Facts about Computer Viruses
The term, “virus”, in computer technology, refers to a self replicating
application that spreads by making copies of itself by inserting into other
programs, other executables or documents, and when executed begins to
perform harmful actions on the system. All computer viruses are
deliberately created, not always malicious and some of them may be benign
and simply annoying.
Non-Memory Resident and Memory Resident Viruses:
Non-Memory resident viruses, when they are executed, immediately look for
other hosts that can be infected. When they infect these targets, they
transfer control to the application program they infected. A non-resident
virus has a finder module and a replication module. The finder module, once
it finds a new file to infect, calls upon the replication module to infect
that file.
Memory-Resident virus stays in the memory and do not look for hosts to
infect when they are executed. It stays active in the background after its
host program is terminated, and infects files as soon as they are opened
or accessed by other programs or the operating system. It does have the
replication module like the non-memory resident virus, but without the
finder module.
Types of Computer Viruses:
File Viruses:
These types of viruses are the most common, and mostly infect open files
and program libraries on an operating system. The virus functions by
inserting itself into a host file, modifies it in such a way that the virus
is executed when the file is opened. They are also known as left viruses.
Today, there are known viruses infecting all kinds of executables of
standard DOS: batch command files (BAT), loadable drivers (SYS, including
special purpose files IO.SYS and MS- DOS.SYS) and binary executables
(EXE, COM). There are also viruses targeting executables of other operating
systems - Windows 3.x, Windows95/NT, OS/2, Macintosh, Unix, including the
VxD drivers of Windows 3.x and Windows95.
Macro viruses:
Macros are used in most word processing programs such as Microsoft Office
in order to automate or simplify recurring tasks in documents.
Macro viruses are those viruses that use the application's own macro
programming language to distribute themselves, in which an unwanted
sequence of actions is performed automatically when the application is
started or something else triggers it. These macro viruses may inflict
damage to the document or to other computer software but are relatively
harmless, and are often spread as an e-mail virus.
Boot Viruses:
These were once the most common viruses prevalent during the early and
mid 1990s, when the use of diskettes was popular.
These viruses infect or substitute their own code for either the DOS boot
sector or the Master Boot Record (MBR), which controls the boot sequence
of the PC. The MBR is executed every time a computer is booted so the
virus will also be loaded into memory on every startup and spreads to
every disk that the system reads. They are typically very difficult to
remove, and most antivirus programs cannot clean the MBR while Windows is
running. So, bootable antivirus disks are needed to fix boot sector viruses.
Script viruses:
They are a division of file viruses, written in a variety of script
languages such as VBS, JavaScript, BAT, PHP, HTML etc.
They can form a part of multi-component viruses or infect other scripts
such as Windows or Linux command and service files. If the file format,
such as HTML, allows the execution of scripts, they can infect it.