Page 349 - Computer Science Class 11 Without Functions
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●  A boot sector virus is a virus that affects the boot sector of the disks. It gets activated when the drive is started
              (booted up). The Michelangelo virus is an example of a boot sector virus.




                   A boot sector is a part of secondary storage that contains instructions on how to boot (start) a device.


            ●  File Infectors are viruses that attach themselves to executable files by overwriting a part of their code or appending
              their code to the files. For example, the Romeo and Juliet virus.
            ●  Macro viruses embed themselves into the documents. A macro refers to specific instructions within an application.
              The macro viruses are executable files that may be received as email attachments. When a recipient opens an
              attachment, the viruses gets activated and starts affecting the system programs (deleting, creating, or overwriting
              other files). For example, the Melissa Virus got spread through a Microsoft Word document sent as an attachment.
              When the Word file was opened, the virus got activated. Using the Microsoft Outlook program, the attachment
              would be sent to the first 50 contacts in the address book.
            14.4.2 Trojan Horse

            A Trojan horse is malware that presents itself as desirable and helpful program 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. Fig 15.5 shows how a Trojan horse makes a backdoor entry 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.

                                                                     BackDoor Attack














                                                       Fig 15.5: Trojan Horse

            14.4.3 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.

            14.4.4 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.






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