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Computer Viruses
Computer Viruses Computer Viruses A virus is a program that copies itself without the knowledge of the computer user. Typically, a virus spreads from one computer to another by adding itself to an existing piece of executable code so that it is executed when its host code is run. If a virus if found, you shouldn't panic or be in a hurry, and you should work systematically. Don't rush! A Viruse may be classified by it's method of concealment (hiding). Some are called stealth viruses because of the way that they hide themselves, and some polymorphic because of the way they change themselves to avoid scanners from detecting them. The most common classification relates to the sort of executable code which the virus attaches itself to. These are: ¨ Partition Viruses ¨ Boot Viruses ¨ File Viruses ¨ Overwriting Viruses As well as replicating, a virus may carry a Damage routine. There is also a set of programs that are related to viruses by virtue of their intentions, appearances, or users likely reactions. For example: ¨ Droppers ¨ Failed viruses ¨ Packagers ¨ Trojans ¨ Jokes ¨ Test files THE DAMAGE ROUTINE Damage is defined as something that you would prefer not to have happened. It is measured by the amount of time it takes to reverse the damage. Trivial damage happens when all you have to do is get rid of the virus. There may be some audio or visual effect; often there is no effect at all. Minor damage occurs when you have to replace some or all of your executable files from clean backups, or by re-installing. Remember to run FindVirus again afterwards. Moderate damage is done when a virus trashes the hard disk, scrambles the FAT, or low-level formats the drive. This is recoverable from your last backup. If you take backups every day you lose, on average, half a day's work. Major damage is done by a virus that gradually corrupts data files, so that you are unaware of what is happening. When you discover the problem, these corrupted files are also backed up, and you might have to restore a very old backup to get valid data. Severe damage is done by a virus that gradually corrupts data files, but you cannot see the corruption (there is no simple way of knowing whether the data is good or bad). And, of course, your backups have the same problem. Unlimited damage is done by a virus that gives a third party access to your network, by stealing the supervisor password. The damage is then done by the third party, who has control of the network. CLASSIFICATION OF VIRUSES Stealth Viruses If a stealth virus is in memory, any program attempting to read the file (or sector) containing the virus is fooled into believing that the virus is not there, as it is hiding. The virus in memory filters out its own bytes, and only shows the original bytes to the program. There are three ways to deal with this: 1. Cold Boot from a clean DOS floppy, and make sure that nothing on the hard disk is executed. Run any anti-virus software from floppy disk. Unfortunately, although this method is foolproof, relatively few people are willing to do it. 2. Search for known viruses in memory. All the virus scanners do this when the programs are run. 3. Use advanced programming techniques to probe the confusion that the virus causes. A process known as the "Anti-Stealth Methodology" in some scanners can be used for this. Polymorphic Viruses A polymorphic virus is one that is encrypted, and the decryptor/loader for the rest of the virus is very variable. With a polymorphic virus, two instances of the virus have no sequence of bytes in common. This makes it more difficult for scanners to detect them. Many scanners use the "Fuzzy Logic" technique and a "Generic Decryption Engine" to detect these viruses. The Partition and Partition Viruses The partition sector is the first sector on a hard disk. It contains information about the disk such as the number of sectors in each partition, where the DOS partition starts, plus a small program. The partition sector is also called the "Master Boot Record" (MBR). When a PC starts up, it reads...This is ONLY a preview of the article. If you would like to view the entire document, you must subscribe to Electronic References. Please register below now! Get This Full Article After Registration
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