Difference b/w Viruses,Worms & Trojans

What is a virus?

A computer virus is a small program written to alter the way a computer operates, without the permission or knowledge of the user. A virus must meet two criteria:

It must execute itself.
It will often place its own code in the path of execution of another program.

It must replicate itself. For example, it may replace other executable files with a copy of the virus infected file. Viruses can infect desktop computers and network servers alike.

Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply to replicate themselves and make their presence known by presenting text, video, and audio messages. Even these benign viruses can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.


There are five recognized types of viruses:

File infector viruses: File infector viruses infect program files. These viruses normally infect executable code, such as .com and .exe files. The can infect other files when an infected program is run from floppy, hard drive, or from the network. Many of these viruses are memory resident.
After memory becomes infected, any non-infected executable that runs becomes infected.
Examples of known file infector viruses include Jerusalem and Cascade.


Boot sector viruses: Boot sector viruses infect the system area of a disk--that is, the boot record on floppy disks and hard disks. All floppy disks and hard disks (including disks containing only data) contain a small program in the boot record that is run when the computer starts up.
Boot sector viruses attach themselves to this part of the disk and activate when the user attempts to start up from the infected disk. These viruses are always memory resident in nature.

Master boot record viruses: Master boot record viruses are memory resident viruses that infect disks in the same manner as boot sector viruses. The difference between these two virus types is where the viral code is located. Master boot record infectors normally save a legitimate copy of the master boot record in an different location.

Multi-partite viruses: Multi-partite (also known as polypartite) viruses infect both boot records and program files. These are particularly difficult to repair. If the boot area is cleaned, but the files are not, the boot area will be reinfected. The same holds true for cleaning infected files. If the virus is not removed from the boot area, any files that you have cleaned will be reinfected.
Examples of multi-partite viruses include One_Half, Emperor, Anthrax and Tequilla.

Macro viruses: These types of viruses infect data files. They are the most common and have cost corporations the most money and time trying to repair. With the advent of Visual Basic in Microsoft's Office 97, a macro virus can be written that not only infects data files, but also can infect other files as well. Macro viruses infect Microsoft Office Word, Excel, PowerPoint and Access files. Newer strains are now turning up in other programs as well. All of these viruses use another program's internal programming language, which was created to allow users to automate certain tasks within that program. Because of the ease with which these viruses can be created, there are now thousands of them in circulation.
Examples of macro viruses include W97M.Melissa,WM.NiceDay and 97M.Groov.

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What is a Trojan horse?
Trojan Horses are impostors--files that claim to be something desirable but, in fact, are malicious.
A very important distinction between Trojan horse programs and true viruses is that they do not replicate themselves. Trojans contain malicious code that when triggered cause loss, or even theft, of data. For a Trojan horse to spread, you must, invite these programs onto your computers--for example, by opening an email attachment or downloading and running a file from
the Internet.Trojan.Vundo is a type of a Trojan.


What is a worm?
Worms are programs that replicate themselves from system to system without the use of a host file. This is in contrast to viruses, which requires the spreading of an infected host file.
Although worms generally exist inside of other files, often Word or Excel documents, there is a difference between how worms and viruses use the host file. Usually the worm will release a document that already has the "worm" macro inside the document. The entire document will travel from computer
to computer, so the entire document should be considered the worm. W32.Mydoom.AX@mm is an example of a worm.


What is a virus hoax?
Virus hoaxes are messages, almost always sent by email, that amount to little more than chain letters.

Some of the common phrases used in these hoaxes are:
If you receive an email titled [email virus hoax name here], do not open it!
Delete it immediately!
It contains the [hoax name] virus.
It will delete everything on your hard drive and [extreme and improbable danger specified here].
This virus was announced today by [reputable organization name here].
Forward this warning to everyone you know!

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What is not a virus?
Because of the publicity that viruses have received, it is easy to blame any computer problem on a virus. The following are not likely to be caused by a virus or other malicious code:

Hardware problems. There are no viruses that can physically damage computer hardware, such as chips, boards, and monitors.
The computer beeps at startup with no screen display. This is usually caused by a hardware problem during the boot process. Consult your computer documentation for the meaning of the beep codes.

The computer does not register 640 K of conventional memory. This can be a sign of a virus, but it is not conclusive. Some hardware drivers such as those for the monitor or SCSI card can use some of this memory. Consult with your computer manufacturer or hardware vendor to determine if this is the case.

You have two antivirus programs installed and one of them reports a virus. While this could be a virus, it can also be caused by one antivirus program detect the other program's signatures in memory.


You are using Microsoft Word and Word warns you that a document contains a macro.
This does not mean that the macro is a virus.
You are not able to open a particular document. This is not necessarily an indication of a virus. Try opening another document or a backup of the document in question. If other documents open correctly, the document may be damaged.

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What is safe computing?
With all the hype, it is easy to believe that viruses lurk in every file, every email, every Web site.

However, a few basic precautions can minimize your risk of infection. Practice safe computing and
encourage everyone you know to do so as well.

General precautions

* Be suspicious of email attachments from unknown sources.
* Verify that attachments have been sent by the author of the email. Newer viruses can send email messages that appear to be from people you know.
* Do not set your email program to "auto-run" attachments.
* Obtain all Microsoft security updates.
* Back up your data frequently. Keep the (write protected) media in a safe place--preferably in a different location than your computer.
* Always use an Antivirus Program.
* Keep your antivirus updated.

 

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