Computer Viruses: The Nasty Truth
by George Royal
The term, "virus", in computer technology, refers to a selfreplicating application that spreads by making copies of itselfby inserting into other programs, other executables ordocuments, and when executed begins to perform harmful actionson the system. All computer viruses are deliberately created,not always malicious and some of them may be benign and simplyannoying.
Non-Memory Resident and Memory Resident Viruses:
Non-Memory resident viruses, when they are executed, immediatelylook for other hosts that can be infected. When they infectthese targets, they transfer control to the application programthey infected. A non-resident virus has a finder module and areplication module. The finder module, once it finds a new fileto infect, calls upon the replication module to infect that file.
Memory-Resident virus stays in the memory and do not look forhosts to infect when they are executed. It stays active in thebackground after its host program is terminated, and infectsfiles as soon as they are opened or accessed by other programsor the operating system. It does have the replication modulelike the non-memory resident virus, but without the findermodule.
Types of Computer Viruses:
File Viruses: These types of viruses are the most common, andmostly infect open files and program libraries on an operatingsystem. The virus functions by inserting itself into a hostfile, modifies it in such a way that the virus is executed whenthe file is opened. They are also known as left viruses. Today,there are known viruses infecting all kinds of executables ofstandard DOS: batch command files (BAT), loadable drivers (SYS,including special purpose files IO.SYS and MS- DOS.SYS) andbinary executables (EXE, COM). There are also viruses targetingexecutables of other operating systems – Windows 3.x,Windows95/NT, OS/2, Macintosh, Unix, including the VxD driversof Windows 3.x and Windows95.
Macro viruses: Macros are used in most word processing programssuch as Microsoft Office in order to automate or simplifyrecurring tasks in documents. Macro viruses are those virusesthat use the application's own macro programming langauge todistribute themselves, in which an unwanted sequence of actionsis performed automatically when the application is started orsomething else triggers it. These macro viruses may inflictdamage to the document or to other computer software but arerelatively harmless, and are often spread as an e-mail virus.
Boot Viruses: These were one of the most common virusesprevalent during the early and mid 1990s, when the use ofdiskettes was popular. These viruses infect or substitute theirown code for either the DOS boot sector or the Master BootRecord (MBR), which controls the boot sequence of the PC. TheMBR is executed every time a computer is booted so the viruswill also be loaded into memory on every startup and spreads toevery disk that the system reads. They are typically verydifficult to remove, and most antivirus programs cannot cleanthe MBR while Windows is running. So, bootable antivirus disksare needed to fix boot sector viruses.
Script viruses: They are a division of file viruses, written ina variety of script langauges such as VBS, JavaScript, BAT, PHP,HTML etc. They can form a part of multi-component viruses orinfect other scripts such as Windows or Linux command andservice files. If the file format, such as HTML, allows theexecution of scripts, they can infect it.
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