University of Malta

Computer Viruses
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A computer virus is a program designed and written to make additional copies of itself and spread from location to location, typically without user knowledge or permission.

Viruses are written by programmers with malicious intent to annoy computer users. There are different categories of viruses - some of these may corrupt or destroy data files stored in the hard disk. These include:

  • Boot-sector viruses - The boot sector is the part of system software containing most of the instructions for starting up the computer system. The boot-sector virus replaces these instructions with some of its own. Once the system is turned on, the virus is loaded into main memory before the operating system. From there it is in a position to infect all files. USB flash disks are also vulnerable to viruses and other potential threats if connected to an infected computer, spreading the virus/es if moved to another computer.
  • File viruses - attach to executable program files (filenames with extensions .com, .exe) - those that actually begin a program. When the program is run, the virus starts working, trying to get into main memory and infecting other files.
  • Macro viruses - Many application programs such as MS Word and MS Excel embed miniature programs, known as macros, inside files and templates. Macros are executed each time the file or template is opened. Macro viruses attach their macros to files and templates. Therefore when an application loads the file and executes the instructions in it, the first instructions to execute are those of the virus.
  • Trojan horses - covertly place illegal, destructive instructions in the middle of a computer program. Once the program is run, the Trojan horse becomes active. Trojans do not replicate themselves like other viruses.

Worms are similar to viruses in that they make copies of themselves, but differ in that they need not attach to particular files or sectors at all. Once a worm is executed, it seeks other systems - rather than parts of systems - to infect, then copy its code to them. Typically worms slow down computer systems.

Typically, viruses spread by:

  • Introducing infected secondary backing media (USB flash/pen disks) into the computer.
  • Opening infected files attached to email messages.
  • Sharing infected files over the intranet i.e. the internal campus computer network.
  • Downloading infected files from ftp (file transfer protocol) sites on the Internet.

Last Updated: 6 January 2010

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