Server Intrusion Detection for Every Platform
Open Source HIDS
OSSEC is fully open source and free. You can tailor OSSEC for your security needs through its extensive configuration options, adding custom alert rules and writing scripts to take action when alerts occur.
OSSEC offers comprehensive host-based intrusion detection across multiple platforms including Linux, Solaris, AIX, HP-UX, BSD, Windows, Mac and VMware ESX.
OSSEC helps organizations meet specific compliance requirements such as PCI DSS. It detects and alerts on unauthorized file system modification and malicious behavior that could make you non-compliant.