ModSecurity

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 inside a compatible web server as a module for Apache, Nginx, or IIS, rather than as a standalone appliance or proxy. Its rules language and API provide a way to implement protections, and it can run alone or alongside the OWASP Core Rule Set, which covers common web attacks. Listed uses include 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, and then enabling blocking. Inspecting incoming requests may have a small performance impact, which rule tuning can help manage. OWASP maintains the project with community contributions, and discussions and development are directed to Slack and GitHub. The listed plan costs nothing; managed rule sets are not included.