What Is A Virus?

A parasitic program written intentionally to enter a computer without the users permission or knowledge. The word parasite is used because a virus attaches to files or boot sectors and replicates itself, thus continuing to spread. Though some virus's do little but replicate others can cause serious damage or effect program and system performance. A virus should never be assumed harmless and left on a system.

A functional definition of a computer virus must include the fact that it creates damage to the computer system "infected"; either inadvertently or deliberately. This definition is somewhat simplified and does not cover all virus types, but is sufficient to show the major differences between viruses and so-called "Trojan" programs, which is that the virus replicates, but the Trojan program does not.

What Is Heuristic Scanning And Why Is It Important?

Generally speaking, there are two basic methods to detect viruses - specific and generic. Specific virus detection requires the anti-virus program to have some pre-defined information about a specific virus (like a scan string). The anti-virus program must be frequently updated in order to make it detect new viruses as they appear. Generic detection methods however are based on generic characteristics of the virus, so theoretically they are able to detect every virus, including the new and unknown ones.

Why is generic detection gaining importance? There are four reasons:

1) The number of viruses increases rapidly. Studies indicate that the total number of viruses doubles roughly every nine months. The amount of work for the virus researcher increases, and the chances that someone will be hit by one of these unrecognizable new viruses increases too.

2) The number of virus mutants increases. Virus source codes are widely spread and many people can't resist the temptation to experiment with them, creating many slightly modified viruses. These modified viruses may or may not be recognized by the anti-virus product. Sometimes they are, but unfortunately often they are not.

3) The development of polymorphic viruses. polymorphic viruses like MtE and TPE are more difficult to detect with virus scanners. It is often months after a polymorphic virus has been discovered before a reliable detection algorithm has been developed. In the meantime many users have an increased chance of being infected by that virus.

4) Viruses directed at a specific organization or company. It is possible for individuals to utilize viruses as weapons. By creating a virus that only works on machines owned by a specific organization or company it is very unlikely that the virus will spread outside of the organization. Thus it is very unlikely that any virus scanner will be able to detect the virus before the payload of the virus does its destructive work and reveals itself.

Each of these scenarios demonstrates the fact that virus scanners can not recognize a virus until the virus has been discovered and analyzed by an anti-virus vendor.

These same scenarios do not hold true for generic detectors, and therefore many people are becoming more interested in generic anti-virus products. Of the many generic detection methods, heuristic scanning is currently becoming the most important.

How Does Heuristic Scanning Perform ?

Heuristics is a relatively new technique and still under development. It is however gaining importance rapidly. This is not surprising as heuristic scanners are able to detect over 90% of the viruses without using any predefined information like signatures or checksum values. The amount of false positives depends on the scanner, but a figure as low as 0.1% can be reached easily.