Malicious code attacks include a number of types of computer programs that were created with the intention of causing data loss or damage. The three main types of malicious code attacks are viruses, Trojan horses, and worms.
A virus is malicious software that is attached to another program to execute a particular unwanted function on a workstation. An example is a program that is attached to command.com (the primary interpreter for Windows systems) and deletes certain files and infects any other versions of command.com that it can find.
A Trojan horse is different only in that the entire application was written to look like something else, when in fact it is an attack tool. An example of a Trojan horse is a software application that runs a simple game on a workstation. While the user is occupied with the game, the Trojan horse mails a copy of itself to every address in the user's address book. The other users receive the game and play it, thereby spreading the Trojan horse to the addresses in each address book.
Viruses normally require a delivery mechanism, a vector, such as a zip file or some other executable file attached to an email, to carry the virus code from one system to another. The key element that distinguishes a computer worm from a computer virus is that human interaction is required to facilitate the spread of a virus.
Worms are self-contained programs that attack a system and try to exploit a specific vulnerability in the target. Upon successful exploitation of the vulnerability, the worm copies its program from the attacking host to the newly exploited system to begin the cycle again.
A virus consists of a set of instructions that attaches itself to other computer programs, usually in the computer's operating system, and becomes part of them. In most cases, the corrupted programs continue to perform their intended functions but surreptitiously execute the virus's instructions as well. A virus is usually designed to execute when it is loaded into a computer's memory. Upon execution, the virus instructs its host program to copy the viral code into, or “infect,” any number of other programs and files stored in the computer. The infection can then transfer itself to files and code on other computers through magnetic disks or other memory-storage devices, computer networks, or online systems. The replicating viruses often multiply until they destroy data or render other program codes meaningless. A virus may simply cause a harmless joke or cryptic message to appear on a computer user's video monitor each time he turns on his computer. A more damaging virus can wreak havoc on an extremely large computer system within a matter of minutes or hours, causing it to crash and thereby destroy valuable data.
Clearly writing and spreading virus programs are unethical acts; they have very serious consequences, and cause systems to crash and organisations to cease operating for certain periods. One of the most concerning consequences of such actions is when viruses interrupt the smooth functioning of an organisation which could in extreme cases even cause people to die. Logic bombs are also sometimes planted. There is obviously a lot of anti-virus software on the market now though that helps to deal with this ever-growing problem.
