ModSecurity

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What ModSecurity is

ModSecurity is a free, open-source web application firewall that inspects incoming HTTP(S) traffic and can block malicious requests before they reach an application. It runs as a module inside Apache, Nginx, or IIS, rather than as a standalone appliance or proxy. The project provides visibility into traffic plus a rules language and API for implementing protections. ModSecurity can run on its own or with the OWASP Core Rule Set, which the FAQ says provides broad coverage against common web attacks. Listed uses include real-time application security monitoring and access control, HTTP traffic logging, passive security assessment, and web application hardening. It supports Linux, Unix-like systems, Windows, and macOS when paired with a compatible web server. Installation guidance recommends starting in detection-only mode, reviewing and tuning events, then enabling blocking. Inspecting every request can have a small performance impact, and rule tuning can help manage it. OWASP maintains the project with community contributors; users are directed to Slack and GitHub for community discussions and projects.

Who it suits

ModSecurity suits people configuring web application protections within a compatible Apache, Nginx, or IIS server. It can be used for monitoring, logging, assessment, access control, or hardening.

What it does well

Runs with Apache, Nginx, or IIS
Can be used with the OWASP Core Rule Set
Supports traffic logging and passive security assessment
Free and open source

What to know first

Runs inside a web server, not as a standalone appliance or proxy
Inspecting every request can have a small performance impact
Managed rule sets are not included

Limits

The listed plan does not include managed rule sets.

Plans

Plan	Billed and limits	Price
ModSecurity	Open-source WAF engine · runs as a module inside a web server	Free

As modsecurity.org lists them, read 4 Oct 2026. The maker's own page has the final word.

Getting started

Open the ModSecurity website.
Choose a compatible web server: Apache, Nginx, or IIS.
Install ModSecurity as a module for that web server.
Start in detection-only mode and review and tune events.
Enable blocking after reviewing and tuning events.

Verdict

ModSecurity offers an open-source WAF engine that runs within a compatible web server and can operate with or without the OWASP Core Rule Set. The project advises tuning detection events before enabling blocking.

Questions

How much does ModSecurity cost?

ModSecurity is free.

Is ModSecurity open source?

Yes. It is described as an open-source web application firewall.

Which web servers can use ModSecurity?

It can be installed as a module for Apache, Nginx, or IIS.

Does ModSecurity run as a standalone appliance?

No. It runs inside a web server rather than as a standalone appliance or proxy.

Can it use the OWASP Core Rule Set?

Yes. ModSecurity can run with the OWASP Core Rule Set, which is described as covering common web attacks.

Which operating systems does ModSecurity support?

It supports Linux, Unix-like systems, Windows, and macOS when paired with a compatible web server.

Facts from ModSecurity's own pages

Community: The project directs users to Slack and GitHub for community discussions and projects and says it welcomes contributors and developers. · modsecurity.org, 4 Oct 2026
Deployment: The FAQ says ModSecurity runs inside a web server rather than as a standalone appliance or proxy. · modsecurity.org, 4 Oct 2026
Maintenance: The FAQ says ModSecurity is maintained by OWASP with support from a wider community of contributors. · modsecurity.org, 4 Oct 2026
Performance: The FAQ says inspecting every incoming request can have a small performance impact and that tuning rules can help keep the site running smoothly. · modsecurity.org, 4 Oct 2026
Purpose: ModSecurity is an open-source, rule-based web application firewall that analyzes incoming traffic and helps block malicious requests before they reach an application. · modsecurity.org, 4 Oct 2026
Recent security fixes: A September 2026 security update digest describes fixes in ModSecurity 2.9.15 and libmodsecurity 3.0.17 for multiple reported issues. · modsecurity.org, 4 Oct 2026
Rule sets: ModSecurity can run on its own or with the OWASP Core Rule Set, which the FAQ says provides broad coverage against common web attacks. · modsecurity.org, 4 Oct 2026
Supported operating systems: The FAQ lists Linux, Unix-like systems, Windows, and macOS when paired with a compatible web server. · modsecurity.org, 4 Oct 2026
Traffic inspection: The project describes ModSecurity as a cross-platform WAF module that provides visibility into HTTP(S) traffic and a rules language and API for implementing protections. · modsecurity.org, 4 Oct 2026
Tuning: The installation guide recommends starting in detection-only mode, reviewing and tuning events, and then enabling blocking. · modsecurity.org, 4 Oct 2026
Usage scenarios: The project lists real-time application security monitoring and access control, full HTTP traffic logging, continuous passive security assessment, and web application hardening as usage scenarios. · modsecurity.org, 4 Oct 2026
Web server integrations: ModSecurity can be installed as a module for Apache, Nginx, or IIS. · modsecurity.org, 4 Oct 2026

Where ModSecurity stands on Freedom251

Website Security Software#3 of 27
Web Application Firewall Software#5 of 35