial rules to provide comprehensive app protection. It can also be configured with the OWASP Top 10 Core Rule Set available on GitHub.

It was created to monitor web traffic for applications hosted on the Apache HTTP server. But the newer version, ModSec 3.0, is not limited to Apache. It has a central library that can easily connect to different servers, such as Nginx, Apache, and Microsoft IIS.

Note: ModSecurity went into end-of-sale on April 1, 2022, and will reach end-of-life on March 31, 2024.

Key Features of ModSecurity WAF

  • Core and Customized Ruleset

  • Persistent Storage

  • Layer 7 Protection

  • IP Reputation

  • Audit Logging

  • OWASP Top 10 Protection

IronBee Web Application Firewall by Qualys

The IronBee WAF is an open-source web application firewall that uses managed and custom rules to ensure the security of your web application. Qualys is fundamentally known as a security company that scans web applications for vulnerabilities at intervals. They've had cases of vulnerability exploitation between web app scans, so they developed the IronBee WAF to give real-time visibility on the activities of a web app and offer effective security.

It can be used in multiple deployment modes and has an easy layout where admins can change rules, create exceptions, respond to events, etc. IronBee was designed as a highly portable framework that can easily be extended and embedded. At its core, IronBee is just a shared library that exposes an API and loads external modules to extend functionality. This framework separates data acquisition and configuration from its core.

Moreover, its architecture and mode of operation are very similar to that of ModSecurity, as it was led by the team that built ModSecurity. The only difference is that IronBee uses an Apache 2.0 software license with no copyright limitations. It routes all incoming and outgoing requests to the Qualys server to scan for threats before it applies the configured rules.

Additionally, the IronBee WAF is easy to implement even if you don't have an in-depth knowledge of its architecture.

Key Features of IronBee WAF

  • Flexible Data Acquisition, Operation, and Deployment Modes

  • Data Model Based on Real-Life User Activities

  • Apache Software License v2.0

  • Inbound and Outbound Traffic Analysis

  • Multiple Request Pattern Matching

  • User-Agent Profiling

Octopus Web Application Firewall

This is a unique open-source web application that is written in C language. It uses libevent to make multiple connections and is optimized for many keep-alive connections, which are essential for high-speed AJAX applications and help speed up a web page. It does this by maintaining a connection between a client and server to reduce the time needed to serve files.

The Octopus WAF focuses on providing tight security to web apps while keeping its latency as low as possible. Furthermore, to ensure the security of web apps, it compares incoming web requests against a set of rules configured by the administrator. It allows you to flexibly customize rules and exceptions to protect specific endpoints or your app's security in general. It is flexible, lightweight, and protects web apps against common attacks.

Key Features of Octopus WAF

  • Reverse Proxy

  • Security Detection Using Algorithms

  • Log-Saving Options

  • Extremely Low Latency

  • Minimal Installation Requirements

Coraza Web Application Firewall by Zup IT Innovation

Coraza WAF is one of the few enterprise-grade WAFs. It is open-source, can be used commercially (Apache 2.0 license), and has been written in the Go language. Coraza is popular for its scalability and flexibility, as it can handle large loads, requests, and traffic with minimal effort.

It is also compatible with the OWASP core ruleset and ModSecurity ruleset and allows administrators to customize these rules to suit their app's security requirements. This Core Rule Set (CRS) protects from many common attack categories, including the following:

  • SQL Injection (SQLi)

  • Cross-Site Scripting (XSS)

  • PHP and Java Code Injection

  • HTTPoxy

  • Shellshock

  • Scripting, Scanner, and Bot Detection

  • Metadata and Error Leakages

Furthermore, Coraza WAF is simple to use and can be imported as a library or used with connectors like Coraza-Caddy, Docker, Coraza-Server, etc. You can extend its capabilities (including its operators, audit engine, etc.) to suit your app's security needs. The simplicity of its source code allows everyone to understand it at its core.

Finally, you can embed the Coraza WAF in your web server as a reverse proxy, transport proxy, or traffic mirror.

Key Features of Coraza WAF

  • App Security Using the Core Ruleset

  • Flexible Deployment and Integration

  • Scalability and Accessibility

Shadow Daemon Web Application Firewall

Shadow Daemon is an open-source WAF that uses customized rules and signatures to protect web apps against attacks like XML, SQLi, code and command injections, backdoor access, etc.

It is situated at the application level and uses small connectors to intercept requests. Its request parsing procedure feature is divided into whitelisting, blacklisting, and integrity checking. To successfully whitelist, Shadow Daemon WAF checks all incoming requests to ensure they look like they should.

Similarly, to successfully blacklist, it searches for common attack patterns using sophisticated regular expressions configured by the admins in its core rules. Lastly, to successfully carry out integrity checks, it compares encrypted requests against the checksum values in the configured rules. Although the integrity check prevents the execution of a known script, it will not prevent dynamically encoded scripts.

Here's how Shadow Daemon differs from other WAFs: it doesn't completely block malicious requests. Rather, it filters out the harmful part and allows the harmless part to access the web server. This approach reduces false positives while ensuring the app's security is prioritized.

Key Features of Shadow Daemon WAF

  • Request Blocking (uses Integrity Checks, Whitelisting, and Blacklisting)

  • Secure Architecture

  • Simple, Secure, and Auditable Modules

WebKnight Web Application Firewall by AQTRONIX

This is an open-source WAF that protects IIS-hosted web applications. It protects web apps from several common attacks, including the following:

  • SQL Injection

  • Cross-Site Scripting

  • Cross-Site Request Forgery

  • Data Leakage

It scans GET and POST payloads and compares them against configured rules (no attack signatures) to phish out and block malicious payloads.

Due to the absence of attack signatures, it is easy to maintain and doesn't require you to manually restart IIS when you make changes to the WAF. Also, it is compatible with WebDAV, SharePoint, OWA, and ColdFusion.

Furthermore, WebKnight WAF also offers protection against bot and brute force attacks and has an admin web interface where you can check the security status of your web app and configure rules. Unlike some other open-source web application firewalls, WebKnight effectively scans and decrypts encrypted traffic to check for malicious activities in requests. It is one of the few free open-source WAFs that provides email support.

Key Features of WebKnight WAF

  • Hotlinking Protection

  • SSL Session Encryption

  • Run-Time Update

  • Brute Force Attack Protection

  • GET and POST Payloads Scanning

  • SQLi, XSS, and Information leak Protection

Vulture Web Application Firewall by The Honeynet Project

Vulture WAF uses an AI engine to discover anomalies in web requests. It also works as a load balancer, network sniffer, and log normalization, and was built on FreeBSD, Apache, Redis, and MongoDB. It works as a cluster and is horizontally scalable, allowing you to add as many nodes as you want.

It works with three main modules:

  1. mod_vulture

  2. mod_defender

  3. mod_SVM.

The job of mod_vulture is to authenticate users (using LDAP or active directory) and check IP reputation. It uses ModSecurity WAF architecture with additional patches to make it work in clusters.

On the other hand, Mod_defender uses Apache-ported NAXSI WAF architecture to defend against attacks. While the last and most important module, the mod_SVM (Support Vector Machine), uses machine learning to collect logs and create a mathematical representation of good traffic. When this is done, Vulture WAF checks all incoming traffic against these mathematical expressions and marks all requests without these expressions as malicious.

Key Features of Vulture WAF

  • Darwin Engine for AI-Based Anomaly Detection

  • Caching and Compression

  • Stream Encryption

  • HAProxy for Load Balancing

Lua-resty Web Application Firewall by OpenResty

Lua-resty is an open-source reverse proxy WAF built on the OpenResty stack. It uses the ModSecurity ruleset (with a tooling feature for translation) and customized rules to protect against known attacks. The translation feature allows users to input rules from previous WAFs without learning a new ruleset. It also protects web applications against unknown attacks using a virtual patch set.

In addition to this, it uses the Nginx architecture to parse large amounts of traffic smoothly during peak times, Lua Ngx API to analyze HTTP requests to detect and block malicious attacks, and Nginx asynchronous processing mode to reduce web latency.

Key Features of Lua-resty WAF

  • Traffic Behavioral Analysis

  • Interactive Platform Monitoring and Protection

  • Real-Time DNS Listing for Malicious Requests

  • Memcached Storage

  • Ruleset Translation from Other WAFs


Open-source web application firewalls are popular for their transparency, flexibility, community support, and cost-effectiveness. Not to mention that they provide as much app security, if not better, than proprietary WAFs. Most open-source WAFs are free, but unique ones like open-appsec offer a pricing plan to give additional technical support to their customers. Try open-appsec in the Playground today.


Experiment with open-appsec for Linux, Kubernetes or Kong using a free virtual lab

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