Virus
Tutorial Table of Contents
Table
of Contents
The tutorial is designed to be read
start to finish but should you wish to jump directly to a tutorial page, come
back here and use the table of contents below...
c)
--Needs more--
A
virus reproduces, usually without your permission or knowledge. In general
terms they have an infection phase where they reproduce widely and an attack
phase where they do whatever damage they are programmed to do (if any). There
are a large number of virus types.
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Viruses are a cause of much
confusion and a target of considerable misinformation even from some virus
experts. Let's define what we mean by virus:
A
virus is a program that reproduces its own code by attaching itself to other
executable files in such a way that the virus code is executed when the
infected executable file is executed.
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You could probably also say that the
virus must do this without the permission or knowledge of the user, but that's
not a vital distinction for purposes of our discussion here. We are using a broad
definition of "executable file" and "attach" here.
An obvious example of an executable
file would be a program (COM or EXE file) or an overlay or library file used by
an EXE file. Less obvious, but just as critical, would be the macro portion of
what you might generally consider to be a data file (e.g., a Microsoft Word
document). It's important to also realize that the system sectors on either a
hard or floppy disk contain executable code that can be infected--even those on
a data disk. More recently, scripts written for Internet Web sites and/or
included in E-mail can also be executed and infected.
To attach might mean physically
adding to the end of a file, inserting into the middle of a file, or simply
placing a pointer to a different location on the disk somewhere where the virus
can find it.
Most viruses do their job by placing
self-replicating code in other programs, so that when those other programs are
executed, even more programs are infected with the self-replicating code. This
self-replicating code, when triggered by some event, may do a potentially harmful
act to your computer.
Another way of looking at viruses is
to consider them to be programs written to create copies of themselves. These
programs attach these copies onto host programs (infecting these programs).
When one of these hosts is executed, the virus code (which was attached to the
host) executes, and links copies of itself to even more hosts.
Similar to viruses, you can also
find malicious code in Trojan Horses, worms, and logic bombs. Often the
characteristics of both a virus and a worm can be found in the same beast;
confusing the issue even further.
Note: The balance between viruses, worms, and Trojan Horses
changes from time to time. In the early days of such malware viruses tended to
dominate. Various macro viruses/worms appeared later as the dominate form and
by around 2005 or so Trojan Horses started to be more prominent and by early
2008 they were, by far, the dominant malware.
Before looking at specific virus
types you might also want to consider the following general discussions:
Summary
- A virus is a program that reproduces its own code.
- Generally, the first thing a virus does is to reproduce
(i.e., infect).
- Viruses balance infection versus detection
possibility.
- Some viruses use a variety of techniques to hide
themselves.
- On some defined trigger, some viruses will then
activate.
- Viruses need time to establish a beachhead, so even if
they activate they often will wait before doing so.
- Not all viruses activate, but all viruses steal system
resources and often have bugs that might do destructive things.
- The categories of viruses are many and diverse. There
have been many made and if you get one it should be taken seriously. Don't
be fooled by claims of a good virus; there is no reason at the moment to
create one.
Virus
writers have to balance how and when their viruses infect against the
possibility of being detected. Therefore, the spread of an infection may not
be immediate.
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Viruses need time to infect. Not all
viruses attack, but all use system resources and often have bugs.
Viruses come in a
great many different forms, but they all potentially have two phases to their
execution, the infection phase and the attack phase.
Infection Phase
When the virus executes it has the
potential to infect other programs. What's often not clearly understood is
precisely when it will infect the other programs. Some viruses infect other
programs each time they are executed; other viruses infect only upon a certain
trigger. This trigger could be anything; a day or time, an external event on
your PC, a counter within the virus, etc. Virus writers want their programs to
spread as far as possible before anyone notices them.
It is a serious mistake to execute a
program a few times - find nothing infected and presume there are no viruses in
the program. You can never be sure the virus simply hasn't yet triggered its
infection phase.
Many viruses go resident in the
memory of your PC in the same or similar way as terminate and stay resident
(TSR) programs. (For those not old enough to remember TSRs, they were programs
that executed under DOS but stayed in memory instead of ending.) This means the
virus can wait for some external event before it infects additional programs.
The virus may silently lurk in memory waiting for you to access a diskette,
copy a file, or execute a program, before it infects anything. This makes
viruses more difficult to analyze since it's hard to guess what trigger
condition they use for their infection.
On older systems, standard (640K)
memory is not the only memory vulnerable to viruses. It is possible to
construct a virus which will locate itself in upper memory (the space between
640K and 1M) or in the High Memory Area (the small space between 1024K and
1088K). And, under Windows, a virus can effectively reside in any part of
memory.
Resident viruses frequently take
over portions of the system software on the PC to hide their existence. This
technique is called stealth. Polymorphic
techniques also help viruses to infect yet avoid detection.
Note that worms often take the opposite approach and spread as fast as
possible. While this makes their detection virtually certain, it also has the
effect of bringing down networks and denying access; one of the goals of many
worms.
Attack Phase
Many viruses do unpleasant things
such as deleting files or changing random data on your disk, simulating typos
or merely slowing your PC down; some viruses do less harmful things such as
playing music or creating messages or animation on your screen. Just as the
infection phase can be triggered by some event, the attack phase also has its
own trigger.
Does this mean a virus without an
attack phase is benign? No. Many viruses have bugs in them and these bugs often
cause unintended negative side effects. In addition, even if the virus is
perfect, it still steals system resources. (Also, see the "good" virus discussion.)
Viruses often delay revealing their
presence by launching their attack only after they have had ample opportunity
to spread. This means the attack could be delayed for days, weeks, months, or
even years after the initial infection.
The attack phase is optional, many
viruses simply reproduce and have no trigger for an attack phase. Does this
mean that these are "good" viruses? No! Anything that writes itself
to your disk without your permission is stealing storage and CPU cycles. (Also
see the "good" virus discussion.) This is made worse since viruses that "just
infect," with no attack phase, often damage the programs or disks they
infect. This is not an intentional act of the virus, but simply a result of the
fact that many viruses contain extremely poor quality code.
An an example, one of the most common
past viruses, Stoned, is not intentionally harmful. Unfortunately, the author
did not anticipate the use of anything other than 360K floppy disks. The
original virus tried to hide its own code in an area of 1.2MB diskettes that
resulted in corruption of the entire diskette (this bug was fixed in later
versions of the virus).
There
were over 50,000 computer viruses in 2000 and that number was then and still
is growing rapidly. Sophos, in a print ad in June 2005 claims "over
103,000 viruses." And, Symantec, in April 2008 is reported to have
claimed the number is over one million. Fortunately, only a small percentage
of these are circulating widely.
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There are more MS-DOS/Windows
viruses than all other types of viruses combined (by a large margin). Estimates
of exactly how many there are vary widely and the number is constantly growing.
In 1990, estimates ranged from 200
to 500; then in 1991 estimates ranged from 600 to 1,000 different viruses. In
late 1992, estimates were ranging from 1,000 to 2,300 viruses. In mid-1994, the
numbers vary from 4,500 to over 7,500 viruses. In 1996 the number climbed over
10,000. 1998 saw 20,000 and 2000 topped 50,000. It's easy to say there are more
now. Indeed, in April 2008, the BBC reported
that Symantec now claims "that the security firm's
anti-virus programs detect to 1,122,311" viruses and that "almost two
thirds of all malicious code threats currently detected were created during
2007."
The confusion exists partly because
it's difficult to agree on how to count viruses. New viruses frequently arise
from someone taking an existing virus that does something like put a message
out on your screen saying: "Your PC is now stoned" and changing it to
say something like "Donald Duck is a lie!". Is this a new virus? Most
experts say yes. But, this is a trivial change that can be done in less than
two minutes resulting in yet another "new" virus.
More confusion arises with some
companies counting viruses+worms+Trojans as a unit and some not.
Another problem comes from viruses
that try to conceal themselves from scanners by mutating. In other words, every
time the virus infects another file, it will try to use a different version of
itself. These viruses are known as polymorphic
viruses.
One example, the Whale (an early,
huge, clumsy 10,000 byte virus), creates 33 different versions of itself when
it infects files. At least one vendor counted this as 33 different viruses on
their list. Many of the large number of viruses known to exist have not been
detected in the wild but probably exist only in someone's virus collection.
David M. Chess of IBM's High
Integrity Computing Laboratory reported in the November 1991 Virus Bulletin
that "about 30 different viruses and variants account for nearly all of
the actual infections that we see in day-to-day operation." In late 2007,
about 580 different viruses, worms, and Trojans (and some of these are members
of a single family) account for all the virus-related malware that actually
spread in the wild. To keep track visit the Wildlist, a list which reports virus sightings.
How can there be so few viruses
active when some experts report such high numbers? This is probably because
most viruses are poorly written and cannot spread at all or cannot spread
without betraying their presence. Although the actual number of viruses will
probably continue to be hotly debated, what is clear is that the total number
of viruses is increasing, although the active viruses not quite as rapidly as
the numbers might suggest.
A
virus' name is generally assigned by the first researcher to encounter the
beast. The problem is that multiple researchers may encounter a new virus in
parallel which often results in multiple names.
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What's in a name? When it comes to
viruses it's a matter of identification to the general public. An anti-virus
program does not really need the name of a virus as it identifies it by its
characteristics. But, while giving a virus a name helps the public at large it
also serves to confuse them since the names given to a particular beast can
differ from anti-virus maker to anti-virus maker.
How? Why? Much as they would like
to, the virus writers do not get to name their beasts. Some have tried by
putting obvious text into the virus but most of the anti-virus companies tend
to ignore such text (mostly to spite the virus writers
).
And, any virus writer that insists on a particular name has to identify
themselves in the process--something they usually don't want to do. So, the
anti-virus companies control the virus naming process. But, that leads to the
naming problem.
Viruses come into various anti-virus
companies around the world at various times and by various means. Each company
analyzes the virus and assigns a name to it for tracking purposes. While there
is cooperation between companies when new viruses are identified, that
cooperation often takes a back seat to getting a product update out the door so
the anti-virus company's customers are protected. This delay allows alternate
names to enter the market. Over time these are often standardized or, at least,
cross-referenced in listings; but that does not help when the beast makes its
first appearance.
This problem/confusion will
continue. One practical and well documented example of how it affects a
real-world virus listing can be seen at the WildList site on the page...
One attempt at bringing some order
to the naming problem is Ian Whalley's VGrep [registration required to view page]. VGrep attempts
to collect all of the various virus names and then correlates them into a single
searchable list. While useful, there is, again, the lag time necessary to
collect and correlate the data.
So, get used to viruses having
different names. As Shakespeare said...
What's in a name? That which we call a rose
By any other name would smell as sweet...
Another attempt is the database at VirusPool which "...tries to put information from all known
infections and antivirus creators into one place so you can compare names and
results." I wish them the best of luck.
A new site to try to correlate
malware names: CME - Common Malware Enumeration.
CME provides single, common identifiers to new virus threats to reduce public
confusion during malware outbreaks. CME is not an attempt to solve the
challenges involved with naming schemes for viruses and other forms of malware,
but instead aims to facilitate the adoption of a shared, neutral indexing
capability for malware.
Finally, some vendors have largely
given up with naming specific malware and resorting to generic names for the
type of malware (e.g., Troj/Agent). The malware is being generated faster than
the naming system can reasonably keep up. Look for this to probably continue.
Of course, this will then mean changes to the specific methods of disinfection
as you would no longer be able to download a specific disinfector for a named
beast. Time will tell how this develops.
While
serious if you have one, viruses are only one way your data can be damaged.
You must be prepared for all threats; many of which are more likely to strike
than viruses.
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It's important to keep viruses in
perspective. There are many other threats to your programs and data that are
much more likely to harm you than viruses. A well known anti-virus researcher
once said that you have more to fear from a cup of coffee (which may spill)
than from viruses. While the growth in number of viruses, the introduction of
the Microsoft Word macro viruses, VisualBasic Script worms, and
socially-engineered Trojans now puts this statement into question (even though
you can avoid these by just not clicking on them to open them!), it's still
clear that there are many dangerous occurrences of data corruption from causes
other than from viruses.
So, does this mean that viruses are
nothing to worry about? Emphatically, no! It just means that it's
foolish to spend much money and time on addressing the threat of viruses if
you've done nothing about the other more likely threats to your files. Because
viruses and worms are deliberately written to invade and possibly damage your
PC, they are the most difficult threat to guard against. It's pretty easy to
understand the threat that disk failure represents and what to do about it
(although surprisingly few people even address this threat). The threat of
viruses is really no different; lost concentration or a zero-day attack
can give you the same kind of problem a lost hard drive
can. There are no "cures" for the virus problem. One just has to take
protective steps with anti-virus software and use common sense when dealing
with unknown files and Web links.
There
are many reasons from simple boredom to criminal activity for making money.
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Back in the dim mists of time, most
virus writers were people who just wanted to test the system and push the
envelope. They delighted in finding a way to insert their code into places
where others might not find it and held contests of sorts to see who could do
what the fastest during various conferences.
Another common reason for writing
viruses was to "punish" users for some perceived infraction. The
Brain virus, for example, was said to have been written to punish users of
illegal copies of software (software pirates). Users could become legitimate by
contacting Brain Computer Services for help.
The early virus writer Dark Avenger,
in an interview with Sarah Gordon, put it this way:
The innocent users would be much less affected if they
bought all the software they used (and from an authorized dealer) and if they
used it in the way they are allowed to by the license agreement. If somebody
instead of working plays pirated computer games all day long, then it's quite
likely that at some point they will get a virus. ... Besides, viruses would
spread much less if the 'innocent users' did not steal software, and if they
worked a bit more at the workplace, instead of playing games.
With the advent of virus writing
kits more people entered into the picture. These were largely the bored who had
too much time on their hands and decided to spend it making and distributing
viruses just for the heck of it. Many of these people could not actually program
one if they had to; they just used the kits and put in different parameters and
then sent whatever came out on their way in the hope of getting their name
("handle" actually -- a person's true name on a virus caused them
great problems) mentioned somewhere.
This sort of activity expanded as
the virus and worm and Trojan world expanded and script worms became common.
Indeed, the term "script kiddie" was more or less coined during this time to indicate
someone who would just take an existing script worm, modify a small part of it,
and then release that as a "new" worm.
As spyware and adware started to
appear motives started to change. Money started to enter into the picture.
First came botnets; networks of worms/viruses or Trojans designed to sit on a
system and wait for a central command to do something, maybe crash the
system(s) they were installed on. Then, the botnets evolved; or, at least,
their purpose evolved. The botnet creators realized that they could use the
botnets to make the infected computers send out spam. Since spammers would pay
to send out spam money started to enter into the equation. The botnets were
sending out messages based on the infected users computers' stored address
lists so the spammers had an automatic source of valid E-mail addresses and a
possible way to get through blacklists because they could put the infected
user's return address on the E-mail and the receiver might very well have that
user whitelisted. So, the spammer got what they wanted and the botnet creators
started to get paid.
Once money came into the game,
however, so did crime. Trojans were developed to quickly infect users and then
sent out in the spam so the new users would not only get spam but if they
responded they would be infected by the Trojan as well. Scripts and
Windows/Internet Explorer holes made this form of malware even easier to send
to and infect others who might not have updated their computer system recently.
The use of social engineering to make these message appear "real"
increased so the clickthroughs increased.
The malware sent evolved as well.
Newer malware tended toward collecting information from systems instead of
crashing them or destroying data. This stolen data became even more valuable to
criminals than just the fact that spam was getting through. Identity theft
based on the stolen information increased as the attacks became more targeted.
Some of this malware is designed to
target specific banks in specific countries and is quite professional looking.
And, it's not limited to crimes of identity theft for banking purposes; some
malware targets the massively multiplayer on-line games. Why target games?
Because once you steal someone's credentials in such a game you can pretend to
be that person and sell virtual items to other players. The games have become
so popular that virtual items are going for large prices (a virtual space
station went for $100,000 if you can believe that). Of course, the person doing
the buying is getting scammed and the person who's credentials have been stolen
gets the blame. The criminal, meanwhile, walks with the money.
Rootkit
installation to do the data collection is one of the newer threats and promises
to increase the revenue of the criminal groups behind some of the latest
attacks.
Peer-to-peer networking is also a
target as that allows massive data to be moved. Criminals need to do that
efficiently and anonymously and that's exactly what P2P networks do.
Hardware
is a common cause of data problems. Power can fail, electronics age, add-in
boards can be installed wrong, you can mistype, there are accidents of all
kinds, a repair technician can actually cause problems, and magnets you don't
know are there can damage disks.
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Hardware problems are all too
common. We all know that when a PC or disk gets old, it might start acting
erratically and damage some data before it totally dies. Unfortunately,
hardware errors frequently damage data on even young PCs and disks. Here are
some examples.
Power
Faults
Your PC is busy writing data to the
disk and the lights go out! "Arghhhh!" Is everything OK? Maybe so,
maybe not; it's vital to know for sure if anything was damaged.
Other power problems of a similar
nature would include brownouts, voltage spikes, and frequency shifts. All can
cause data problems, particularly if they occur when data is being written to
disk (data in memory generally does not get corrupted by power problems; it
just gets erased if the problems are serious enough).
- Brownout:
Lower voltages at electrical outlets. Usually they are caused by an
extraordinary drain on the power system. Frequently you will see a
brownout during a heat wave when more people than normal have air
conditioners on full. Sometimes these power shortages will be
"rolling" across the area giving everyone a temporary brownout.
Maybe you'll get yours just as that important file is being written to
disk.
- Voltage Spikes:
Temporary voltage increases are fairly common. Large motors or circuit
breakers in industry can put them on the electrical line. Sudden losses
(e.g., a driver hits a power pole) can causes spikes as the circuits
balance. An appliance in your home can cause a spike, particularly with
older wiring. Lightning can put large spikes on power lines. And, the list
goes on. In addition to current backups and integrity information for your
software and data files, including a hardware voltage spike protection
device between the wall and your computer hardware (don't forget the printer
and monitor) can be very helpful.
- Frequency Shifts:
While infrequent, if the line frequency varies from the normal 60 Hertz
(or 50 Hertz in some countries), the power supply on the computer can be
affected and this, in turn, can reflect back into the computer causing
data loss.
Solution: Consider a combined surge protector and uninterruptible
power supply.
Age
It's not magic; as computers age
they tend to fail more often. Electronic components are stressed over time as
they heat up and cool down. Mechanical components simply wear out. Some of
these failures will be dramatic; something will just stop working. Some,
however, can be slow and not obvious. Regrettably, it's not a question of
"if", but "when" in regard to equipment failure.
Solution: Keep an eye on the specials after three to five years.
Incompatibilities
You can have hardware problems on a
perfectly healthy PC if you have devices installed that do not properly share
interrupts. Sometimes problems are immediately obvious, other times they are
subtle and depend upon certain events to happen at just the wrong time, then
suddenly strange things happen! (Software can do this too!)
Solution: Make a really good backup before installing anything
(hardware or software) so you can revert the system back to a stable state
should something crop up.
Finger
Faults
(Typos and "OOPS! I didn't mean
to do that!")
These are an all too frequent cause
of data corruption. This commonly happens when you are intending to delete or
replace one file but actually get another. By using wild cards, you may
experience a really "wild" time. "Hmmm I thought I deleted all
the *.BAK files; but they're still here; something was deleted; what was it? Or
was I in the other directory?" Of course if you're a programmer or if you
use sophisticated tools like a sector editor, then your fingers can really get
you into trouble!
Another finger fault problem arises
with touchpads below the space bar on notebook computers. It's very easy to
brush the touchpad when you are typing away and suddenly find yourself entering
characters in a screen location very different from where you were before you
touched the pad.
Solution: Be careful and look up now and again to make certain your
cursor is where you want it.
Malicious
or Careless Damage
Someone may accidentally or
deliberately delete or change a file on your PC when you're not around. If you
don't keep your PC locked in a safe, then this is a risk. Who knows what was
changed or deleted? Wouldn't it be nice to know if anything changed over the
weekend? Most of this type of damage is done unintentionally by someone you
probably know. This person didn't mean to cause trouble; they simply didn't
know what they were doing when they used your PC.
Solution: Never run the computer as an administrative user and have
guest accounts available for others who use the computer. Keep up-to-date
backups as well.
Typhoid
Mary
One possible source for computer
infections is the Customer Engineer (CE), or repairman. When a CE comes for a
service call, they will almost always run a diagnostic program from diskette.
It's very easy for these diskettes to become infected and spread the infection
to your computer. Sales representatives showing demonstrations via floppy disks
are also possibly spreading viruses. Always check your system after other
people have placed their floppy disk into it. (Better yet, if you can, check
their disk with up-to-date anti-virus software before anything is run.)
Solution: Insist on testing their disk before use or make certain
they've used an up-to-date anti-virus before coming to your location.
Magnetic
Zaps
Computer data is generally stored as
a series of magnetic changes on disks. While hard disks are generally safe from
most magnetic threats because they are encased within the computer compartment,
floppy disks are highly vulnerable to magnets. The obvious threat would be to
post a floppy disk to the refrigerator with a magnet; but there are many other,
more subtle, threats.
Some of the more subtle sources of
magnetism include:
- Computer Monitor.
Don't put floppy disks anywhere near the monitor; it generates a magnetic
field.
- Telephone.
When ringing, telephones (particularly older phones with a bell) generate
a magnetic field.
- Bottom Desk Drawer.
While the desk drawer does not generate a magnetic field, the vacuum
cleaner that the maintenance people slide under the desk to clean the floor
does.
- Bottom Bookcase Shelf and File Cabinet Drawer. Same comment as the desk drawer just above.
- Pets.
Pet fur generates a strong electrostatic charge which, if discharged
through a disk, can affect files on the disk. Instead of "The dog ate
my homework," today it could just as easily be: "The cat sat on
my homework." (I once had a student where this exact problem
happened; a cat sat on her floppy disk and static wiped out the data on
the disk.)
Solution: Stay away from magnets or sources of static of all kinds
when working with a computer.
Bottom line: There are tools to assist in recovery from disk problems,
but how do you know all the data is OK? These tools do not always recover good
copies of the original files. Active action on your part before disaster
strikes is your best defense. It's best to have a good, current backup and, for
better protection, a complete up-to-date integrity-check map of everything on
your disk.
Software
interactions are a significant source of problems; but these are inadvertent.
Software attacks are deliberate and can also be significant.
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Software threats can be general
problems or an attack by one or more types of malicious programs.
Software
Problems
This category accounts for more
damage to programs and data than any other. We're talking about non-malicious
software problems here, not viruses. Software conflicts, by themselves, are
much more likely threats to your PC than virus attacks (unless you do something
like click on a link you should not have or install unknown/cracked software).
We run our PCs today in a complex
environment. There are many resident programs (e.g., anti-virus, video drivers)
running simultaneously with various versions of Windows, DOS, BIOS, and device
drivers. All these programs execute at the same time, share data, and are
vulnerable to unforeseen interactions between each other. Naturally, this means
that there may be some subtle bugs waiting to "byte" us. Any time a
program goes haywire, there's the risk it may damage information on disk.
There's the further problem that not
all programs do what we hope they will. If you have just undeleted a file, you
don't really know if all the correct clusters were placed back in the right
order. When SCANDISK or CHKDSK "fixes" your disk for you, you have no
way of knowing exactly what files it changed to do its job. It becomes even
more complex if you use other utilities to do similar tasks.
Software problems happen and can be
very serious if you have not taken appropriate action in advance of the
problem.
Software
Attacks
These are programs written
deliberately to vandalize someone's computer or to use that computer in an
unauthorized way. There are many forms of malicious software; sometimes the
media refers to all malicious software as viruses. This is not correct and it's
important to understand the distinction between the various types as it has
some bearing on how you react to the attack. The discussions that follow attempt
to make clear distinctions between malicious software types. Realize that often
a malicious program may have characteristics of more than one of these types
(e.g., a virus that attacks files but also spreads itself across a network).
Don't get wrapped up in the semantics, just try to understand the major
differences.
In addition to viruses, the main
thrust of this tutorial, there are:
- Logic Bombs.
Just like a real bomb, a logic bomb will lie dormant until triggered by
some event.
- Trojans.
These are named after the Trojan horse, which delivered Greek soldiers
into the city of Troy.
- Worms.
A worm is a self-reproducing program that does not infect other programs
as a virus will, but instead creates copies of itself, that create even
more copies.
Finally, a type of malicious
software that could be classified under Trojan but we've put on a page of its
own as a special case:
- Virus Droppers.
A dropper is a program that, when run will attempt to install a regular
virus onto your hard disk.
A
logic bomb will lie dormant until triggered by some event.
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Just like a real bomb, a logic bomb
will lie dormant until triggered by some event. The trigger can be a specific
date, the number of times executed, a random number, or even a specific event
such as deletion of an employee's payroll record.
When the logic bomb is triggered, it
will usually do something unpleasant. This can range from changing a random
byte of data somewhere on your disk to making the entire disk unreadable.
Changing random data may be the most insidious attack since it generally causes
substantial damage before anyone notices that something is wrong. It's vital to
have software in place that quickly detects such damage.
Although you can detect it after the
fact, there is unfortunately no way to prevent a well written logic bomb from
damaging your system. This is one reason (among many) that having good
backups of important data is so important.
If you've had someone in to do any
system work on your computer (e.g., custom programming) it's particularly
important that you independently verify the work was done correctly and to
verify no trap doors or logic bombs were inserted into your systems. Work like
custom programming require programmers to have detailed access to your systems;
just the kind of access someone who wanted to insert a logic bomb into your
system would love to have. (This is not to say independent contractors are
worse than any other person who has low-level access to your systems; it's just
one obvious example.) And, with today's remote desktop built into Windows; it's
even easier to give such control over to a support person at some remote
software vendor's location or someone posing as such.
Some historic logic bombs include...
- In 1982, the CIA was tipped to a plan to steal control
system plans from a Canadian firm for use in the Trans-Siberian pipeline.
They had the company insert a logic bomb which resulted in a large
explosion when triggered.
- In June 1992 a defense contractor employee was arrested
for inserting a logic bomb into a rocket project. Supposedly his plan was
to come back as a consultant and "solve" the problem for a large
fee.
- In February 2000 a programmer was indicted before a
grand jury; accused of planting a logic bomb at Deutsche Morgan Grenfell.
It was planted in 1996 and supposed to trigger in mid-2000 but was
discovered before it went off.
- In October 2003 a Unix administrator changed code on a
server at Medco Health Solutions Inc. that was supposed to go off on his
birthday in 2004. An error caused it to fail so he wrote another for the
next year but it was discovered before it could go off.
- In June 2006 a system administrator for UBS was charged
with using a logic bomb to commit securities fraud. He was convicted.
- On 29 January 2009 it was reported that a Fannie Mae contractor who had been let go
managed to insert a script designed to execute on 31 January 2009 in to
the Fannie Mae system. Apparently, the contractor was allowed to keep his
access and computer for a short time after he was notified of the
termination (a major error on the part of Fannie Mae IT/security). The script,
found before it executed, would have wiped clean some 4,000 servers.
Like
the horse, a Trojan program is a delivery vehicle; a program that does
something undocumented and often malicious.
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These malicious programs are named
after the Trojan horse, which delivered soldiers into the city of Troy.
Like the horse, a Trojan program is
a delivery vehicle; a program that does something undocumented which the
programmer intended, but that the user would not approve of if s/he knew about
it. The Trojan program appears to be a useful program of some type, but when a
certain event occurs, it does something nasty and often destructive to the
system.
Most of the "classic"
Trojan programs were delivered to users on disks which advertised themselves as
something useful. As an example, a disk that was supposed to contain Aids
information was once distributed. Unfortunately, when a program on the disk was
run the user's hard disk was encrypted and rendered useless. Many newer Trojan
programs make their way to you as E-mail attachments with the text in the
E-mail program enticing you to run the attachment.
There have been many Trojan programs
and new ones crop up every day. It's important to know and trust the source of
any program you receive because most anti-virus programs can't detect new
Trojans. These programs, while potentially destructive, still use common
DOS/Windows commands and any attempt to trigger an alert on these commands
would result in massive false alarms.
Most anti-virus programs today
include Trojans as soon as they are circulating as Trojans make up much of the
malware in 2005/2006; but it may still be too late for you as it takes some
time to update their databases. Trojans are, however, simple to avoid if you don't sucumb to the lures
of the E-mails that send them to you.
Just to give you some examples of
what sort of thing to watch out for, here are some Trojan examples, some
historical and some recent. Brief descriptions are given here with more detail
is available in the link.
- ANSI Bomb. (rare
today). This sort of Trojan used the ANSI.SYS driver in DOS to remap
various display and keyboard functions.
- Windows Help Macros. (rare but
demonstrated). The Windows HLP help file format allowed macros to be
attached to help files. The macros could contain malicious code.
- Social Engineering Messages. A wide variety of Trojans use social engineering to
attempt to get you to run the malware associated with the message.
- Double
File Extensions. Windows
generally comes with the display of common file extensions turned off by
default. Files of the form README.TXT.EXE would show up as README.TXT but
if you clicked on the file it would run as a program.
- Screen Savers.
Windows screen savers are basically executable code and malicious software
in one can run in the background during the display.
- Road Apple. A Trojan
may be given a name the curious would naturally be interested in and then
left where the curious can find it.
- Physical Media. A Trojan
could be widely distributed using physical media sent to many around the
world. The subject would have to be compelling (an AIDS Trojan distributed
via CD is one example that has happened).
- And, many more. See Wikipedia for more examples.
Some researchers consider a virus a
particular case of a Trojan horse; others believe that if a virus does not do
any deliberate damage it cannot be classed as a Trojan. In common use, most
people (including Computer Knowledge) use Trojan to refer to a non-replicating
malicious program.
A
worm is a self-reproducing program that does not infect other programs as a
virus will.
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A worm is a self-reproducing program
that does not infect other programs as a virus will, but instead creates copies
of itself, and these create even more copies.
Worms are usually seen on networks
and on multi-processing operating systems, where the worm will create copies of
itself that are also executed. Each new copy will create more copies quickly
clogging the system. Keep in mind, however, that most PCs are connected to a
network (the Internet) and so are targets for worms.
The so-called ARPANET/INTERNET
"virus" was actually a worm. It created copies of itself through the
network, eventually bringing the network to its knees. It did not infect other
programs as a virus would, but simply kept creating copies of itself that would
then execute and try to spread to other machines.
Some newer macro viruses also send
their infected documents over the Internet to others who then infect their
systems and spread the virus further. Some have classed these as worms.
However, because these programs require a host in order to spread (even though
they send themselves and the host over a network) Computer Knowledge (and most
anti-virus researchers) puts these beasts into the virus category. But, you can
see where distinctions between categories can get blurred.
The newer script worms don't help
clarify the classification issue. Many of these are sent as a VisualBasic
Script (VBS) file attached to an E-mail message. If you click on the attachment
to open it the script runs and will often send the script to addresses in your
E-mail address book; thus spreading itself. Technically, these would be worms
but are often called viruses.
Bottom line: Don't really try to make a firm distinction between a worm
and a virus. You'll just get frustrated. Call it a virus and be done with it
but understand, deep down, that it just might be a worm.
A
dropper is a program that, when run will attempt to install a regular virus
onto your hard disk.
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Normally, you obtain a virus by
either attempting to boot from an infected floppy disk, by running an infected
file, or by loading an infected document with viral macro commands in it. There
is another way you can pick up a virus: by encountering a virus dropper. These
are rare, but now and again someone will attempt to be clever and try to
program one.
Basically, a dropper is just what
the name implies: a program designed to run and install (or "drop") a
virus onto your system. The program itself is not infected nor is it a virus
because it does not replicate. So, technically, a dropper should be considered
a Trojan. Often,
because the virus is hidden in the program code, a scanner will not detect the
danger until after the virus is dropped onto your system. (It's technically
possible to write a virus that also drops other viruses, and several have been
tried. Most are very buggy, however.)
It's a technical point, but there is
a class of dropper that only infects the computer's memory, not the disk. These
are given the name injector by some virus researchers.
A dropper is a program (malware
component) that has been designed to "install" some sort of malware
(virus, backdoor, etc) to a target system. The malware code can be contained
within the dropper (single-stage) in such a way as to avoid detection by virus
scanners or the dropper may download the malware to the target machine once
activated (two stage).
There are two major types of
droppers, those that do not require user interaction which perform through the
exploitation of a system by some vulnerability and those that require user
interaction by convincing the user that it is some legitimate or benign
program. A dropper which installs a malware program to memory only is sometimes
called an injector.
Viruses
come in many types; written using many different infection strategies.
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Computer viruses come in a variety
of types. Breaking them into categories is not easy as many viruses have
multiple characteristics and so would fall into multiple categories. We're
going to describe two different types of category systems: what they infect
and how they infect. Because they are so common, we're also going to include a
category specific to worms.
What They Infect
Viruses can infect a number of
different portions of the computer's operating and file system. These include:
How They Infect
Viruses are sometimes also
categorized by how they infect. These categorizations often overlap the
categories above and may even be included in the description (e.g., polymorphic
file virus). These categories include:
Blended Threats
And, as you might expect, not all
malware operates according to a single rule. Combinations like a Trojan with
embedded virus and many other combinations exist. Plus, a single virus may have
multiple attack vectors. The categories above are more for understanding a
technique than to say these are the single techniques used or even the only
techniques used.
Now either click on the virus topic
you are interested in or read about each in sequence...
Viruses
can infect a number of different portions of the computer's operating and
file system.
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Viruses can infect a number of
different portions of the computer's operating and file system. These include:
That's a summary, now see how each
of these might work by continuing on in the tutorial.
System
sectors (Master Boot Record and DOS Boot Record) are often targets for
viruses. These boot viruses use all of the common viral techniques to infect
and hide themselves. While mostly obtained from an infected disk left in the
drive when the computer starts, they can also be "dropped" by some
file infectors or Trojans.
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System sectors are special areas on
your disk containing programs that are executed when you boot (start) your PC.
Every disk (even if it only contains data) has a system sector of some sort.
Sectors are simply small areas on your disk that your hardware reads in single
chunks. System sectors are invisible to normal programs but are vital for
correct operation of your PC. In the early days, they were a common target for
viruses; as floppy drives went out of fashion the instance of these diminished
to almost zero and then about 2008 infection of the system sectors started to
rise as a way to get rootkits running
on a system.
There are two types of system
sectors found on DOS/Windows PCs:
System sector viruses modify the
program in either the DOS boot sector or the Master Boot Record. Since there
isn't much room in the system sector (only 512 bytes), these viruses usually
have to hide their code somewhere else on the disk. These viruses sometimes
cause problems when this spot already contains data that is then overwritten.
To make themselves harder to find a system sector virus will sometimes find the
"end" of the disk and write itself to the disk in an area beyond this
with special routines to access that area to get its code back out.
Some viruses, such as the Pakistani
Brain virus, mark the spot where they hide their code as bad. This is one
reason to be suspicious if any utility suddenly reports additional bad sectors
on your disk and you don't know why (don't panic, bad sectors occur frequently
for a wide variety of reasons). These viruses usually go resident in memory on
your PC, infect the hard disk, and infect any floppy disk that you access.
Simply looking at the directory of a floppy disk may cause it to be infected if
one of these viruses is active in memory. The more modern of these beasts exist
as rootkits which can load either before, with, or as part of the operating
system.
On Macintosh systems, some of these
viruses even infected a diskette immediately upon inserting a diskette into the
floppy drive. (PCs generally do not access a disk automatically as the
Macintosh does.)
Since viruses are active in memory
(resident), they can hide their presence. If Brain is active on your PC, and
you use a sector editor to look at the boot sector of an infected diskette, the
virus will intercept the attempt to read the infected boot sector and instead
return a saved image of the original boot sector. You will see the normal boot
sector instead of the infected version. Viruses that do this are known as stealth viruses.
In addition to infecting diskettes,
some system sector viruses also spread by infecting files. Viruses of this type
are called multipartite
(multiple part) viruses. Since they can infect both files and system sectors
they have more avenues to spread. (Note: Some file viruses also infect
system sectors to complete the circle.)
While
more in number, file infectors are not the most commonly found. They infect
in a variety of ways and can be found in a large number of file types.
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In terms of sheer number of viruses,
these were the most numerous for some time. However, because of bugs in the
virus code, they have not been the most widely spread.
The simplest file viruses work by
locating a type of file they know how to infect (usually a file name ending in
.COM or .EXE) and overwriting part of the program they are infecting. When this
program is executed, the virus code executes and infects more files. These
overwriting viruses do not tend to be very successful since the overwritten
program rarely continues to function correctly and the virus is almost
immediately discovered.
The more sophisticated file viruses
save (rather than overwrite) the original instructions when they insert their
code into the program. This allows them to execute the original program after
the virus finishes so that everything appears normal.
Just as system sector viruses can
remain resident in memory and use stealth techniques to hide their presence,
file viruses can also hide this way. If you do a directory listing, you will
not see any increase in the length of the file and if you attempt to read the
file, the virus will intercept the request and return your original uninfected
program to you.
Some file viruses (such as 4096)
also infect overlay files as well as the more usual *.COM and *.EXE files.
Overlay files have various extensions, but .OVR and .OVL are common (overlay
files are almost never used today; they are something you found in the MS-DOS
days). Files with the extension .DLL are also capable of being infected (but
generally are not; typically they are only libraries of functions). Indeed, as
operating systems become more advanced, typically more files become able to
contain executable code and thus be vulnerable to infection. (See the file
extension list for a more complete summary.)
Pure
data files cannot propagate viruses, but with extensive macro languages in
some programs the line between a "data" file and executable file
can easily become blurred to the average user. While text E-mail messages
can't contain viruses they may have attachments that do, some will run active
code in a message, and some E-mail programs will automatically load and run
attachments. Don't let them. Finally, be careful of programs that use other
programs for reading E-mail.
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As indicated throughout this
tutorial, in order for a virus to do anything, first a program of some type
must execute. A virus, no matter what type, is still a program and it must load
into memory and run in order to do anything. Simply reading it into memory is
not sufficient. Pure data files are not viruses simply because, by their
nature, they do not execute.
The problem, however, is that many
modern programs contain some form of macro language; in some cases a very
powerful macro language with commands that include opening, manipulating, and
closing files. More and more, these programs allow a user to extend their
capabilities by writing powerful macros and then attaching these to data files
produced by that program. In many cases, in order to make things easy for
users, the macros are set up to run automatically whenever the data file is
loaded. It's in cases like this where the line between a data file and program
starts to blur.
Note: There are many triggers (other
than loading the document) that viral code can exploit. And, once running, various
elements of the program's macro language can be exploited so that all future
data files produced by that program version could contain the viral macro code.
Most scanners have default settings
that check the most common executable files and data files from programs that
have a macro language. So, when using those programs it's a good idea to not
change the default extension so scanners can find the files they need to. Also,
scanners can be set to check every file instead of just files that normally execute;
but most do not do this by default--that would make the scanning process too
long for most people.
In order to know when to turn full
scanning on you need to know something about the software you use. In
particular, you need to make yourself aware of any software that uses the sort
of "automatic macro" feature described here. Never use a piece of
software until you've explored its manual for some time just to see its full
capabilities. If these include some sort of "programming" (macro)
language, be aware there is an opportunity for problems. Common programs with
macro capability that can be exploited by virus writers are Microsoft Word®,
Excel® and other Office programs. Windows Help files can also contain macro
code (but are rarely exploited because of the difficulty in doing so). And, at
one time some macro code to be exploited existed in the full version of the
Acrobat program which reads and writes PDF files (the free reader is not
affected; only the full version).
A second vulnerability exists on the
Internet. Some E-mail programs and Internet browsers allow you to click on a
data file or program that might be attached to a message or displayed on a web
page and have that file or program load and/or run automatically. You should
not allow this to happen. Always save the file or program to disk and then
check it with anti-virus software before loading or executing it (or have an
anti-virus program that "attaches" to your programs such that it
checks files before the program loads them or checks E-mail as it comes in).
And, even more insidious are newer
E-mail programs that allow one to use programs like Microsoft Word to read and
write messages. You may not even know you are using Word. But, since the E-mail
program does use Word, macros can be encoded into the message and be made to
run on your system when you open the message to read it. It is very important
that you know the characteristics of programs you use! Only then will you be
able to determine if you are at risk.
Companion
viruses make use of a DOS quirk that runs COM files before EXE files. The
virus infects EXE files by installing a same-named COM file.
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Would you believe that a virus can
infect your files without changing a single byte in the infected file? Well,
it's true; two different ways in fact! The more common of the two ways is
called the companion or spawning virus (the other is a cluster virus).
The companion virus infects your files by locating all files with names ending
in EXE. The virus then creates a matching file name ending in COM that contains
the viral code.
Here's what happens: Let's say a
companion virus is executing on your PC and decides it's time to infect a file.
It looks around and happens to find a file called PGM.EXE. It now creates a
file called PGM.COM containing the virus. The virus usually plants this file in
the same directory as the .EXE file but it could place it in any directory on
your DOS path. If you type PGM and hit enter, DOS will execute PGM.COM instead
of PGM.EXE. (In order, DOS will execute COM, then EXE, then BAT files of the
same root name, if they are all in the same directory.) The virus executes,
possibly infecting more files and then loads and executes PGM.EXE. The user
probably won't notice anything wrong.
This type of virus is fairly easy to
detect by the presence of the extra COM files. Sometimes the virus attempts to
hide the extra files by either placing them into a different directory (but one
on the PATH) or gives them a hidden attribute so a normal DIR command will not
show them. And, of course, when the virus is active in memory it can
effectively hide the COM files as well (but, unlike many viruses, a companion
infector need not remain in memory to do its work).
A good integrity map of what should
be on the hard disk can be used to easily detect and clean companion viruses.
Note: There are some instances where it is normal to have both
COM and EXE files of the same name (such as DOS 5's DOSSHELL) but this is
relatively rare. When this is the case, the companion virus will usually not
change the existing COM file (although some are sloppy and will).
Companion viruses were never
particularly common and under Windows where specific files are associated with
icons you likely won't see them.
Viruses are sometimes also
categorized by how they infect. These categorizations often overlap the
categories above and may even be included in the description (e.g., polymorphic
file virus). These categories include:
- Tunneling Viruses
Viruses that try to "tunnel" under anti-virus software while
infecting.
- NTFS ADS Viruses
Viruses that ride on the alternate data streams in the NT File System.
- Buffer Overflow
Viruses can sneak into the system when data overflows a buffer holding it.
- Botnet
Botnets can spread viruses and other malware automatically or on command.
- Social Engineering
This is probably the fastest way for a virus, worm or Trojan to spread.
- Search Poisoning
Viruses can be spread by Web pages constructed to be high in search
results.
These
pages detail some techniques that various historical viruses/worms use to
infect. A current threat feed is also provided.
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These pages detail some techniques
that various historical viruses/worms use to infect. There is not enough
information here to enable you to write a virus (and please don't ask for more
-- it won't be sent!); however, you should get an idea of some past attempts at
malware.
These are the viruses described
here...
Legacy
Viruses
Current
Threats
The feeds here are provided by the
sites mentioned and the information content belongs to those sites. Links here
open in a new page/tab.
Current Virus News
Sep 10, 2010
Apple released iOS version 4.1
yesterday and it patches 24 security vulnerabilities. 20 of the
vulnerabilities are related to ...
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Not
all computer viruses are designed to be destructive. They all do, however,
use various resources that could be put to other uses by a computer so in
that regard, all computer viruses and other malware have some effect on a
computer.
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I think computer viruses should
count as life. I think it says something about human nature that the only form
of life we have created so far is purely destructive. We've created life in our
own image.
Despite Hawking's quote, not all
computer viruses are designed to be destructive. They all do, however, use
various resources that could be put to other uses by a computer so in that
regard, all computer viruses and other malware have some effect on a computer.
Those that are programmed to have an effect, however, do so in a wide variety
of ways that can be subtle or very destructive.
Among the subtle effects are various
screen display changes. Early viruses tended to uses these sorts of effects to
call attention to themselves when the author wanted that to happen. For
example, random icons might be displayed on the desktop (e.g., the Windows 95
virus Marburg). Or, in the extreme, the virus might even want you to play
a game (e.g., Playgame).
Not all viruses are so benign
however. There are many that will attempt to be very destructive to data. Some
(e.g., Michelangelo) attempt to overwrite the data on your computer. Others
(e.g., Hungarian Filler) just overwrite the file allocation table or directory.
Still others (e.g., Disk Killer) may attempt to encrypt your data and then hold you hostage
by offering the decryption key for a fee. There is little limit to the
imagination of the people who write these beasts and so just about anything is
fair game.
I'll get an argument on this but
there are even viruses that attack the computer's hardware. Many say you can't
destroy computer hardware with malware but Computer Knowledge contends that
rendering the hardware unusable is as good as destroying it and it's in this
regard that malware can attack hardware. In particular, today, the BIOS in a
computer (the small program on a chip that is called when a computer starts up)
usually comes on a chip that can be rewritten using software in order to
provide the rare update to the BIOS without requiring the user to change the
hardware chips to perform the upgrade. Such BIOS chips can be easily
overwritten or cleared and that renders the computer useless until and unless
you can use a low-level utility on removable media to restore the BIOS and
allow the computer to then boot normally. While this damage is not permanent,
it is very inconvenient and not everyone will be capable of doing this sort of
restore. An example of this sort of virus is the CIH Spacefiller
virus.
Most today, however, are not
attempting to destroy data but, instead, are after some sort of profit. Virus
writing has turned a commercial corner and money can often be the driving force
behind the various payloads. Here are some more specific actions that malware
can take. Keep in mind that this is not an exclusive list.
6.Virus
History Summary
Below is an expanded summary of the
history of viruses from the start to today.
Note: There are endless arguments about the "first"
virus. There were a number of malware attacks in the 1970s and some count these
among the virus attacks. The description of the malware, however, would
indicate these were worms and not viruses by general definition. Just to be
complete, however, the questionable entries from the 1970s are included here
with the caveat that Computer Knowledge considers virus history to start in
1981.
Pre 1981 Period
In the early 1970s Creeper
was found an ARPANET. It was a worm that moved through modems to other systems
where it displayed the message "I'M THE CREEPER : CATCH ME IF YOU
CAN." A similar program called Reaper followed Creeper. It appeared
to attempt to find and delete Creeper. In 1974 malware called Rabbit
which multiplied so fast making copies of itself that systems crashed. In 1975
a game written for the UNIVAC 1108 called Pervading Animal. The game
asked questions in an attempt to determine what animal the user had thought of.
The game, however, attempted to write itself to every writable program file,
changing the creation time to be able to determine if it had already written to
that file or not. It was never determined if this Trojan-like behavior was
intentional or just an unintended bug. In the theoretical arena Jürgen Kraus
wrote a master thesis called Selbstreproduktion bei Programmen
(Self-reproduction of programs).
Now, on to the modern history.
1981 - The First Virus In The Wild
As described in Robert Slade's
history, the first virus in the wild actually predated the experimental work
that defined current-day viruses. It was spread on Apple II floppy disks (which
contained the operating system) and reputed to have spread from Texas A&M.
[Side note: Thanks to a pointer from anti-virus pioneer Fridrik Skulason
we know the virus was named Elk Cloner and displayed a little rhyme on the
screen:
It
will get on all your disks
It will infiltrate your chips
Yes it's Cloner!
It will stick to you like glue
It will modify ram too
Send in the Cloner!
For more info on Elk Cloner see the
author's (Richard Skrenta) page at:
1983 - The First Documented
Experimental Virus
Fred Cohen's seminal paper Computer
Viruses - Theory and Experiments from 1984 defines a computer virus and describes the
experiments he and others performed to prove that the concept of a computer
virus was viable. From the paper...
On November 3, 1983, the first virus
was conceived of as an experiment to be presented at a weekly seminar on
computer security. The concept was first introduced in this seminar by the
author, and the name 'virus' was thought of by Len Adleman. After 8 hours of expert
work on a heavily loaded VAX 11/750 system running Unix, the first virus was
completed and ready for demonstration. Within a week, permission was obtained
to perform experiments, and 5 experiments were performed. On November 10, the
virus was demonstrated to the security seminar.
1986 - Brain, PC-Write Trojan, &
Virdem
The common story is that two
brothers from Pakistan (Basit Farooq Alvi and Amjad Farooq Alvi) analyzed the
boot sector of a floppy disk and developed a method of infecting it with a virus
dubbed "Brain" (the origin is generally accepted but not absolutely).
Because it spread widely on the popular MS-DOS PC system this is typically
called the first computer virus; even though it was predated by Cohen's
experiments and the Apple II virus. That same year the first PC-based Trojan
was released in the form of the popular shareware program PC-Write. Some
reports say Virdem was also found this year (it was presented to the
underground Chaos Computer Club in Germany in December by Ralf Burger according
to reports); it is often called the first file virus.
Finding
a virus on your system may not be easy; they often don't cooperate. Using
anti-virus tools is important.
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A virus may or may not present
itself. Viruses attempt to spread before activating whatever malicious activity
they may have been programmed to deliver. So, viruses will often try to hide
themselves. Sometimes there are symptoms that can be observed by a trained
casual observer who knows what to look for (but, don't count on it).
Virus authors often place a wide
variety of indicators into their viruses (e.g., messages, music, graphic
displays). These, however, typically only show up when the virus payload
activates. With DOS systems, the unaccounted for reduction of the amount of RAM
known to be in the computer is an important indicator resident viruses have a
hard time getting around. But, under Windows, there is no clear indicator like
that. The bottom line is that one must use anti-virus software to detect (and fix)
most viruses once they are on your system.
Your main defense is to detect and
identify specific virus attacks to your computer. There are three methods in
general use. Each has pros and cons and are discussed via these links. Often, a
given anti-virus software program will use some combination of the three
techniques for maximum possibility of detection.
In a more general sense, check here
for some ideas about using the above-referenced methods and other useful
information:
Another line of defense is
continuing education. Click below to see some sources of on-going information.
