Different Types of Malicious Software

Facts and Myths about Computer Viruses,
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All three of these are potentially malicious forms of software; many people simply refer to the all three types as “computer viruses”. However, there are some key differences between them.

Viruses

A computer virus is a program (a block of executable code), which attaches itself to, overwrites or otherwise replaces another program in order to reproduce itself without the knowledge of the computer user. (See chart on next page for more details.) Many viruses are comparatively harmless, and may be present for years with no noticeable effect. Some, however, may cause random damage to data files (sometimes insidiously, over a long period) or attempt to destroy files or make disks unreadable. Still others cause unintended damage. Even so-called benign viruses cause significant damage by occupying disk space and/or main memory, by using up CPU processing time, and by the time and expense wasted in detecting and removing them. One of the most famous computer viruses was the Michelangelo virus. This virus received wide attention in the media, and sent waves of panic and hysteria through the computer user community in 1992. While Michelangelo itself did relatively little damage to computer systems, the reaction to Michelangelo resulted in a lot of wasted time, effort and money.

Trojan Horses

A Trojan Horse is a program intended to perform some covert and usually malicious act, which the victim did not expect or want. It differs from a destructive virus in that it doesn’t reproduce — though this distinction is by no means universally accepted. An infamous “trojan horse” is a fake version of a popular “shareware” archiving program, “PKZIP”. This “Trojan Horse” first surfaced in May of 1995, but warnings about it are still circulating on the Internet.

(Tape) Worms

A worm is a program, which spreads on its own. Unlike a virus, it does not attach itself to a host program. Unlike a trojan horse, it reproduces and spreads by itself. In practice, worms are not normally associated with personal computer systems. The most famous worm is probably the one set loose on the Internet in 1988 by Robert Morris, Jr. Morris’s worm was a small program, which wreaked havoc on machines across the country by overloading them with invisible tasks, preventing users from being able to use the machines effectively.
Distinguishing Features of Different Types of Malicious Software

Features	Viruses	Trojan Horses	Tapeworms
Reproduction	Viruses reproduce by modifying or replacing other software. The “infected” (or “host”) software then acts as a “vector”, infecting other software.	Trojan Horses do not reproduce.	Tapeworms reproduce on their own by making copies of themselves.
Transportation	“Infected” software is transported to another computer, usually by disk or downloading, where the infection process starts again.	Computer users are duped into installing Trojan Horses by claims that they do something good.	“Network worms” find their own way to other computers over a network. Other worms spread via “infected” disks.
Dependencies	Viruses function by “infecting” other software. They are essentially code fragments.	Trojan Horses are self-contained programs.	Tapeworms are self-contained programs, or systems of programs.

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