OSSEC is an Open Source Host-based Intrusion Detection System that performs log analysis, file integrity checking, policy monitoring, rootkit detection, real-time alerting and active response.
Elkeid is a Cloud-Native Host-Based Intrusion Detection solution project to provide next-generation Threat Detection and Behavior Audition with modern architecture.
Libellux: Up & Running provides documentation on how-to install open-source software from source. The focus is Zero Trust Network to enhance the security for existing applications or install tools to detect and prevent threats.
Some of my security-related coding projects for OpenBSD: A kernel-based user-profile intrusion detection system (FUPIDS) and an ICMP-based "port-knocking" service (openportd).
Lightweight file integrity based - Host Intrusion Detection System (HIDS) built in pure Python3. Recursively enumerates directories, stores hashes of files, detects and logs file modifications.