What is Virus?

Also Learn What is Trojan and Worm

Even though Computer Virus is a popular topic only few really know what is virus. This page will provide everything you must know about various types of Computer viruses.

So what is virus? A Computer virus is a parasitic program written to intentionally enter a computer without the user's permission or knowledge. The word parasitic is used because a virus attaches to files or boot sectors and replicates itself which allows it to spread. Though some viruses do little more than copy themselves, others can cause serious damage or affect program and system performance. A virus should never be assumed harmless or left on a system.

Types of viruses

Viruses are classified by the ways they infect computer systems:

1) Program: Executable program files such as those ending with the

extensions .Com, .Exe, .Ovl, .Drv, .Sys, .Bin

2) Boot: Boot Record, Master Boot, FAT and Partition Table.

3) Multipartite: Both program and boot infector.

What is virus can do to your Computer?

What is Trojan Horse?

Besides being a large wooden horse used in the Trojan war 2500 years ago (and more recently by Brad Pitt), a Trojan Horse or Trojan is a computer program that seems innocuous or harmless, but conceals another more nasty function. Generally a Trojan is contained in a part of program brought into your computer from an outside source - (e.g. floppy disk,CD, internet download, infected email). Trojans can be dangerous sometimes. For example a program may appear to be a computer game demo, but while you enjoy the game, it may be happily formatting your hard disk or emailing porn sites to everyone in your email address book.

Trojans are also used as a 'back door' or 'trap door' to sneak into a computer's operating system's information. An example is a program that pretends to be the system log-in program (such as you find in Windows NT/XP or Linux). When an unsuspecting user tries to log-in, the trojan program captures the user-name & password. Then it may indicate a failed log-in attempt and exit to the real log-in program. The user may successfully log into the system next time, but by that time Trojan has also got enough details to log into the system.

Example Trojans

Notroj - This Trojan horse pretends to be a program that guards against Trojans. It's actually a time bomb that wipes out the hard disk after it's more than 70 percent full.

PWSteal.Refest - A Trojan Horse that installs itself as a BHO (Browser Helper Object) for Internet Explorer and steals online banking information when it is submitted in web forms. PWSteal.Likmet.A - A Trojan horse that displays a fake MSN Messenger logon window and steals the password provided.

Run.me - This is a graphics program which plays the Star Spangled Banner and displays the American Flag while it worms its way into the hard disk and erases the data on it.

Have you realized what is vurus can do to your computer?

What is Worm?

A Worm is a self-propagating program that works its way through a system or network (like the Internet), often causing damage. It does not require a host program to activate it. Someone has to insert a worm directly into network of interconnected computers where messages can be sent from one to another and data files and programs exchanged. An example is a local area network where each computer has its own files, programs operating systems and hard discs such as would be found in a university or corporate setting.

Example Worms

Alarm Clock Worm - A worm that reaches out through the network to an outgoing terminal (one equipped with a modem), and places wake-up calls to a list of users.

Worm Watcher - A special program which automatically takes steps to limit the size of a worm, or shut it down, it grows beyond a certain limit. The worm watcher also maintains a running log recording changes in the state of individual segments. This information can be used to analyze what might have gone wrong with a worm.

W32.Hardoc@mm - A mass-mailing worm that sends itself to email addresses found in .html files and the Windows address book on the infected computer. This worm uses a vulnerability in the Windows™ family of operating systems to allow the automatic execution of the attachment on an unprotected computer.

Mydoom.A@mm - A mass-mailing worm that arrives as an attachment with the file extension .bat, .cmd, .exe, .pif, .scr, or .zip.

When a computer is infected with Mydoom, the worm sets up a backdoor into the system which can potentially allow an attacker to connect to the computer and use it to gain access to its network resources. In addition, the backdoor can download and execute arbitrary files.

Now you know what is virus. In the next article I will explain the symptoms of infected computers.

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