This document discusses operating system (OS) security. It defines protection as controlling access to system resources and security as ensuring integrity, confidentiality, and availability. It identifies threats like unauthorized access, unauthorized resource use, and malicious data modification. It discusses security issues like physical security, authentication, software vulnerabilities, and malware. It recommends precautions like BIOS security, user account security, data security, antivirus software, and firewalls. Finally, it states that Linux and other UNIX-based systems are among the most secure operating systems currently.