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The Best Open Source Web Application Firewalls

The Internet is comparable to the Wild West. When a website goes live, it is immediately inundated with negativity from all angles. The vast majority of this traffic consists of bots (from robots, and automated programs that trawl the internet for vulnerable websites), which do not adhere to speed constraints.

When vulnerabilities are exploited by bots, this can lead to a website being defaced, data being stolen (usernames and passwords, sensitive private information, etc.), or even becoming a zombie bot among other zombie bots that go on to infect other vulnerable websites.

Botnets, with the computing power and internet bandwidth held captive by them, are eventually rented out to the highest bidder on the black market. There, vulnerabilities may inflict a variety of very real-world harms.

If you own an internet business, you must prevent hackers from destroying your website. If your website becomes infected with hacker code, search engines will not link to it. Protect your organization with a firewall for web applications.

So, on top of the regular security best practices to prevent bad things from happening, occasionally a WAF is implemented as a first line of protection.

A web application firewall (WAF) is a form of application firewall that provides visibility and analysis of HTTP(S) traffic to and from an online application. Its purpose is to thwart attacks designed to refuse service and steal data. It grants the administrator direct control over the requests and replies flowing through the system without requiring modification of the backend code. A WAF differs from a conventional firewall in that it protects a particular online application or group of web apps. And it does so without interacting with online apps.

There are several free WAFs to protect your web applications. The most delicate aspect of open-source WAF is the ability to customize the code based on your projects.

Through this article, the pros and cons of using open source WAF solutions, the best open source WAF solutions, must-have WAF solution features, and security concerns with open source WAF solutions will all be covered.

What is a Web Application Firewall (WAF)?

Figure 1. What is a Web Application Firewall (WAF)?

What are the Advantages and Disadvantages of Free Open Source WAFs?

Open-source WAFs are extremely adaptable and configurable, making WAF technology accessible to companies that cannot afford commercial WAFs. Here are the advantages of using an open-source WAF solution.

  • Cost Benefits: Use an open-source Web App Firewall if you do not wish to spend a significant amount on web application security.
  • No Vendor Lock-in: In addition to cost benefits, there is no possibility of vendor lock-in, which makes switching to a different product or provider difficult in the future. Depending on the underlying license, businesses can alter open source WAF solutions to build unique solutions
  • Developer Community Support: Another important advantage is the developer community's normally high degree of dedication. Since a large number of independent developers collaborate on the code and regularly test it, flaws and vulnerabilities are frequently discovered and eradicated rapidly. This has a favorable impact on the software's quality, stability, and security.
  • Start Small, then Grow: With open source, you may start small and rapidly with community versions before migrating to a commercially supported solution as your company's needs dictate. If the project does not require assistance, the community version can be used indefinitely. You have the option of evaluating many options, selecting the one that will work, and then scaling up with a commercial solution.

Besides the advantages of open source WAF solutions, there are some disadvantages as well:

  • Distributed Attacks: WAFs utilizing open-source frameworks and code are vulnerable to widespread flaws. Due to the fact that multiple systems utilize the same framework, hundreds of thousands of applications are susceptible to any newly discovered vulnerabilities. Once these vulnerabilities are uncovered, firms scurry to develop a fix before they are exploited by attackers. This indicates that open-source WAFs cannot be depended upon to stop attacks in real-time.
  • Can be Bypassed: The majority of open-source WAFs utilize software with exploitable flaws. Open-source WAFs feature fail open and fail close events when excessive traffic is detected. During a fail open, a WAF does merely monitoring and so allows all traffic, including possibly malicious data, to pass through. In the event of a fail-close, all traffic is halted. A DoS or DDoS attack might circumvent the WAF, limiting full application access.
  • Zero Day Vulnerabilities: A zero-day vulnerability is an attack that is unknown to a cybersecurity professional and only known to the attacker. Therefore, it takes time for a cybersecurity expert to develop a patch against a zero-day vulnerability. In the meantime, an attacker can compromise the system using this opportunity. The majority of open-source WAFs are unable to defend against these types of cyber attacks. To keep a WAF current with invasive zero-day attacks, developers must often and comprehensively change system rules, which is impossible.
  • Configuration and Maintenance Issues: Open-source WAFs often need to be configured immediately after installation. In certain cases, they need considerably more labor than regular firewalls. For optimal protection, knowledge of both the open-source WAF and the application it is being installed on is essential. If you lack this type of security knowledge in-house, you must outsource it, which is expensive. Given the value of the assets they protect, professionals in this field command a hefty salary. You wouldn't want a novice to muck up your cybersecurity frameworks via incompetence. Open-source WAF network maintenance is extremely labor-intensive. Due to the adaptability of web apps, they are always evolving and need maintenance. Users are often in need of new features and regular updates. Not to mention the rapidly changing environment of computers.

What are the Best Open Source WAFs?

The following open-source Web Application Firewall might be helpful if you are seeking a free alternative to commercial WAF to safeguard your website:

  • WebKnight
  • Shadow Daemon
  • Coraza
  • OctopusWAF
  • IronBee
  • ModSecurity


NAXSI is an acronym for Nginx Anti XSS and SQL Injection. Technically, it is a third-party Nginx module that is bundled with several UNIX-like systems. By default, this module reads a limited selection of basic (and understandable) rules encompassing 99 percent of known patterns associated with website vulnerabilities.

Naxsi has a basic ruleset and is expandable with user-defined rulesets. The configuration occurs in the context of Nginx. The WAF is adaptable to various contexts and web applications by virtue of scores for individual rules and configurable thresholds for blocking operations.

Naxsi may examine many data, including URLs, request parameters, cookies, headers, and the POST body, and it can be enabled or disabled at the location level in the Nginx configuration. Automatic whitelist generation simplifies upstream firewall deployment and eliminates any false positives. Other applications, such as NX-Utils and Doxi, simplify administration, report production, and ruleset modifications.

NX-utils, which are included with Naxsi, are highly useful for producing whitelists and reports. First, the NX-utils collection consists of intercept mode, which enables Naxsi to record requests stopped by the WAF for future reports and whitelists in a database, and report mode, which visualizes the saved events. A future version of NX-Utils will enable enhanced report processing and filtering in order to evaluate WAF events with greater precision.

NAXSI, which is a loadable module for the Nginx web server, is utilized by the OPNsense WAF. NAXSI has two sorts of rules:

  • Main Rules: These rules apply internationally. Blocking code snippets that might be exploited to obtain unauthorized access to the server (such as SQL-/XPATH-injection for data access) or to take control of a foreign client (for example XSS).
  • Basic Rules: These rules are typically used in places to whitelist primary rules by ID or for supplementary rules.

NAXSI, unlike the majority of Web Application Firewalls, does not rely on a signature database like an antivirus, and so cannot be bypassed by an "unknown" attack method. Naxsi means free software (as in freedom) and free to use.

NAXSI works as a DROP-by-default firewall. NAXSI only filters GET and PUT requests, and its default setup acts as a DROP-by-default firewall, thus you must add the ACCEPT rule for it to function properly.


Given the rise in online dangers, protecting a web application is always a challenge. You should investigate every option for protecting your website from hackers. If you wish to secure an IIS-hosted website, you should consider using WebKnight WAF.

AQTRONiX's WebKnight is an open-source web application firewall for IIS web servers. It prevents harmful requests from reaching the IIS by scanning all requests.

All blocked requests are logged by default, and you may modify this to suit your needs. WebKnight 3.0 has an admin web interface for configuring rules and doing administrative chores, including statistics.

Some of the prominent features of WebKnight are:

  • Interface for administration - handy for managing WebKnight and statistics
  • Logging - log requests that are blocked or handled by WebKnight
  • Use with WebDAV, Cold Fusion, OWA, Share Point, etc
  • Using brute force against defenses
  • Block IP that is handy for blocking incoming requests from known malicious IP addresses
  • Hotlinking security
  • Robots obstructing
  • Examine both GET and POST payloads
  • Run-time update - there is no need to restart IIS while updating WebKnight
  • SSL session encryption
  • SQLi, XSS, CSRF, and data loss prevention(DLP)

Let's take a look at what benefits it has in addition to the features listed above. Here are some of the benefits of WebKnight:

  • Protects your web application from threats and also stops malicious robots.
  • Provides information about the ongoing attacks and increases the blue team's visibility.
  • Achieve PCI DSS compliance requirements.
  • Open source (GNU GPL) with just support fees.

Shadow Daemon

Shadow Daemon is a suite of tools designed to identify, record, and prevent web application attacks. The Shadow Daemon is technically a web application firewall that intercepts requests and removes harmful parameters. It is a modular solution that isolates online applications, analyses, and interfaces in order to boost security, flexibility, and scalability.

Shadow Daemon is a free application. It is offered under the GPLv2 license, meaning the source code can be studied, updated, and distributed by anybody.

The Shadow Daemon is simple to install and maintain using a well-organized online interface that enables in-depth analysis of threats.

The interface also includes shell scripts for sending weekly reports via email, rotating logs, and similar tasks.

The Shadow Daemon can identify common attacks such as:

  • SQL injections
  • XML injections
  • Code injections
  • Command injections
  • Cross-site scripting
  • Local/remote file inclusions
  • Backdoor access

The Shadow Daemon, unlike many other web application firewalls, does not entirely block malicious requests whenever feasible. Instead, it removes just the potentially harmful components of a request before allowing it to proceed. This prevents attacks while not needlessly frustrating visitors in the event of false positives.

Shadow Daemon is for those who wish to host their own dynamic website without always worrying about vulnerabilities and attacks.

Shadow Daemon is for those who wish to determine whether and how their website is under attack.

Shadow Daemon is for those who do not wish to blindly rely on closed-source, expensive, and secretive software.


Coraza is an open-source, enterprise-grade, high-performance Web Application Firewall (WAF) designed to safeguard your most cherished apps. It is developed in the Go programming language, supports ModSecurity and SecLang rule sets, and is fully compatible with the OWASP Core Rule Set.

Coraza is a drop-in replacement for the soon-to-be-discontinued Trustwave ModSecurity Engine, and it supports SecLang rule sets as an industry standard.

The main features of Coraza are as follows:

  • Security: Coraza utilizes the OWASP Core Rule Set (CRS) to defend your web applications from a broad variety of threats, including the OWASP Top Ten, with minimal false positives. CRS defends against a variety of typical attack types, including SQL Injection (SQLi), Cross Site Scripting (XSS), PHP & Java Code Injection, HTTPoxy, Shellshock, Scripting/Scanner/Bot Detection, and Metadata & Error Leakages.
  • Extensible: At its heart, Coraza is a library with several connectors for deploying on-premise Web Application Firewall instances. Create your own audit loggers, persistence engines, operators, and actions to expand Coraza as much as you like.
  • Performance: Coraza can manage large websites and tiny blogs with a minimum performance effect.
  • Accessibility: Anyone may comprehend and alter the Coraza source code. It is simple to add additional features to Coraza.
  • Community: Coraza is a community-driven endeavor; contributions are welcome and all suggestions are examined.

A Golang compiler v1.16+ is a prerequisite. Linux distribution (Debian or Centos recommended, Windows not supported yet) is available now.


OctopusWAF is an open-source Web application firewall written completely in C that makes numerous connections using libevent. The event-driven design is geared for many concurrent connections (keep-alive), which is essential for AJAX applications with high speed. This tool is quite lightweight. You may use it in any desired manner. This resource is ideal for securing particular endpoints that require customized security.

  • Reverse proxy capability
  • Detect anomalies using regular expressions and lib PCRE resources
  • Detect security anomalies using string-matching algorithms such as DFA, horspool, and karp-rabin
  • Detect anomalies in security using libinjection
  • Options for log saving


At the 2011 RSA Conference, Qualys, Inc., the leading supplier of on-demand IT security risk and compliance management solutions, introduced IronBee, a new open source project that will deliver the next generation of web application firewall (WAF) technology.

Increasing web application usage and the shift to cloud computing require the deployment of WAF technology to secure data and comply with requirements such as payment card industry (PCI) compliance. With the release of IronBee, Qualys is establishing a community of commercial and open source contributors that will enable businesses of all sizes to adopt next-generation WAF technology to safeguard their data and IT assets.

IronBee supply:

  • Modern application security assessment engine that offers new processing capabilities and HTTP traffic analysis.
  • Apache Software License v2 is a non-viral open source license that permits participation from both people and commercial enterprises, thus building a community of both users and creators.
  • Built from the ground up for numerous deployment options, including passive, embedded, out-of-process, and reverse proxy.
  • Modular architecture enables contributors to simply build their own modules without having a thorough grasp of the IronBee architecture, as well as facilitates the packaging of configuration information and modules based on user requirements.
  • Community-based effort to collect, consolidate, and distribute the information required to defend web applications.


ModSecurity sometimes referred to as Modsec, is an open-source web application firewall (WAF). Originating as a module for the Apache HTTP Server, it has grown to include a variety of Hypertext Transfer Protocol request and response filtering capabilities as well as other security features across several platforms, including Apache HTTP Server, Microsoft IIS, and Nginx. It is free software distributed under the Apache 2.0 license.

Effective July 1, 2024, Trustwave will no longer provide support for ModSecurity. The maintenance of the ModSecurity code will thereafter be returned to the open-source community.

The NGINX ModSecurity WAF is a web application firewall (WAF) based on ModSecurity 3.0, a rewrite of the original ModSecurity software that functions as a native dynamic module for NGINX Plus. The NGINX ModSecurity WAF may be used to prevent a wide variety of Layer 7 attacks and adapt to new threats with virtual patching. Despite having a free version, NGINX ModSecurity is not an open-source project, so keep that in mind.

What are Must-Have Features of the WAF?

When searching for a WAF for your server, several open source choices will appear in the search results. Open source projects give a clear image of what is required in a web application firewall and how they function, making this a suitable starting point.

The Open Web Application Security Project (OWASP) is a non-profit organization devoted to making software and server security "transparent so that individuals and businesses may make educated decisions".

You may learn about the top ten web application security issues on their wiki. The OWASP Top 10 emphasizes the most important security problems when designing or deploying a WAF for a server. This is the primary attack that a WAF is meant to prevent, and the list explains how a WAF secures your server.

Here are five aspects that are essential when selecting a WAF.

  • Integration: As with other aspects of security, providing the proper protection begins with an analysis of the object being protected. You may already have assets protected by a hardware WAF. A cloud-based WAF can be placed in front of these devices to offload more common web attack traffic. You likely have some cloud-based assets, and if you're like the majority of businesses today, you're either pursuing or considering a multi-cloud strategy. In this circumstance, it is essential to consider an environment-independent security solution.
  • Positive and Negative Security: It is necessary to consider many sorts of defensive postures, beginning with positive or negative security. A negative security posture presumes that all communication is permitted unless it contains a previously detected danger or attack. This is the most common deployment style for WAFs, and it is clear to understand why: a negative approach is significantly less likely to block genuine traffic. Clearly, the effectiveness of this strategy depends on the status of the security vendor's signature rule database and awareness of impending threats, since this defines the expected level of protection. If you adopt this posture, it is crucial that your databases keep up with evolving attacks as they are produced and mutate over time. This model will not detect zero-day threats since, by definition, these cyber attacks have no associated signatures. The positive security approach, on the other hand, holds that traffic is rejected unless it is expressly approved. This strategy will identify zero-day threats as well as assaults using malformed packets or non-RFC-compliant traffic. A positive security strategy relies on traffic heuristics and automatic learning, enabling you to fit the profile to the traffic.
  • Learning Mode: To achieve optimal security, it is essential that the service/device "learns" from its own experiences. This is a crucial aspect to which the service is well positioned to contribute. Because security teams are frequently sufficiently isolated from development teams, they may lack knowledge of program components or what defines "acceptable conduct". Learning mode observes the traffic traversing the device and gives recommendations regarding which relaxation rule if any, should be implemented.
  • Customization: Vulnerabilities and attack signatures are common WAF components. It is crucial that businesses remain at the forefront of vulnerability research and periodically provide signatures for securing vulnerable services and open source libraries. It is essential to be able to include signatures from other sources, such as industry ISACs and third-party vendors. Additionally, it should be straightforward for you to add your own signatures to the WAF rules based on your own expertise and knowledge.
  • Easy to use: You should be able to choose from a wide variety of controls and apply them in granular form, including the ability to apply policies to groups of apps. While deploying a WAF may be a given, having a WAF that can be readily updated and maintained is a competitive advantage. Due to the ongoing evolution of threats, a WAF must be adaptable enough to stay up.