Virus-L/comp.virus FAQ: Definitions and General Information

= Section B.   Definitions and General Information =

B1)  What are computer viruses (and why should I worry about them)?

Fred Cohen "wrote the book" on computer viruses, through his Ph.D.
research, dissertation and various related scholarly publications.  He
developed a theoretical, mathematical model of computer virus behaviour,
and used this to test various hypotheses about virus spread.  Cohen's
formal definition (model) of a virus does not easily translate into
"human language", but his own, well-known, informal definition is "a
computer virus is a computer program that can infect other computer
programs by modifying them in such a way as to include a (possibly
evolved) copy of itself".  Note that a program does not have to perform
outright damage (such as deleting or corrupting files) in order to be
classified as a "virus" by this definition.

The problem with Cohen's human language definition is that it doesn't
capture many of the subtleties of his mathematical model--as indeed, few
informal definitions do--and questions arise that can only be answered
by checking his formal model.  Using his formal definitions, Cohen
classifies some things as viruses that most readers of Virus-L/
comp.virus (and many experts) would not consider viruses.  For example,
given certain circumstances on an IBM PC running DOS, the DISKCOPY
program is classified as a virus by Cohen's formalisms.

This has led to some tension between what Cohen considers a "virus" and
what is usually discussed on Virus-L.  Several other definitions of
"virus" have been proposed, but it is probably fair to say that most of
us are concerned about things that are viruses by the following

A computer virus is a self-replicating program containing code that
explicitly copies itself and that can "infect" other programs by
modifying them or their environment such that a call to an infected
program implies a call to a possibly evolved copy of the virus.

Probably the major distinction between Cohen's definition and "viruses"
as we tend to use the word is that we see them as deliberately designed
to replicate (although there is some debate over this too).  Cohen's
definition does *not* require this (and this would be difficult to build
into his formal model).

Note that many people use the term "virus" loosely to cover any sort of
program that tries to hide its possibly malicious function andor tries
to spread onto as many computers as possible, though some of these
programs may more correctly be called "worms" (see B2) or "Trojan
Horses" (see B3).  Also be aware that what constitutes a "program" for a
virus to infect may include a lot more than is at first obvious--don't
assume too much about what a virus can or can't do!

These software "pranks" are very serious; they are spreading faster than
they are being stopped, and even the least harmful of viruses could be
life-threatening.  For example, in the context of a hospital life-
support system, a virus that "simply" stops a computer and displays a
message until a key is pressed, could be fatal.  Further, those who
create viruses can not halt their spread, even if they wanted to.  It
requires a concerted effort from computer users to be "virus-aware",
rather than continuing the ambivalence that has allowed computer viruses
to become such a problem.

Computer viruses are actually a special case of something known as
"malicious logic" or "malware", and other forms of malicious logic are
also discussed in Virus-L/comp.virus.  It can be important to understand
the distinctions between viruses and these other forms of malware.

B2)  What is a Worm?

A computer WORM is a self-contained program (or set of programs), that
is able to spread functional copies of itself or its segments to other
computer systems (usually via network connections).

Note that unlike viruses, worms do not need to attach themselves to a
host program.  There are two types of worms--host computer worms and
network worms.

Host computer worms are entirely contained in the computer they run on
and use network connections only to copy themselves to other computers.
Host computer worms where the original terminates itself after launching
a copy on another host (so there is only one copy of the worm running
somewhere on the network at any given moment), are sometimes called

Network worms consist of multiple parts (called "segments"), each
running on different machines (and possibly performing different
actions) and using the network for several communication purposes.
Propagating a segment from one machine to another is only one of those
purposes.  Network worms that have one main segment which coordinates
the work of the other segments are sometimes called "octopuses."

The infamous Internet Worm (perhaps covered best in "The Internet Worm
Program: An Analysis," Eugene H. Spafford, Purdue Technical Report CSD-
TR-823) was a host computer worm, while the Xerox PARC worms were
network worms (a good starting point for these is "The Worm Programs--
Early Experience with a Distributed Computation," Communications of the
ACM, 25, no.3, March 1982, pp. 172-180).

B3)  What is a Trojan Horse?

A TROJAN HORSE is a program that does something undocumented that the
programmer intended, but that some users would not approve of if they
knew about it.  According to some people, a virus is a particular case
of a Trojan Horse, namely one which is able to spread to other programs
(i.e., it turns them into Trojans too).  According to others, a virus
that does not do any deliberate damage (other than merely replicating)
is not a Trojan.  Finally, despite the definitions, many people use the
term "Trojan" to refer only to *non-replicating* malware, so that the
set of Trojans and the set of viruses are disjoint.

B4)  What are the main types of PC viruses?

Generally, there are two main classes of viruses.  The first class
consists of the FILE INFECTORS which attach themselves to ordinary
program files.  These usually infect arbitrary COM and/or EXE programs,
though some can infect any program for which execution or interpretation
is requested, such as SYS, OVL, OBJ, PRG, MNU and BAT files.  There is
also at least one PC virus that "infects" source code files by inserting
code into C language source files that replicates the virus's function
in any executable that is produced from the infected source code files
(see E5 for a more detailed discussion of the issue of "executable"

File infectors can be either DIRECT-ACTION or RESIDENT.  A direct-action
virus selects one or more programs to infect each time a program
infected by it is executed.  A resident virus installs itself somewhere
in memory (RAM) the first time an infected program is executed, and
thereafter infects other programs when *they* are executed (as in the
case of the Jerusalem virus) or when other conditions are fulfilled.
Direct-action viruses are also sometimes referred to as NON-RESIDENT.
The Vienna virus is an example of a direct-action virus.  Most viruses
are resident.

The second main category of viruses is SYSTEM or BOOT-RECORD INFECTORS:
these viruses infect executable code found in certain system areas on a
disk.  On PCs there are ordinary boot-sector viruses, which infect only
the DOS boot sector, and MBR viruses which infect the Master Boot Record
on fixed disks and the DOS boot sector on diskettes.  Examples include
Brain, Stoned, Empire, Azusa and Michelangelo.  All common boot sector
and MBR viruses are memory resident.

To confuse this classification somewhat, a few viruses are able to
infect both files and boot sectors (the Tequila virus is one example).
These are often called "MULTI-PARTITE" viruses, though there has been
criticism of this name; another name is "BOOT-AND-FILE" virus.

Aside from the two main classes described above, many antivirus
researchers distinguish either or both of the following as distinct
classes of virus:

FILE SYSTEM or CLUSTER viruses (e.g. Dir-II) are those that modify
directory table entries so that the virus is loaded and executed before
the desired program is.  The program itself is not physically altered,
only the directory entry of the program file is.  Some consider these to
be a third category of viruses, while others consider them to be a sub-
category of the file infectors.  LINK virus is another term occasionally
used for these viruses, though it should be avoided, as "link virus" is
commonly used in the Amiga world to mean "file infecting virus."

KERNEL viruses target specific features of the programs that contain the
"core" (or "kernel") of an operating system (3APA3A is a DOS kernel
virus and is also multipartite).  A file infecting virus that *can*
infect kernel program files is *not* a kernel virus--this term is
reserved for describing viruses that utilize some special feature of
kernel files (such as their physical location on disk or a special
loading or calling convention).

B5)  What is a stealth virus?

A STEALTH virus is one that, while "active", hides the modifications it
has made to files or boot records.  This is usually achieved by
monitoring the system functions used to read files or sectors from
storage media and forging the results of calls to such functions.  This
means programs that try to read infected files or sectors see the
original, uninfected form instead of the actual, infected form.  Thus
the virus's modifications may go undetected by antivirus programs.
However, in order to do this, the virus must be resident in memory when
the antivirus program is executed and *this* may be detected by an
antivirus program.

Example:  The very first DOS virus, Brain, a boot-sector infector,
monitors physical disk I/O and re-directs any attempt to read a Brain-
infected boot sector to the disk area where the original boot sector is
stored.  The next viruses to use this technique were the file infectors
Number of the Beast and Frodo (aka 4096, 4K).

Countermeasures:  A "clean" system is needed so that no virus is present
to distort the results of system status checks.  Thus the system should
be started from a trusted, clean, bootable diskette before any virus-
checking is attempted; this is "The Golden Rule of the Trade" (see G8
for help with making a clean boot disk and booting clean).

B6)  What is a polymorphic virus?

A POLYMORPHIC virus is one that produces varied but operational copies
of itself.  These strategies have been employed in the hope that virus
scanners (see D1) will not be able to detect all instances of the virus.

One method of evading scan string-driven virus detectors is self-
encryption with a variable key.  These viruses (e.g. Cascade) are not
termed "polymorphic", as their decryption code is always the same.
Therefore the decryptor can be used as a scan string by the simplest
scan string-driven virus scanners (unless another virus uses the
identical decryption routine *and* exact identification (see B15) is

A technique for making a polymorphic virus is to choose among a variety
of different encryption schemes requiring different decryption routines:
only one of these routines would be plainly visible in any instance of
the virus (e.g. the Whale virus).  A scan string-driven virus scanner
would have to exploit several scan strings (one for each possible
decryption method) to reliably identify a virus of this kind.

More sophisticated polymorphic viruses (e.g. V2P6) vary the sequences of
instructions in their variants by interspersing the decryption
instructions with "noise" instructions (e.g. a No Operation instruction
or an instruction to load a currently unused register with an arbitrary
value), by interchanging mutually independent instructions, or even by
using various instruction sequences with identical net effects (e.g.
Subtract A from A, and Move 0 to A).  A simple-minded, scan string-based
virus scanner would not be able to reliably identify all variants of
this sort of virus; rather, a sophisticated "scanning engine" has to be
constructed after thorough research into the particular virus.

One of the most sophisticated forms of polymorphism used so far is the
"Mutation Engine" (MtE) which comes in the form of an object module.
With the Mutation Engine any virus can be made polymorphic by adding
certain calls to its assembler source code and linking to the mutation-
engine and random-number generator modules.

The advent of polymorphic viruses has rendered virus-scanning an ever
more difficult and expensive endeavor; adding more and more scan strings
to simple scanners will not adequately deal with these viruses.

B7)  What are "fast" and "slow" infectors?

A typical file infector (such as the Jerusalem) copies itself to memory
when a program infected by it is executed, and then infects other
programs when they are executed.

A FAST infector is a virus that, when it is active in memory, infects
not only programs which are executed, but even those that are merely
opened.  The result is that if such a virus is in memory, running a
scanner or integrity checker can result in all (or at least many)
programs becoming infected.  Examples are the Dark Avenger and the Frodo

The term "SLOW infector" is sometimes used to refer to a virus that only
infect files as they are modified or as they are created.  The purpose
is to fool people who use integrity checkers into thinking that
modifications reported by their integrity checker are due solely to
legitimate reasons.  An example is the Darth Vader virus.

B8)  What is a sparse infector?

The term "sparse infector" is sometimes used to describe a virus that
infects only occasionally (e.g. every tenth program executed), or only
files whose lengths fall within a narrow range, etc.  By infecting less
often, such viruses try to minimize the probability of being discovered.

B9)  What is a companion virus?

A COMPANION virus is one that, instead of modifying an existing file,
creates a new program which (unknown to the user) is executed instead of
the intended program.  On exit, the new program executes the original
program so that things appear normal.  On PCs this has usually been
accomplished by creating an infected .COM file with the same name as an
existing .EXE file.  Integrity checking antivirus software that only
looks for modifications in existing files will fail to detect such

B10) What is an armored virus?

An ARMORED virus is one that uses special tricks to make tracing,
disassembling and understanding of its code more difficult.  A good
example is the Whale virus.

B11) What is a cavity virus?

A CAVITY VIRUS is one which overwrites a part of the host file that is
filled with a constant (usually nulls), without increasing the length of
the file, but preserving its functionality.  The Lehigh virus was an
early example of a cavity virus.

B12) What is a tunnelling virus?

A TUNNELLING VIRUS is one that finds the original interrupt handlers in
DOS and the BIOS and calls them directly, thus bypassing any activity
monitoring program (see D1) which may be loaded and have intercepted the
respective interrupt vectors in its attempt to detect viral activity.
Some antivirus software also uses tunnelling techniques in an attempt to
bypass any unknown or undetected virus that may be active when it runs.

B13) What is a dropper?

A DROPPER is a program that has been designed or modified to "install" a
virus onto the target system.  The virus code is usually contained in a
dropper in such a way that it won't be detected by virus scanners that
normally detect that virus (i.e., the dropper program is not *infected*
with the virus).  While quite uncommon, a few droppers have been
discovered.  A dropper is effectively a Trojan Horse (see B3) whose
payload is installing a virus infection.  A dropper which installs a
virus only in memory (without infecting anything on the disk) is
sometimes called an "injector".

B14) What is an ANSI bomb?

An "ANSI bomb" is a sequence of characters, usually embedded in a text
file, that reprograms various keyboard functions of computers with ANSI
console (screen and keyboard) drivers.  In theory a special sequence of
characters could have been included in this FAQ sheet to reprogram your
Enter key to issue the command "format c:" with a return character
tacked on the end.

Such a possibility however, need not translate into much of a threat.
It is rare for modern software to require the computer it runs on to
have an ANSI console, so few PCs or other machines should load ANSI
drivers.  Also, few people use software that simply "types" output to
the terminal device, so such an ANSI bomb in an e-mail or News posting
would most likely not reprogram your keyboard anyway.  Further, although
FORMAT C: may be catastrophic under certain versions of DOS, it won't
hurt Macintoshes and would probably have very unexpected, or no, effects
on other systems.

If you are at all worried about the possibility of having something
untoward happen on your PC due to an ANSI bomb *and* you have to load an
ANSI driver (some communications software still requires it), look for
one of the third-party ANSI drivers which abound on BBSes and FTP sites.
Most of these have improved performance over DOS's ANSI.SYS *and* either
do not support, or let you disable, keyboard re-mapping.

B15) Miscellaneous Jargon and Abbreviations

AV = antivirus.  A commonly used shorthand on Virus-L/comp.virus, as in
"av software".

BSI = Boot Sector Infector: a virus that takes control when the computer
attempts to boot.  These are found in the boot sectors of floppy disks,
and the MBRs or boot sectors of hard disks (see B4 for more details).
BSIs are also known as BSVs (Boot Sector Viruses).

CMOS = Complementary Metal Oxide Semiconductor: A memory area that is
used in AT class, and higher, PCs for storage of system information.
CMOS is battery backed RAM (see below), originally used to maintain date
and time information while the PC was turned off.  CMOS memory is not in
the normal CPU address space and cannot be executed (see E2 for further
discussion of issues concerning CMOS memory and viruses).

DBS = DOS Boot Sector: The first sector of a logical DOS partition on a
hard disk or the first absolute sector of a diskette.  This sector
contains the startup code that actually loads DOS.  This is often
confused with the MBR.  Some boot sector viruses infect the DBS rather
than the MBR when infecting hard disks.

DETECTION = The ability of an antivirus program to detect that a virus
is present, without necessarily reporting which particular virus it is
(also see IDENTIFICATION and RECOGNITION, in this section).

DOS = Disk Operating System.  We use the term "DOS" to mean any of the
MS-DOS, PC-DOS, DR DOS or Novell DOS systems for PCs and compatibles,
even though there are operating systems called "DOS" on other, unrelated

GERM = The first generation of a virus.  It normally cannot be produced
again during the replication process and is usually created by compiling
the source of the virus.

GOAT FILES = Programs which usually do nothing special (e.g., just exit,
or simply display a message), that are used by antivirus researchers to
capture samples of viruses.  This is done to make it easier to
disassemble and understand the virus, because the infected "goat"
program is (usually) simple and does not clutter the disassembly.
Alternative terms are BAIT FILES, DECOY FILES and VICTIM FILES.  In any
of these terms, the word "programs" often replaces the word "files".

IDENTIFICATION = The ability of an antivirus program (usually a scanner)
to not only detect the virus and recognize it by name, but also to
recognize it to a high degree of uniqueness.  This allows third parties
to understand which particular virus it is without seeing a sample of
the virus.  EXACT IDENTIFICATION occurs when every section of the non-
modifiable parts of the virus body are uniquely identified.  ALMOST
EXACT IDENTIFICATION occurs if the identification is only good enough to
ensure that an attempt to remove the virus will not result in damage to
the host object by the use of an inappropriate disinfection method (also
see DETECTION and RECOGNITION, in this section).

MBR = Master Boot Record: the first absolute sector (track 0, head 0,
sector 1) on a PC hard disk, that usually contains the partition table
but on some PCs may only contain a boot sector.  The MBR is also known
as the MBS (Master Boot Sector).  This is *not* the same as the DOS Boot
Sector, logical sector 0 (see above).

PARTITION TABLE = A 64-byte data structure that defines the way a PC's
hard disk is divided into logical sections known as partitions.  While
there is often more than one partition table on a PC's hard disk, the
most important is the one stored *in* the MBR.  This one contains
important extra information such as which partition (if any) should be
booted from.  The partition table is purely data, so is not executed.
Some people erroneously use the term "partition table virus" as a
synonym for "MBR virus".

RAM = Random Access Memory: the place programs are loaded into in order
to execute; the significance for viruses is that, to be active, they
must load themselves into part of the RAM.  However, some virus scanners
may declare that a virus is active when it is found in RAM, even though
it may only be left in a buffer area following a disk read operation,
rather than truly being active (see C8 for further discussion of this

RECOGNITION = The ability of an antivirus program (usually a scanner) to
detect a virus and to recognize it by name (also see DETECTION and
IDENTIFICATION, in this section).

TARGETING VIRUS = A virus that tries to bypass or hinder the operation
of one or more *specific* antivirus programs.  Also known as RETALIATOR,

SCAN STRING = A sequence of bytes (characters) that occur in a known
virus but not, one hopes, in legitimate programs.  Some scanners allow
"wildcards"--positions that are matched by any character--in their scan
strings.  Authors of virus scanners reduce the likelihood of false
positives (see B7) by carefully selecting their scan strings and often
by only searching "likely" parts of target files.

SEARCH STRING = A synonym for scan string.

SIGNATURE = A poor synonym for scan string.  We recommend that you avoid
using this term and use "scan string" or "search string" instead.

TOM = Top Of Memory: the end of conventional memory--an architectural
design limit--at the 640KB mark on most PCs.  Some early PCs may not
have a full 640KB, but the amount of memory is always a multiple of
64KB.  A boot-record virus on a PC typically resides just below this
mark and changes the value which will be reported for the TOM to the
location of the beginning of the virus so that it won't be overwritten.
Checking this value for changes can help detect a virus, but there are
also legitimate reasons why it may change (see C10).  A very few PCs
with unusual configurations or memory managers may report in excess of

TSR = Terminate but Stay Resident: these are PC programs that stay in
memory while you continue to use the computer for other purposes; they
include pop-up utilities, network software, and the great majority of
common viruses.  These can often be seen using utilities such as MEM and

VX = Virus eXchange.  A shorthand usually reserved for those BBSes and
FTP sites, and their community of users, that make their virus
collections "openly" available for downloading.  Exchange of virus
samples between bona fide members of the antivirus community is not
tagged with the VX label.
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