McAfee VIRUSSCAN 5.1 User Manual page 20

About VirusScan Software
file, a boot sector, or a master boot record that viruses tend to infect, either
because they can hide within them, or because they can hijack their execution
routines. This way, the scanner avoids having to examine the entire file for
virus code; it can instead sample the file at well defined points to look for virus
code signatures that indicate an infection.
The development environment brings as much speed to .DAT file construction
as it does to scan engine routines. The environment provides tools researchers
can use to write "generic" definitions that identify entire virus families, and
that can easily detect the tens or hundreds of variants that make up the bulk of
new virus sightings. Continual refinements to this technique have moved
most of the hand-tooled virus definitions that used to reside in .DAT file
updates directly into the scan engine as bundles of generic routines.
Researchers can even employ a Virtran architectural feature to plug in new
engine "verbs" that, when combined with existing engine functions, can add
functionality needed to deal with new infection techniques, new variants, or
other problems that emerging viruses now pose.
This results in blazingly quick enhancements the engine's detection
capabilities and removes the need for continuous updates that target virus
variants.
Encrypted polymorphic virus detection
Along with generic virus variant detection, the scan engine now incorporates
a generic decryption engine, a set of routines that enables VirusScan software
to track viruses that try to conceal themselves by encrypting and mutating
their code signatures. These "polymorphic" viruses are notoriously difficult to
detect, since they change their code signature each time they replicate.
This meant that the simple pattern-matching method that earlier scan engine
incarnations used to find many viruses simply no longer worked, since no
constant sequence of bytes existed to detect. To respond to this threat, McAfee
VirusScan researchers developed the PolyScan Decryption Engine, which
locates and analyzes the algorithm that these types of viruses use to encrypt
and decrypt themselves. It then runs this code through its paces in an
emulated virtual machine in order to understand how the viruses mutate
themselves. Once it does so, the engine can spot the "undisguised" nature of
these viruses, and thereby detect them reliably no matter how they try to hide
themselves.
20 McAfee VirusScan