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Trojan Horse

            A Trojan horse is malware that presents as desirable and helpful but actually causes harm to the system. A Trojan horse
            generally attaches itself to a safe application. For example, it may be attached to a game downloaded over the Internet.
            Figure 9.4 shows how a Trojan horse  makes a backdoor into the system that hackers can use to get into the system.
            Hackers can keep track of everything that happens on the system and take control of it by looking at or damaging its data.

















            Worms
            A worm is a malicious program that replicates itself.  So, a networked device could have several copies of a worm
            infecting many files.  Like viruses, to spread a worm for the first time, an email or an instant chat is sent with attachments
            from seemingly trustworthy senders. Once the infected email attachment is opened, the worm is downloaded onto
            the recipient's system. Then it replicates itself and spreads to the devices of the contacts (again via mail) in the user's
            mail account. For example, the Code Red worm can make around 3 lakh copies in a few hours.
            Spyware

            The spyware secretly collects information from a system.  One may unknowingly install spyware on their device when
            downloading files or software from dubious websites or by  opening  suspicious email attachments. Therefore, one
            must exercise caution while  opening emails or downloading files from suspicious sources. Some common signs of a
            system under the spyware attack are: getting constant error messages, random icons being displayed on the desktop,
            or unwanted  opening of tabs while  browsing the Internet

            Adware
            An adware is a software that displays advertisements to users of the Internet and mobile applications. Advertisements
            can show up in different ways in an app, such as a static box, a banner, a full screen, a video, or a pop-up (Figure 9.5).
            These advertisements generate revenue for developers. Adware may also record user activities and pass them on to
            the relevant businesses for money. Adware may also act as spyware by collecting private and sensitive information.






















                                                        Figure 9.5: Adware

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