**1. Introduction**

Along with the tremendous progress in Internet technology in the last few decades, the sophistication of the exploits and thereby the threats to computer systems have also equally increased. The exploitation is done by malicious hackers who find vulnerabilities or weaknesses, which are the pre-existing errors in the security settings in the computer systems. The common types of vulnerabilities are errors in the design or configuration of network infrastructure, protocols, communication media, operating systems, web-based applications and services, databases, etc.

Threat is a potential risk that exploits a vulnerability to infringe security and cause probable damage/disruption to the information/service stored/offered in/by computer systems or through communication links. A threat to a computer systems occurs when the confidentiality (preventing exposure to unauthorized parties), integrity (not modified without authorization), and availability (readily available on demand by authorized parties) of information on systems are affected. Thus, a computer system threat in general can include anything deliberate, unintended, or caused by natural calamity that effects in data loss/manipulation or physical destruction of hardware. Accordingly, the threats on computer system are classified as physical threats and nonphysical threats. Physical threats cause impairment to hardware or theft to system or hard disk that holds critical data. Nonphysical threats target the data and the software on the computer systems by corrupting the data or by exploiting the errors in the software.

The exploits when successful result in security attacks on computer systems. Hence, threat is a possible danger caused by system vulnerability, while attack is the attempt of unauthorized action or a harmful action. The realization of a threat is usually detrimental and is termed an attack.

In this introductory chapter, the computer security threats are defined as probable attacks from hackers that let them to gain illicit entree to a computer. In this chapter, a detailed introduction is given on the common computer system threats. The logical threats are a main cause of security incidents on computer systems. Knowing these threats and their characteristics helps in identifying the threats and to proactively devise steps in protecting the systems. The organization of this chapter is as follows. Section 2 introduces the motivation and objective of the hackers. Section 3 is on the classification of threats, which also includes an exhaustive coverage of all the threats. The details of the top security menaces of 2020 and the expectation for the latter half of 2020 are introduced in Section 4. Section 5 concludes the chapter.
