This chapter examines one of the more insidious threats to your system security: malicious code.
Malicious code is
Unauthorized code (contained within a legitimate program) that performs functions unknown to (and probably unwanted by) the user
A legitimate program that has been altered by the placement of unauthorized code within it that performs functions unknown (and probably unwanted)
Any program that appears to perform a desirable and necessary function but that (because of unauthorized code within it) performs functions unknown to (and probably unwanted by) the user
Unauthorized code designed to conceal itself and destroy your data
Many different kinds of malicious code exist but the following are ...