5.1. Direct-Action Viruses
Some of the simpler computer viruses do not actively manifest themselves in computer memory. The very first file infector viruses on the IBM PC, such as Virdem and Vienna, belong to this category. Usually direct-action viruses do not spread fast and do not easily become in the wild.
Direct-action viruses load with the host program into computer memory. Upon getting control, they look for new objects to infect by searching for new files. This is exactly why one of the most common kinds of computer virus is the direct-action infector. This kind of virus can be crafted with relative ease by the attacker on a variety of platforms, in binary or in script languages.
Direct-action viruses typically use a FindFirst, FindNext sequence to look for a set of victim applications to attack. Typically such viruses only infect a couple of files upon execution, but some viruses infect everything at once by enumerating all directories for victims. In other cases, direct-action viruses simply copy themselves between the diskettes and the hard disk without waiting for the user to copy an infected file to the diskette. This technique, however, makes them much more likely to be noticed by a user because the extra diskette activity is a noisy operation.
Depending on the location of the actual host, the virus might become luckier in network environments. On the network, the virus might enumerate network shares or simply attack files, assuming that writeable network resources are available in the A: to Z: range. In this way, direct-action viruses can be extremely slow infectorsunless they appear in a networked environment.