OSSEC is an Open Source Host-based Intrusion Detection System. It performs log analysis, integrity checking, Windows registry monitoring, rootkit detection, real-time alerting and active response.

It runs on most operating systems, including Linux, OpenBSD, FreeBSD, MacOS, Solaris and Windows. A list with all supported platforms is available here.

OSSEC v1.6 released! (Sep 02, 2008)