What Is A Computer Virus ?.........................!!!!

A Computer Virus is a piece of Software that has been written to surprisingly enter your computer system and infect / damage your files. A Virus reproduce , usually without your permission or knowledge. In general terms they have an infection phase where they reproduce widely and an attack phase where they do whatever damage they are programmed to do. There are a number of different viruses.

A program reproduces its own code by attaching itself to other executable files in such a way that the virus code is executed when the infected executable file is executed.



Common Symptoms Of Viruses?.....................!!!!

Some common symptoms that could indicate your system's been infects are:

• Unusual messages or displays on your monitor
  
• Unusual sounds or music played at random times
  
• Your system has less available memory than it should
  
• A disk or volume name has been change
  
• Programs or files are suddenly missing
  
• Unknown programs or files have been crated
  
• Some of your files become corrupted or suddenly don't work properly
  
  
  
  
   

Behaviours Of Viruses………………..!!!!

All viruses potentially have two phases to their execution.

• Infection Phase
  
• Attack phase
  
  
  
  
   

Infection Phase…………………..!!!!

Virus writers have to balance how and when their viruses infect against the possibility of being detected. Therefore , the spread of an infection may not be immediate.

When the virus executes it has the potential to infect other programs. What's often not clearly understood is precisely when it will infect the other programs. Some viruses infect other programs each time they are executed , other viruses infect only upon a time , an external event on your PC , a counter within the virus , etc. Virus writers want their programs to spread as far as possible before anyone notices them.

Many viruses go resident in the memory of your PC in the same or similar way as terminate and stay resident (TSR) programs.

This means the virus can wait for some external event before it infects additional programs. The virus may silently lurk in memory & it is called as "stealth" capability, waiting for you to access a diskette, copy a file, or execute a program, before it infects anything. This makes viruses more difficult to analyze since it's hard to guess what trigger condition they use for their infection.



Attack Phase…………………!!!!

Viruses need time to infect. Not all viruses attack, but all use system resources and often have bugs.

Many viruses do unpleasant things such as deleting files or changing random data on your disk, simulating typos or merely slowing your PC down; some viruses do less harmful things such as playing music or crating phase also has its own trigger.

Viruses often delay revealing their presence by launching their attack only after they have had ample opportunity to spread. This means the attack could be delayed for days , weeks , months, or many viruses simply reproduce and have no trigger for an attack phase. Does this mean that these are "good" viruses? No! Anything that writes itself to your disk without your permission is stealing storage and CPU cycle. This is made worse since viruses that "just infect" , with no attack phase, often damage the programs or disks they infect.



Types Of Viruses…………………..!!!!

Viruses come in many types, written using many different infection strategies. Breaking them into categories is not easy as many viruses have multiple categories. Viruses can infect a number of different portions of the computer's operating and file system.



1.System Sector/ Boot Viruses………….!!!!

System sectors are special areas on your disk containing programs that are executed when you boot (start) you're PC. Every disk has a system sector. System sectors are invisible to normal programs but are vital for correct operation of your disk.



2.File Viruses…………………………..!!!!



While more in number, file infectors are not the most commonly found. They infect in a variety of ways and can be found in a large number of file types.

The simplest file viruses' work by locating a type of file they know how to infect and overwriting part of the program they are infecting. When this program is executed, the virus code executed and infects more files. These overwriting viruses do not tend to be very successful since the overwritten program rarely continues to function correctly and the virus is almost immediately discovered.

The more sophisticated file viruses save the original instructions when they insert their code into the program. This allows them to execute the original program after virus finishes so that everything appears normal. Just as system sector viruses can remain resident in memory.


3.Macro Viruses………………………..!!!!



Pure data files cannot propagate viruses, but with extensive macro languages in some programs the line between a "data" file and "executable" file can easily become blurred to the average user. While text e-mail messages can't contain viruses they may have attachments that do and some e-mails programs will automatically load and run these. Don't let them. Finally, be careful or programs that use other programs for reading E-Mails.

A virus, no matter what type, it still a program and it must load into memory and run in order to do anything. Simply reading it into memory is not sufficient. Pure data files are not viruses simply because, by their nature, they do not execute.

The problem, however , is that many modern programs contain some form of macro languages ; in some cases a very powerful macro with commands that include. Opening, manipulating, and closing files. More and more, these programs allow a user to extend their capabilities by writing powerful macros and then attaching these data files produced by that program. In many cases, in order to make things easy for users, the macro are set up to run automatically whenever the data file is loaded. It's in cases like this where the line between a data file and program starts to blur.

Most virus scanners have default settings that check the most common executable file and data files from programs that have a macro language. So when using those programs it's a good idea to not change the default extension so scanners can find the files they need to. Common programs with macro capability that can be exploited by virus writers are Microsoft Word , Excel and other Office Programs. And, the latest macro code to be exploited exists in the full version of the Acrobat program which reads and writes PDF files.

A second vulnerability exists on the Internet. Some e-mail programs and Internet browsers allow you to click on a data file or program that might be attached to a messages or displayed on a web page and have that file or program load and/or run automatically. You should not allow this to happen. Always save the file or program to disk and then check it which anti-virus software before loading or executing it.


4.Cluster Viruses………………………..!!!!

There is a type of virus known as a "cluster" virus that infects your files not by changing the file or planting extra files but by changing the DOS directory information so that directory entries point to the virus code instead of the actual program. When you run a program , DOS first loads and executes the virus code , the virus then locates the actual program, DOS first loads and executes the virus code, the virus then locates the actual program and executes it.

This type of virus can cause serious problems if you don't known it 's there. While the virus is in memory , if control access to the directory structure on the disk.


5.Companion Viruses……………………..!!!!

  Companion viruses make use of a DOS quirk that runs COM files before EXE files. The virus infects EXE files by installing a same-named COM file. Would you believe that a virus can infect your files without changing a single byte in the infected file? Well, it's true; two different ways in fact ! The more common of the two ways is called the companion or spawning virus. The companion virus infects your files by locating all files with names ending in EXE. The virus then crates a matching file name ending in COM that contains that viral code. Let's say a companion virus is executing on your PC and decides it's time to infect a file. It looks around and happens to find a file called PGM.EXE. It now crates a file called PGM.COM containing the virus.


6.Batch Viruses………………………….!!!!

While not often found , it is possible to write a batch file that containing a virus.


Non-Virus Malicious Codes……………...!!!!

Similar to viruses, you can also find malicious code in logic bombs, Trojan Horses & Worms.


Logic Bombs……………………………....!!!!

Just like a real bomb , a logic bomb will lie dormant until triggered by some event. The trigger can be a specific date, the number of times executed, a random number , or even a specific event. When the logic bomb is triggered, it will usually do something unpleasant . This can range from changing a random byte of data somewhere on your disk to making the entre disk unreadable. Changing random data may be the most insidious attack since it generally causes substantial damage before anyone notices that something is wrong.


Trojans……………………………………!!!!


These malicious programs are named after the "Trojan Horse", which delivered soldiers into the city of Troy. Like the horse, a Trojan program is a delivery vehicle, a program that does something undocumented which the programmer intended. The Trojan program appears to be a useful program of some type, but when a certain event occurs, it does something nasty and often destructive to the system.

Most of the "classic" Trojan programs were delivered to users on disks which advertised themselves as something useful. Many never Trojan programs make their way to you as E-Mail attachments.

There have been many Trojan programs and new ones crop us every day. It's important to know and trust the source of any program you receive because most anti-virus programs can't detect new Trojans. These programs, while potentially destructive, still use common DOS / Windows commands and any attempt to trigger an alert on these commands would result in massive false alarms.




Worms…………………………………….!!!!

A worm is a self-reproducing program that does not infect other programs as a virus will, but instead creates copies of it-self and these create even more copies.

Worms are usually seen on networks and on multi-processing operating systems, where the worm will create copies of itself that are also executed. Each new copy will create more copies quickly clogging the system.

The so-called ARPANET / INTERNET "virus" was actually a worm. It created copies of itself through the network, eventually bringing the network to its knees. It did not infect other programs as a virus would, but simply kept creating copies of itself that would then execute and try to spread to other machines.

Some newer macro viruses also send their infected documents over the internet to others who then infect virus further. Some have classed these as worms.

The newer script worms don't help clarify the classification issue. Many of these are sent as a Visual Basic Script file attached to an E-Mail message. If you click on the attachment to open it the script runs and will often send the script to addresses in your E-Mail address book, thus spreading itself. Technically, these would be worms but are often called viruses.



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