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The infection of PC compatible computers
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THE INFECTION OF PC COMPATIBLE COMPUTERS
Stephen E. Kiel
Raymond K. Lee
Georgia Institute of Technology
Summer Quarter 1988
INTRODUCTION
The recent publicity over computer viruses has produced
mixed reactions and much confusion inside, as well as outside, of
the computing industry. The conflicting opinions are caused either
by a misunderstanding of what viruses are or a lack of
understanding of their potential problems. This paper answers
those questions and in addition, gives a description of currently
suggested methods for IBM PC's and compatibles for detecting,
preventing, and eliminating viruses. A highly technical discussion
is not the objective, but rather a broad overview is given along
with sources of additional information and assistance.
THE BEGINNING
On November 3, 1983, an idea was conceived of by Fred
Cohen as an experiment to be presented at a weekly seminar on
computer security [1]. The idea was simple enough: design a
computer program that could modify other programs to include a
possibly evolved copy of itself. This evolved copy would then
modify other programs and thus continue the propagation and
evolution. The program could easily be spread by unknowing users
throughout a computer system or network.
It only took eight hours of expert work on a heavily
loaded VAX 11/750 to complete the first of such programs and
prepare it for demonstration. The program was inserted into the
beginning of a new program on the system called 'vd,' which
displayed Unix structures graphically. A new program was chosen so
that details of its operation and its performance characteristics
would be unknown. Users were introduced to vd via the system
bulletin board.
The program inside of vd used the authorizations of every
user using it to infect their programs. In all of the experiments,
the program that was initially inserted into vd was granted all
system rights in under an hour. The shortest time was under five
minutes, with the average time under 30 minutes. Even people who
knew that the experiments were taking place were unable to defend
themselves. Once the surprising results of the experiments were
announced, the administrators of the VAX 11/750 decided that no
further computer experiments would be performed on their system.
Precautions were taken to keep the experiment under control. No
damage was done and only reports were sent back on the program's
progress. Also, traces were generated to insure that the program
could not spread without detection. All files were purged of the
program after the experiment was completed. It is unfortunate that
an apparent fear reaction on the part of the system administrators
prohibited any further testing.
DEFINING A VIRUS
A name for programs exhibiting the behavior described
above was thought of by Len Adleman: 'viruses.' A computer virus
can generally be defined as a program which hides in computer
systems, usually in larger programs, whose mission is to replicate
and spread until the occurrence of some designated event. When
this event takes place, the program can then perform some action
specified by its creator. The term 'virus' is very appropriate
since computer viruses (here after referred to as simply 'viruses')
behave much like their biological counterparts.
Once in a computer system, a virus can remain quiet for an
incubation and contagion period, during which it infects other
files. After some prespecified event, such as a period of time or
a number of infections, the virus can come to life and begin an
attack. All the while, the offspring of the virus are infecting
other files and systems, also waiting to be triggered to attack.
The software that controls the computer and the devices
connected to it is known as the DOS, an acronym for disk operating
system. DOS commands are the core of the operating system and
instruct the computer to start, stop, or continue an operation.
The most popular DOS for IBM PC compatible computers is Microsoft
Corporation's MS-DOS.
Personal computer viruses typically infect three special
MS-DOS files: IBMBIO.COM, IBMSYS.COM, and COMMAND.COM. These
files are found on every system disk and become part of memory each
time the operating system is loaded into the computer. The system
files IBMBIO.COM and IBMSYS.COM are hidden and read-only and are
not easily infected. The COMMAND.COM file, which is the default
command processor of MS-DOS, is both visible and modifiable. A
number of viruses have been discovered which infect this file.
These three files are copied to other disks and run on other
machines often enough that a virus in any of these files can spread
very quickly.
The action performed by viruses will vary. It could be
simply the flashing of a harmless message on the screen. A virus
in Aldus Publishing's FreeHand, a graphics program for the
Macintosh, printed the message, "We would like to take this
opportunity to convey our universal message of peace to all
Macintosh users around the world" [2]. The company had to recall
about 5,000 infected packages. Unfortunately, all viral behavior
is not benign like this message printing or the simple infection
tracing found in the experiment discussed in the opening paragraphs
of this paper. There have even been reports of viruses which can
slightly modify spreadsheets and other data [3].
Viruses have been found which reformat hard disks and
destroy data. The destructive behavior is only limited to the
warped imagination of its creator. Because of the hidden dangers
involved, apparently safe software packages carrying such viruses
have become known as "Trojan Horses." A viral outbreak of this
sort took place last fall in the microcomputer labs at Lehigh
University in Bethlehem, Pa. [4]. This particular outbreak,
described below, generated a lot of publicity and caused both
corporations and colleges alike to become concerned about the
potential damage that viruses can inflict.
THE LEHIGH VIRUS
The Lehigh virus was typical of many other viruses. It
sat in the COMMAND.COM file and was thus loaded into the computer
whenever it was booted. The virus hid inside this file in a
temporary storage space called the stack space. After infecting
the same file on a number of other disks, the virus would wipe out
all data and program files on the disk it was on. Backup copies
were similarly infected, some users were attacked more than once.
Once the outbreak had come to light, work began
immediately to identify what was happening and to find a cure.
Fortunately, the virus' creator made a mistake: the date on the
COMMAND.COM file was altered by the infection. (It is relatively
simple to keep the date from changing, so the absence of a changed
file date does not guarantee that a file is virus-free.)
Upon examination of the file, the contaminated stack space
was discovered. Since this space is normally all zeros, student
lab consultants wrote a simple program that looked at the stack
space and wrote zeros over any code that was present. The virus
was then erased from approximately 600 disks.
If it was not for the creator's date mistake, it would
have taken much longer for the Lehigh Computing Center to kill its
virus. It is doubtful that any new viruses that crop up will make
a similar mistake. As everything else related to computers
increases in complexity, so will viruses.
SIZING UP THE PROBLEM
It is unknown exactly how many disks and computer systems
are infected in the world. Some experts and officials are trying
to keep track of the world's viruses by documenting their
characteristics and occurances.
For example, four versions of the Israeli virus and seven
versions of the Brain virus [5] have been found. The Israeli virus
was supposed to do some kind of damage on May 13, 1988, the fortieth
anniversary of the founding of Israel. The Brain virus was originally
written to warn would-be software pirates of a software package for
physicians written by Basit Farooq Alvi, a 19-year-old from Pakistan.
The Brain has since evolved to data destruction.
VIRUS HYPE
Fueling the scare is indeed a problem and has led to what
has become known as the "Virus Hype." The press and media has been
notorious for spreading rumors and partial truths about viruses.
Besides causing undue panic and fear amongst computer users, the
virus writer is getting notoriety and fame. This is shown in a
statement from Stephen D. Morrison, a student from the University
of Manitoba. When asked about the future of viruses, he responded
with the following: "The scenario could be a mad-hacker, plugging
away at a keyboard in the back of a dimly lit office, creating a
virus like no virus ever seen before." This view angers
professionals in the computing field.
Ivars Balkits, an official from Computing Services at the
University of California - Davis, stated, "Depicting the virus
writer as a gothic/romantic figure (like pirates have been, like
gangsters have been, like gang members now are) contributes to the
problem. Continuing to fictionalize the virus writer as a mad
scientist, a Doctor Frankenstein whose genius gives us a secret
thrill, whose lawlessness challenges us, is just the wrong way to
go."
Another approach to stopping the hype and actually
tracking the viruses is "The Dirty Dozen" maintained by Eric
Newhouse [6]. This is a file, originally started by Tom Neff,
which lists unlawfully copied or modified programs that have
appeared on various IBM bulletin boards across the country.
Newhouse hopes that this list will act as a "clearing-house" for
the latest examples of "bogusware," i.e. software that is damaging
to one or more parties. Currently there are almost 50 destructive
programs listed.
In addition to the list of bad software, the Dirty Dozen
contains definitions of viruses and other destructive programs,
instructions on what to do if a virus causes damage to a system,
and a glossary of many of the confusing acronyms and terms used in
the computer field. A list of addresses to send additions and
corrections to the Dirty Dozen, along with comments to Eric
Newhouse, is included in APPENDIX 1. Copies of the Dirty Dozen
can also be obtained from the bulletin boards in the list mentioned
above, as well as from many different electronic bulletin boards
across the country.
DETECTION
Fred Cohen, now a member of the Electrical Engineering
faculty at the University of Cincinnati, stated in a lecture at the
IBM Watson Research Laboratory in Hawthorne, NY, that there are
three ways to detect a virus: by its appearance, by its behavior,
or by the changes it causes. Detection by appearance is
undecidable since all viruses do not "look" alike. It is extremely
difficult to look at a good-sized program written in assembly
language and tell what it does. With an executable program, it is
nearly impossible.
Detection by behavior involves examining programs as they
are executing and is also not very promising. Besides being
disruptive by slowing down execution times, it produces too many
false positives and false negatives. Initially, viruses were
caught by having a monitor program watch for certain internal MS-
DOS and BIOS system calls which are normally used to access system
hardware, but now that is no longer the case.
BIOS is an acronym for basic input/output services. Since
hardware varies from machine to machine, the BIOS is used to
abstract the operating system from the specific hardware it's
running on. The BIOS directly controls all of the input/output
devices, such as the monitor and the disk drives, according to
instructions received from MS-DOS or an executing program.
Unfortunately, viruses can bypass MS-DOS and BIOS system
calls. It is relatively simple to go to a computer store and
purchase literature that describes where MS-DOS and the BIOS keep
the information they need about a disk, and also tells what port
addresses do what on a PC. In order to insure compatibility
between different brands of PC's, every computer manufacturer has
to use the same BIOS data areas and the same port addresses. It is
no mystery to find out exactly what a program has to do to get its
hands on the hardware.
Detection by change is easy to forge and can be very
costly. Early viruses were found to simply append themselves onto
files and thus change the file size or possibly change the file
date, as in the Lehigh virus, viruses have become much more
elusive. Existing files can have viruses implanted inside without
changing their file length or modification date. It is also not
very beneficial to use an erased hard disk as an indicator of viral
presence.
PREVENTION STRATEGIES
"Prevention is the best medicine" is a phrase heard many
times before, but this small advice is very true in the case
against viruses. The key is education. There must be an awareness
among users from the hobbyist to system managers of the potential
dangers of viruses. Obviously, paranoia is not the goal but a
general understanding must be achieved.
With today's ever growing dependence on computers,
ignorance will cost a heavy price, if it has not already.
Therefore, steps must be taken to curtail the likelihood of viral
destruction. Governmental legislation needed is already in
progress: a House bill, the Computer Virus Eradication Act of
1988, was introduced in June that will make infesting computers
with viruses a federal crime. A copy of this pending bill is in
APPENDIX 2. Several other legislative acts have also been
proposed. Currently, 48 states have computer crime laws.
Fortunately, there are some guidelines that, if followed,
will go a long way in keeping one's computer system virus-free. Of
course, these guidelines are only as effective as the extent to
which users are willing to implement them. These guidelines are
divided into three areas - protection of diskettes, protection for
the computer, and protection of systems interconnected by a local
area network (LAN).
DISK PROTECTION
The first thing to do is not to use the original or master
diskettes to execute the programs. Copies of all the original
source disks should be made and used instead. The originals should
then be stored in a safe place, out of sight. Although it is
inconvenient, it is better to have the storage place far away from
the computer or system itself. If there ever is any question as to
the integrity of one of these copied files or disks, it can always
be compared against the safely stored-away master copy.
It is a very good idea to start using the write/protect
tabs that so often get thrown away. These little stickers, usually
black or aluminum colored gummed paper tags, can really save the
day when it comes to inadvertent writes. Once a tab is in place,
it is impossible for the computer to write on the disk.
Besides being found on every system disk, the COMMAND.COM
file is also a favorite hiding place for viruses. This file, as
well as most others, can and should be made read-only without
affecting its use. This can be easily done with the MS-DOS
"ATTRIB.COM" program. Many other utility programs, such as those
listed following the paper in APPENDIX 3, can also accomplish this
task.
COMPUTER PROTECTION
The goal of virus protection can only be accomplished by
limiting computer access. This strategy is simple: keep the
computer "clean" by keeping the virus out. First and foremost,
only tested software should be used. Also, a computer should never
be booted up with an unfamiliar disk. This means that a user must
be especially cautious and extremely careful with public-domain or
shareware programs. Most viruses have a hibernation or incubation
period, so even a seemingly good disk from a friend, co-worker, or
other source can be infected.
To protect a computer's existing files, it is advisable to
establish a good method for backing up files on a regular basis.
One strategy is to do incremental backups three times a week and
perform a complete backup every two months. File attribute (FAT)
tables can and should also be backed up. The intervals between
backups should correspond to the amount of activity on the
computer.
When the computer is not in use, turn it off and lock it
up. When a machine is left turned on and unattended, there is no
way to know what has been installed or run on it while it was
unsupervised. This implies that a computer should never be used
unless the user personally boots it up. As far as locks are
concerned, it is usually negligible to have a key lock installed.
Software locks on PC's are easy to bypass and should not be
trusted.
LANS AND VIRUSES
Beside interconnecting users, LAN's can provide a
excellent route of propagation for viruses. In response to their
initial virus attack, the computing center at Lehigh University has
been taking many steps to reduce the possibilities of any new
outbreaks. According to Kenneth van Wyk, a senior consultant at
Lehigh, additional precautions to those mentioned above should be
taken. The procedures in effect at Lehigh University's PC
laboratories, which can also be applied to other distributed
computing environments, are the following:
1) All public microcomputers contain dual floppy drives
and are connected to LANs (Novell on 3COM boards).
The hard disks were removed.
2) All boot disks are notchless and contain nothing
other than the operating system boot files and the
Novell software needed for the LAN.
3) All Novell hard disks on the file servers are read-
only, with the exception of a "scratch" area where
users can place their temporary files.
4) The "scratch" areas get erased periodically by
Lehigh's student employees.
5) Users logging into the LAN are not automatically
placed in the scratch directory.
VACCINES
With the growing publicity and concern over viruses, there
has been a sudden upspring of so called "vaccines". It may even
seem that the number of these programs are quickly catching up to
the number of known viruses. Keep in mind, however, that none of
these programs are 100% cures, and that many take a different
approach in trying to solve the same problem.
Probably the best attitude to take regarding these
"vaccines" is the that of the Paul Mace Software Company -
"Understand, the people who make these (viruses) are clever and we
haven't seen their worst. We're clever too, and will keep on
improving the vaccine." Several of the software/hardware products
of this nature that are designed for personal computer use at home
and in industry are listed in APPENDIX 4.
AFTER THE ATTACK
Even though precautions are taken, the worst sometimes
happens: a virus evades the lines of defense and wreaks havoc.
Even if a hard disk does manage to crash, regardless of whether it
was virus-induced or not, all is not necessarily lost. Some
investment of time may be needed, but the data can usually be
recovered.
There is no better remedy for a crash of any kind than a
recent backup. Unfortunately, if the virus was backed up along
with the rest of the disk, restoring the backup contents may bring
the virus back to life. If this happens and another crash occurs
from the restoration, it is time to do either a lot of detective
work or seek professional help.
Once a crash has occurred, the first step is to remain
calm. The strong urge to shout and destroy nearby office furniture
has to be suppressed. After this is done, the damage must be
surveyed. The crash is probably a result of the virus doing one of
the following:
1) Formatting the disk
2) Scrambling the FAT (File Attribute) table
3) Erasing files
4) Corrupting the disk's boot sector
The amount of data that can be recovered depends on the cause of
the crash.
At this point if you do not know what you are doing, it is
well worth the time and money to find someone who does. Recovering
data from a crashed disk is a highly technical matter. Further
information on the above causes and their remedies are provided in
APPENDIX 5. Any improper attempts by an inexperienced user can
result in permanent data loss.
FURTHER INFORMATION
One of the best ways to learn more about viruses and
related topics is through VIRUS-L, an electronic mail discussion
forum for sharing information about computer viruses. The computer
that handles this forum is located at Lehigh University and is a
result of the need for more information about viruses after the
Lehigh outbreak.
There are currently several hundred subscribers to the
list from academic and corporate institutions from all over the
world. Discussions on the list include current events, virus
"sightings," practical and theoretical virus prevention methods,
and questions/answers about viruses. The discussions on this list
are extremely informative and educational.
The list is non-moderated and non-digested, which means
that any message sent to the forum goes out immediately to all
subscribers. All submissions to VIRUS-L are stored in weekly log
files which can be down-loaded for later reference. Also, there is
a small archive of some of the public anti-virus programs which are
currently available.
In order to get on the mailing list, a user must have
access to the BITNET network, which is possible through ARPANET,
Internet, and several other networks. If this is the case, than
the user only has to send the message "SUB VIRUS-L <user name>" to
<[email protected]>. Questions and comments about VIRUS-L
can sent to the list's moderator, Kenneth van Wyk, at the addresses
listed in APPENDIX 6.
SUMMARY
Computer viruses, like their biological counterparts, are
constantly changing. It is impossible to predict the course that
future viruses will take. According to William H. Murray of Ernst
& Whinney, "if you can conceive it, and if it could be done by any
other program, then it can be done by a virus." The prevention and
protection methods discussed here are not infallible since they
will need to adapt to the dynamic nature of viruses. This paper is
meant to serve as a useful introduction to the nature of viruses
and how they must be confronted. If this information is
understood, the warnings heeded, and the basic precautions taken,
the probability of a virus attack should be lessened.
APPENDIX 1: The Dirty Dozen
Eric Newhouse, the editor of the Dirty Dozen, can be
contacted for more information at the following addresses:
1) The Crest RBBS/CAMS (160/50 MB), 213-471-2518,
1200/2400. (This is Eric Newhouse's bulletin board)
2) The West LA PC-STORE (50 MB), 213-559-6954,
300/1200/2400.
3) Camelot PC-Board (80 MB), 213-204-6158, 300/1200/2400
- leave E-mail to "NORMAN TEETER" and it will be
relayed.
4) The Source - leave E-mail to "Doctor File Finder"
(Mike Callahan) in IBM SIG #4 and it will be relayed.
APPENDIX 2: The Computer Virus Eradication Act of 1988
Whoever knowingly --
(1) inserts into a program for a computer information or
commands, knowing or having reason to believe that
such information or commands will cause loss to users
of a computer on which such program is run or to
those who rely on information processed on such
computer; and
(2) provides such program to others in circumstances in
which those others do not know of the insertion or
its effects;
or attempts to do so, shall, if any of such conduct affects
interstate or foreign commerce, be fined under this title or
imprisoned not more than 10 years, or both.
Entered July 14th 1988 by Mr. Wally Herger (Congressman from CA)
for himself and Mr. Bob Carr (Congressman from MI); referred to
Committee on the Judiciary.
APPENDIX 3: Disk Utility Programs
1) PC-Tools, Central Point Software. $80.
2) Mace+ Utilities, Paul Mace. $100.
3) Advanced Norton Utilities, Peter Norton. $150.
APPENDIX 4: Vaccine Products
1) Antidote by Quaid Software, Toronto, Canada. Detects
viruses but allows the user to correct the problem.
$60.
2) C-4(Cylene-4) by InterPath Corp., Santa Clara, CA. A
program that resides in ROM and looks out for
viruses. If found, computer activity halts and C-4
warns the user. $30.
3) Data Physician by Digital Dispatch Inc., Minneapolis,
MN. Protects and remove viruses from MS-DOS based
computers.
4) Disk Defender by Director Technologies Inc.,
Evanston, IL. An add on board that will guard the
hard disk.
5) Disk Watcher by RG Software Systems, Willow Grove,
PA. A memory resident utility that "watches" the
disk drives to prevent accidental writes or formats.
$80.
6) Dr. Panda Utilities by Panda Systems, Wilmington, DE.
A set of programs that checks files from BBS and
other software before letting them used. $80.
7) FluShot by Byte's BIX. A free utility. Contact BYTE
magazine or BIX for more information. FREE.
8) Mace Vaccine by Paul Mace Software, Ashland, OR. It
provides write protection for system files. $20.
9) NTIVIRUS by Orion Microsystems, Quebec, Canada.
Monitors the system files for viruses. $30.
10) Passcode System by Dynamics Security Inc., Cambridge,
MA. Complete hardware software protection system.
$200-$2000 depending the size and components needed.
11) Syringe,Canary,Infect by Sophco, Boulder, CO. Three
programs that will "quarantine" a bad disk, test and
remove viruses. $30.
12) Vaccinate by Sophco. A "milder virus" that will warn
the user of other viruses. $195.
13) Virusafe by ComNetco Inc., Bernardsville, NJ. Checks
the system memory for viruses then prevents them from
being used. $250.
14) VirAlarm by Lasertrieve Inc., Metuchen, NJ. Stores
programs on CD-ROM after making sure they are virus-
free.
15) Virus Implant Protection by LeeMah DataCom Security
Corp., Hayward, CA. Uses a dedicated PC to "monitor
unauthorized activities" on other networked
computers.
16) Vaccine by FoundationWare, Cleveland, OH. "5 levels"
of protection from write-protect to checksums. $189.
APPENDIX 5: Recovery from a Disk Crash
Recovering information on a formatted disk depends on the
method of formatting. If the disk was low-level formatted, then
the contents of the files and the directories referencing them have
been over-written. The only hope of recovery is a backup. If the
disk was high-level formatted, then the disk contents have not been
erased and are recoverable to some degree.
Unformatting programs have been written to reconstruct the
contents on the disk. Since MS-DOS breaks up or fragments large
files and stores the pieces wherever there is room on the disk,
complete recovery is only possible if the unformatting programs
have a "picture" of the disk before the crash. This picture is
generally taken by a utility accompanying the unformatting program.
Several of these programs are listed above in APPENDIX 3.
If the FAT table has been scrambled, it can be rebuilt.
Two of the three disk utility programs listed below, Norton
Utilities and PC-Tools, include editors that allow an experienced
user to piece together a FAT table. This is not easy and requires
a large amount of experience and a high degree of proficiency. The
other alternative involves finding a FAT backup program and making
periodic backups. A number of FAT backup programs are public
domain and can thus be obtained from a trusted friend or trusted
computer bulletin board.
If files were erased and the FAT tables are still intact,
then the files may simply have to be unerased. All three of the
disk utility programs listed in APPENDIX 3 can do this. When a
file is erased, the first character of its name is usually changed
to a non-printable character to indicate that it is no longer a
valid directory entry. Everything else is left intact. Since the
contents of erased programs are over-written by newer programs, it
is best to unerase the files the most recent files first. If this
is not done, a previously erased program may grab part of a newer
file.
The last cause of a disk crash is when the boot sector is
either erased or formatted. In this case, the data is still safe
on the disk, but the disk cannot be booted from. Another system
disk in a floppy drive can be used to boot the system. Before
proceeding any further, backup the hard disk in case any damage is
done trying to restore the disk to boot status.
The first thing to try is running the MS-DOS "SYS.COM"
program. This program will copy the system files from one disk to
another. After this is done, COMMAND.COM will have to be copied to
the crashed disk using a simple "COPY" command. Information on
this procedure is available in the MS-DOS manual. If this does not
work, Mace+ Utilities has a function called "restore boot sector"
which should be tried.
If all else fails, the disk should be first backed up and
then low-level reformatted. Instructions for this procedure should
either come with the computer or are available from a computer
store. After this is done, the MS-DOS program "FDISK.COM" be run
to prepare the disk for high-level formatting. This formatting is
done with the DOS "FORMAT.EXE" program. The DOS manual should be
consulted before running any of these MS-DOS commands or programs.
When everything is completed, the backup can be restored.
APPENDIX 6: VIRUS-L
The moderator of VIRUS-L, Kenneth van Wyk, can be
contacted for more information at the following addresses:
1) <[email protected]> on Internet
2) <[email protected]> on BITNET
3) Kenneth van Wyk
User Services Senior Consultant
Lehigh University Computing Center
Bethlehem, PA 18015
(215) 758-3900
REFERENCES
[1] Fred Cohen, "Computer Viruses", PhD dissertation,
University of Southern California, 1985.
[2] P. Honan, "Beware: It's Virus Season", Personal Computing,
July 1988, p36.
[3] P. Karon, "The Hype Behind Computer Viruses", PC Week, May
31, 1988, p49.
[4] Fred Cohen, "On The Implications of Computer Viruses and
Methods of Defense", University of Cincinnati,
unpublished.
[5] J. Pournelle, "Computing at Chaos Manor", BYTE, July 1988,
pp198-200.
[6] E. Newhouse, "The Dirty Dozen", Issue #8a, February 21,
1988.
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