User Guide

About VirusScan Software
24 McAfee VirusScan Anti-Virus Software
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
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.
“Double heuristics” analysis
As a further engine enhancement, McAfee researchers have honed early
heuristic scanning technologies—originally developed to detect the
astonishing flood of macro virus variants that erupted after 1995—into a set of
precision instruments. Heuristic scanning techniques rely on the engine’s
experience with previous viruses to predict the likelihood that a suspicious file
is an as-yet unidentified or unclassified new virus.
The scan engine now incorporates ViruLogic, a heuristic technique that can
observe a program’s behavior and evaluate how closely it resembles either a
macro virus or a file-infecting virus. ViruLogic looks for virus-like behaviors
in program functions, such as covert file modifications, background calls or
invocations of e-mail clients, and other methods that viruses can use to
replicate themselves. When the number of these types of behaviors—or their
inherent quality—reaches a predetermined threshold of tolerance, the engine
fingers the program as a likely virus.
The engine also “triangulates” its evaluation by looking for program behavior
that no virus would display—prompting for some types of user input, for
example—in order to eliminate false positive detections. This double-heuristic
combination of “positive” and “negative” techniques results in an
unsurpassed detection rate with few, if any, costly misidentifications.
Wide-spectrum coverage
As malicious agents have evolved to take advantage of the instant
communication and pervasive reach of the Internet, so VirusScan software has
evolved to counter the threats they present. A computer “virus” once meant a
specific type of agent—one designed to replicate on its own and cause a
limited type of havoc on the unlucky recipient’s computer. In recent years,
however, an astounding range of malicious agents has emerged to assault
personal computer users from nearly every conceivable angle. Many of these
agents—some of the fastest-spreading worms, for instance—use updated
versions of vintage techniques to infect systems, but many others make full
use of the new opportunities that web-based scripting and application hosting
present.