Week 12- Computer Viruses

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Week 12- Computer Viruses

Text of Week 12- Computer Viruses

  • 1.Week - 12
    • Computer Viruses
  • Viruses
    • Classification of Viruses
    • Macro viruses
    • Logic bomb
    • Boot sector viruses
    • Multipartite Viruses
    • Polymorphic Virus
    • File infectors
    • Trojan horses
    • Worms
    • Protection from viruses (Anti Viruses)


  • Infact viruses are malicious code.
  • Malicious code means a program that can be harmful for computer, its program and data present in the computer.
  • Malicious code are categorized into three types.
    • Computer Virus
    • Trojan Horse
    • Worms

Introduction 3. Computer Virus

  • A computer virus is program that can affects or infect a computer without permission or knowledge of the user.
  • It can delete files, programs and any data present on computer hard disk.
  • Some times it may damage computer hardware.

4. Classification of Viruses

  • Viruses can be subdivided into a number of types based on their feature.
    • Macro viruses
    • Logic bomb
    • Boot sector viruses
    • Multipartite Viruses
    • Polymorphic Virus
    • File infectors

5. Macro viruses

  • A macro virus is a virus composed of a sequence of instructions that is interpreted rather than executed directly.
  • Macro viruses are designed to infect different types of documents like Word, Excel and Power point presentation.
  • May delete these files or may convert them into unreadable form.
  • First macro virus was written for Microsoft Word and was discovered in August 1995.
  • Today, there are thousand of macro viruses in existence, some example are Relax, Melissa. A and Bablas.

6. Logic bomb

  • A logic bomb employs code that lies inert until specific condition are met.
  • Logic bombs may reside within standalone programs, or they may part of worms or viruses.
  • An example of a logic bomb would be a virus that waits to execute until it has infected a certain number of hosts.
  • A time bomb is a subset of logic bomb, which is set to trigger on a particular date & or time.
  • Their objective is to destroy data on the computer once certain conditions have been met.
  • Logic bombs go undetected until launched, and the result can be destructive.
  • An example of a time bomb is the infamous Friday the 13th virus.

7. Master Boot Record/Boot Sector Viruses

  • Boot sector viruses alters or hide the boot sector, usually the Ist sector of a bootable disk (hard drive, floppy disk|).or
  • Boot Sector Viruses infect the boot sector of a hard disk or floppy disk.
  • They can also affect the Master Boot Record (MBR) of the hard disk.
  • The MBR is the first software loaded onto your computer.
  • The MBR resides on either a hard disk or floppy disk and when your computer is turned on, the hardware locates and runs the MBR.
  • This program then loads the rest of the operating system into memory.
  • Without a boot sector, computer software will not run.

8. Master Boot Record/Boot Sector Viruses

  • A boot sector virus modifies the content of the MBR.
  • It replaces the legitimate contents with its own infected version.
  • A boot sector virus can only infect a machine if it is used to boot up the computer.
  • Boot sector virus (Apple Viruses 1,2,3, Elk Cloner), Pakistani Brain (x86), Polyboot.B, AntiEXE.

9. Multipartite Viruses

  • Multipartite Virusesinfect the boot sector or Master Boot Record and also infect program files.
  • Such a virus typically has two parts, one for each type.
  • When it infects an executable, it acts as an executable infector.
  • When it infects a boot sector, it works as a boot sector infector.
  • Such viruses come in through infected media and reside in memory then they move on to the boot sector of the hard drive. Form there, the virus infects executable files on the hard drive and spreads across the system.
  • A well known multipartite virus is Ywinz.

10. Polymorphic Virus

  • Polymorphic Viruseschange their own code each time they duplicate themselves.
  • In this way, each new copy is a variation of the original virus, in order to evade detection by antivirus software.
  • Polymorphic viruses encrypt or encode themselves in a different way (using different algorithms and encryption keys) every time they infect a system.
  • This makes it impossible for antivirus to find them using string or signature searches (because they are different in each encryption) and also enables them to create a large number of copies of themselves.
  • Some examples include: Elkern, Marburg, Satan Bug, Tuareg and Dark Avenger.

11. File infectors

  • File Virusesinfect program files and device drivers by attaching themselves to the program file or by inserting themselves into the program code.
  • This type of virus infects programs or executable files (files with an .EXE or .COM extension).
  • When one of these programs is run, directly or indirectly, the virus is activated, producing the damaging effects it is programmed to carry out.
  • The majorities of existing viruses belong to this category, and can be classified according to the actions that they carry out.

12. Trojan Horse

  • Malicious program that hides within a friendly program
  • A Trojan horse is a file that appears harmless until executed.
    • Can erase or overwrite data on a computer
    • Corrupting files & Spreading other malware, such as viruses.(Trojan horse is called a 'dropper)
    • Installing a backdoor on a computer system.

13. Trojan Horse(Contd)

  • The Trojan Horse gets its name from Greek history and the story of Trojans war.
  • Greek warriors hide inside a wooden horse, which the Trojans took within the walls of the city of Troy.
  • When night fell and the Trojans were asleep, the Greek warrior came out of the horse and open the gates to the city, letting the Greek army enter the gates and destroy the city of Troy.

14. Worms Attacks

  • A program or algorithm that replicates itself over a computer network or through e-mail
  • Sometimes performs malicious actions such as using up the computer and network resources and possibly destroying data.
  • Can spread itself over a network, doesnt need to be sent
  • Slow down the computer or network
  • Some examples of worms include:
    • Klez, Nimda, Code Red, PSWBugbear.B, Lovgate.F, Trile.C, Sobig.D and Mapson.

15. Famous Viruses

  • Two of the most famous viruses to date were
    • Mellissa, struck in March, 1999,
    • ILOVEYOU virus, hit in May, 2000
  • Both viruses cost organization and individuals billion of dollars
  • Mellissa virus spread in MS Word documents sent via e-mail
    • When the document was opened, the virus was triggered
    • Mellissa accessed the MS Outlook address book on that computer and automatically sent the infected Word attachment by e-mail to the first 50 people in the address book. Each time another person opened the attachment, the virus would send out another 50 messages.
  • The ILOVEYOU virus was sent as an attachment to an e-mail posing as a love letter.
    • The message in the e-mail said Kindly check the attached love letter coming from abc

16. Famous Viruses(Contd)

  • These viruses corrupt all type of files, including system files.
  • Network at companies and government organizations world wide were shut down for days trying to remedy the problem.
  • Estimates for damage caused by the virus were as high as $10 to $15 billion, with the majority of the damage done in just few hours.
  • In June, 2000, a worm named Timofonica that was propagated via e-mail quickly made its way into the cellular phone network in Spain, sending prank calls and leaving text messages on the phones

17. Web Defacing

  • Another popular form of attack by hackers is web defacing
    • Hackers illegally change the content of a Web site
      • One notable case of Web defacing occurred in 1996 when Swedish hackers changed the Central Intelligence Agency Web site (www.odci.gov/cia) to read Cent