The chapter provides a comprehensive exploration of unauthorized access (UA) to information system resources, categorizing UA based on various characteristics such as manual, remote, active, and software-based attacks. It examines violations related to confidentiality, integrity, and accessibility of information. The research also delves into the classification and impacts of computer viruses, offering an in-depth look at their distribution methods, masking techniques, and destructive behaviors like viruses, logic bombs, and backdoors. In addition, the work discusses empirical and theoretical models for assessing information security vulnerabilities, including the Bell-LaPadula and Biba integrity models, and costs associated with protection measures. It further elucidates the essential features and configurations of firewalls for network security, including session layer screening gateways and application layer firewalls, highlighting the necessity for robust interconnection policies, manageability, flexibility, performance, and self-protection mechanisms. The findings serve as a foundational guide for developing integrated and effective information security systems.