Virus: A type of malware that enters a computer system via the hardware orsoftware without the user’s knowledge, and then attaches itself to a program file andreplicate itself. Different replica of a virus can infect other programs, boot sector,or files by inserting or attaching itself to them.Worm 6: A worm is a program that makes copies of itself, typically from onedevice to another one, using different transport mechanisms through an existingnetwork without any user intervention. Worms can have a devastating impact onInternet traffic, web sites, and the user’s own computer, which may be co-opted bythe creator of worm. The infamous Blaster worm in November 2003 was brought toworldwide attention after its devastating impact.Spyware: A type of malware that collects information for advertising purposes,usually for a secret a third party. The presence of spyware is typically hidden fromusers, and is difficult to detect. Spyware can obtain credit card numbers, passwords,and email addresses, and can also monitor a user’s web activity, scan files, createpop-up ads, log keystrokes, or change the default page of web browsers. Spywarefinds its way into computers as programs covertly bundled with downloaded software,through Peer-to-Peer (P2P) file sharing, or as a result of web browsing. For example,spyware with access to a video camera 6 can record video and transmit it usingeither email or MMS, which enables malicious remote surveillance.Trojan: Malware can also come packaged as a Trojan, a software that appearsto provide some functionalities but, instead, contains a malicious program. Trojanis also known to create backdoors to provide malicious users access to the host. Itis usually user-initiated and does not replicate.Backdoor: A backdoor program is a remote administration utility. Once installedon a computer, backdoor allows attackers to access and control the host over anetwork or the internet. A backdoor is usually able to gain control of a systembecause it exploits undocumented processes in the system’s code. These utilitiesmay be legitimate, or being used for legitimate reasons by authorized administrators.At the same time, they are also frequently used by attackers to gain control of auser’s machine without their knowledge or authorization.Rootkit: A special type of malware that hides itself, specific files and processes,and network links in the compromised devices. Rootkits achieve their malicious goalby infecting the OS.Botnet: Finally, a botnet is a set of devices that are infected by a virus that givesan attacker the ability to remotely control them. Attackers often use it to launchlarge scale network attacks, such as a distributed denial of service attack (DDoS),massive spam mail, or to collect privacy information that can be used for illegalpurposes.