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Where There's Smoke, There's Mirrors: The Truth about Trojan Horses on the Internet

David Chess, Sarah Gordon
Virus Bulletin Conference in Munich, Germany
October 1998

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This paper will examine the prevalence, technical structure and impact of non-viral malicious code ("Trojan horses") on the Internet, and its relevance to the corporate and home user. Using user simulations and first-hand reports provided by real users, we will explore the Trojan experience, focusing on the type and scope of actual Trojan threats encountered on the Internet today. We will discuss the status of hostile active content, including Java and ActiveX, on the Internet, and examine its impact in the real world. We will present strategies for minimizing the risk of damage from Trojan horses on the Internet. Finally, we will discuss how simply extending anti-virus software into "bolt-on" detectors of known hostile code is no substitute for ensuring that your systems are secure against all attacks, known or unknown.

A Not So Brief History of Trojan Horses

First Things First

Trojan horse computer programs draw their name from their mythological namesake, "The Trojan horse ". In Greek mythology, the gods, just like the humans they ruled, were often involved in personal and political battles. One such battle began when Eris, the goddess of strife, was not invited to the wedding of the gods Peleus and Thetis. She threw a golden apple inscribed "to the fairest" into the center of the wedding gathering. Athena, the goddess of wisdom, immediately claimed the apple, but she was not the only one. Hera, the goddess of marriage (and wife of Zeus) and Aphrodite, the goddess of love, claimed it as well. The golden apple became known as the Apple of Discord - for obvious reasons. The three goddesses who claimed the apple asked Zeus to determine which of them actually deserved the apple. The request for this information placed Zeus in somewhat of an awkward position. He knew that no matter whom he chose, the other two would be displeased and, given their nature, seek revenge. To avoid being personally involved in the conflict, he sought out another arbitrator - Paris, a simple human shepherd boy.

The three goddesses tried to bribe Paris. Hera had offered to make him a king, but he was not easily swayed. Probably he liked his simple life, spending bucolic days with his flock of sheep. Athena had offered wisdom and discernment in exchange for the prize, but this too failed. Aphrodite offered Paris the opportunity to possess the most beautiful woman in the world: Helen of Sparta. Paris was ruled by his heart. Aphrodite got the apple, and, to make a very long and complex story short, Paris got Helen and promptly carried her back home to Troy. There was one problem. She was already married.

Paris' choice, like most choices, was not without its consequences. Hera and Athena became bitter enemies of Paris and of his clansmen, the people of Troy (the Trojans). As legend has it, the two losing Goddesses sided with the Greeks in the Greeks vs. the Trojans war, masterminding one battle in particular. The Greeks had been waiting for their opportunity to decimate Troy, and within the city gates, the Trojans waited for the battle to begin. They waited, and waited, for years. Suddenly, it appeared to them that the Greeks had given up and gone home, leaving behind a huge wooden horse as a parting gift. The relieved Trojans opened the gates, and rolled in the "gift". That night the Greek warrior Odysseus and his men came forth from the belly of the horse, where they had been hiding. They opened the city gates and led in the rest of the Greek soldiers. While the Trojans slept peacefully in their beds, the Greek soldiers vanquished the city of Troy and reclaimed Helen for Greece.

The point? The horse, referred to thereafter as "The Trojan horse", was let in by the inhabitants of the city. It facilitated the destruction of the city, and ever since, the expression Trojan horse has been used to talk about something that appears to be one thing, which is gladly welcomed, but which hides an attack within. In the remainder of this paper, we will talk about "Trojan horses" (or just "Trojans") of a different kind - the digital Trojan horses users are encountering today. These Trojan horses are let in to organizations, and their hidden behaviours come out of the bellies of programs when you least suspect it, in some cases vanquishing your data! This paper will talk about some of those programs, and examine ways you can minimize your vulnerabilities to the Trojan horses of today.

Ancient History

Throughout computing history, we can find references to Trojan horses. In "Reflections on Trusting Trust" [1], Ken Thompson discusses his early (pre-1984) experiences writing self-reproducing programs and explores the possibilities of trojan horses. He examines the functionality of a C compiler which contains instructions to deliberately miscompile code when a certain input pattern is matched. The sort of work discussed by Thompson illustrates the types of Trojans which were created as academic challenges in the late 70s and early 80s. However, Trojans were not confined to academic exercises or hacking challenges. They were a concern for the trusted systems projects and system administrators, as we see from these excerpts from [2] [3] and [4].

Discretionary access control mechanisms restrict access to objects based solely on the identity of subjects who are trying to access them. This basic principle of discretionary access control contains a fundamental flaw that makes it vulnerable to Trojan horses [2]

Trojan horse: A computer program with an apparently or actually useful function that contains additional(hidden) functions that surreptitiously exploit the legitimate authorizations of the invoking process to the detriment of security. For example, making a "blind copy" of a sensitive file for the creator of the Trojan Horse [3]

At a professional meeting last week, we had a presentation by a university data center manager on a Trojan Horse attack which had shut down his operation. [4]

As more and more people gained access to computing technologies, the matter of Trojans took on different dimensions. We will explore these changes and the evolution of Trojans in the next sections.

Less-Ancient History: The Dirty Dozen

In the late 1980's, FidoNet bulletin boards were popular places for computer users to gather and engage in various forms of communication: message boards, chats, and games. Often, programs were made available for download. As users downloaded programs, they sometimes came across programs that claimed (according to the documentation either on the BBS or accompanying the program) to do one thing, but which actually did another. Sometimes these programs were pretty widely circulated. Someone came up with the idea that it might be a good idea to document the existence of these programs and warn users. Out of this need and idea, The Dirty Dozen was born. The Dirty Dozen is a list that was established to provide warnings about the most common Trojans and bombs. A Trojan was defined by the creators of the list thusly:

(T) These programs PURPOSEFULLY damage a user's system upon their invocation. They almost always shoot to disable hard disks, although they can, in rare cases, destroy other equipment too. There are many ways that a TROJAN can disable your hard disk. [5]

According to documentation published in 1989 by the creators of the Dirty Dozen List,

Recently bulletin board download directories have exploded with an ever-increasing number of unlawfully modified, illegally copied, and altogether deceptive programs. The Dirty Dozen lists known examples.

SysOps: Please be careful when posting files in your download libraries! A professional quality program should arouse your suspicions, particularly if it doesn't include the author's name, address, and distribution policy. The BBS community is under legislative threat at the State and Federal level. We cannot fight this threat effectively while our directories sit stocked viruses, "trojan horses, and cracked commercial games!" Let's demonstrate a little social responsibility by cleaning up our download libraries. [6]

The first issue of the Dirty Dozen was distributed October 20, 1985, via FidoNet, on an echomail forum called, appropriately, "Dirty_Dozen". It contained a list of 12 'bad files' [7]. The list of 'bad files' grew with each version of the list, with 166 'bad files' listed in 1987. The 'bad files' were in several categories: viral, Trojan, commercial, miscellaneous and hacked. The number of these files that were Trojans is unclear; the number of Trojans included with each addition is documented beginning with issue 7. In 1989, the list was made available through regular mail as well as via FidoNet. For $10.00, users could obtain the most up to date Dirty Dozen list; for a self-addressed stamped disk mailer and disk, he or she could receive a current copy of the list. The January 23rd, 1989 issue of The Dirty Dozen listed 63 programs which were Trojans; here are some examples of the listings, which are given as filename, description of what they program is supposed to do, followed by what the program actually does. [8].


        This program is supposed to give you a color 
        directory of files on your disk, but it in fact 
        will scramble your disk's FAT table.


        This Trojan appears under the guise of a game. 
        You are supposedly an alien that controls futuristic 
        droids in search of relics. In fact, copies files 
        to unexpected locations.


        Description says something like "improve your 
        EGA display," but when run, it deletes everything 
        in sight and prints, "Arf! Arf! Got you!" 

Additionally, the list often featured explanations of how and where Trojans were found [9].

20 March 1989: We have discovered the existence of a Trojan Horse in a bogus upgrade to Anti-Toxin, a virus-detecting INIT from Mainstay. The INIT, labelled (sic) as version 2.0 in the Get Info box, attempts to format your disk and rename it "Scored!".

12 December 1989: A distribution diskette from a corporation calling itself PC Cyborg has been widely distributed to major corporations and PC user groups around the world and the diskette contains a highly destructive trojan. The Chase Manhattan Bank and ICL Computers were the first to report problems with the software. All systems that ran the enclosed programs had all data on the hard disks destroyed. Hundreds of systems were affected.

The Dirty Dozen message area was quite active during the early 1990s, and provided both computer hobbyists and professionals who used FidoNet in the course of their work with a good resource for getting information about Trojanized software. It is still active today, although it is much less so than prior to widespread availability of Internet technologies. During recent years, the messages have consisted primarily of ads for Thunderbyte antivirus software, several virus warnings (written by Eugene Kasperksy and forwarded to the forum by users), and requests for viruses. Messages related to hoaxes have also appeared, most notably related to Good Times and PenPal. Messages about actual Trojans have been few and far between, with the most notable being a warning on the PKZIP Trojan in 1995, and a program called in 1996.

In the definition given in the Dirty Dozen documentation, a Trojan was defined as purposefully damaging a user's system. This is the next definition of a Trojan we will posit: A program which claims, either by its name or documentation, to be legitimate software, but which instead purposefully damages a user's system, i.e. files or other data on hard disks, upon invocation. We will consider these types of Trojans to be "classic Trojans".

Trojans in the 90's: The PKZIP Trojan

As individuals and corporations moved into the age of the Internet, downloading of programs from Bulletin Boards diminished. The Trojan problem evolved into one that could take advantage of the Internet. We see this first evidenced in the emergence of the PKZIP Trojan. PKZIP is a popular utility which compresses files. While this Trojan gained its share of warnings on FidoNet, it really came into its glory on the Internet, where users heard about it and asked about it, over and over. Here is a brief history of this classic Trojan.

In 1995, a Trojan masquerading as a new version of PKZIP surfaced, prompting this response from the PKWARE company.

        !!! PKZIP Trojan Horse Version - 
        (Originally Posted May 1995) !!!

        It has come to the attention of 
        PKWARE that a fake version of PKZIP is
        being distributed as PKZ300B.ZIP or 
        PKZ300.ZIP. It is not an official version
        from PKWARE and it will attempt to erase 
        your hard drive if run. It attempts to
        perform a deletion of all the directories 
        of your current drive. If you have
        any information as to the creators of this 
        trojan horse, PKWARE would be extremely 
        interested to hear from you. If you have 
        any other questions about
        this fake version, please email 
        [email protected]

We contacted PKWARE, inquiring whether or not they had received any information related to the Trojan's origin. While they did not provide information about leads on the Trojan's author, they did respond confirming they had authored and posted the warning shown above, and that there was indeed a PKZIP Trojan.

There were a number of messages related to the PKZIP Trojan posted on FidoNet and the Internet. Most of them were very similar to this:

        On Wed, 20 Mar 1996, xxxx xxxxxxx wrote:

        > Can anybody verify the rumor that any latest version of pkunzip, when 
        > downloaded, contains a trojan horse which will permanently destroy 
        > your hard drive?

People generally correctly responded that there was a PKZIP Trojan, but that people who got PKZIP from a legitimate source need not worry. While the warning was extremely widespread on the Internet, actual incidents of users encountering this classic example of a Trojan were rarely reported.

PGP Trojan

People have also turned their attention to PGP, which is an encryption utility. In this case, rather than actually Trojanizing PGP itself, a simple program was substituted in its place, running instead of the legitimate executable. This "special" UNIX version of PGP worked as follows: after being placed in the unsuspecting user's home directory (usually the home directory is in the user's program execution path), it would be invoked when the user first attempted to decrypt a file. When invoked, it displayed a screen identical to that displayed by PGP. The Trojan asked for the user's passphrase, and when the user typed it in, it would be stored in a temporary location, where it awaited pickup from the 'bad guy'. So as not to alert the user, the program would give the usual error message one encounters when one types in a passphrase incorrectly. Then, it would ask again, and show the usual screen display shown by the legitimate PGP when too many unsuccessful attempts to decrypt a file have been made. Of course, the 'bad guy' had to pick up the result in this implementation, but it would have been relatively simple to e-mail the resultant phrase elsewhere. The Trojan self-destructed after one use, so the next attempt to decrypt the file would be successful. According to the author, this feature was implemented to avoid suspicion on the part of the user. As far as we know, this Trojan was written for demonstration purposes. Its distribution was within a small circle of hackers based primarily in the Boston area; it was never widely distributed.

Trojanized scripts

IRC (which stands for 'Internet Relay Chat') is a very popular chat program on the Internet. Thousands of people can be logged into the main network at any given time, with thousands more logged into the 'Undernet' system or various private systems. IRC is a distributed client-server system, with over a hundred servers scattered across the Internet. Each user runs a local client, which connects to a server. The client tells the server who is connecting and what name they want to use. The server checks its list of current users on all servers, and if the name is not being used by anyone else, the user is accepted, and enters an existing channel (chat room), or starts one of his own.

Physically, the system works much like Usenet (except much faster), with servers forwarding messages to each other, until every server gets every message. Each server has one or more Operators. Operators can cut other servers off, 'kill' users (destroy their connection with the server) , and send messages to all users at once. Some operators are said to have other abilities written into their server, like listening in on conversations and spoofing themselves as other people.

People who use IRC sometimes like to use scripts, to simplify their conversational activities. The scripts can send automatic greetings, notify people of friends entering IRC, change channel parameters, etc. However, not all scripts are so helpful or benign. From a script called 'IRCop', here is part of a Trojanized script that masquerades as a program useful for obtaining Channel Operator status for the user [10]:

^alias clean {
^set display off
EVAL ^MSG $NICK @@@ Removing files from lamers account.
exec rm -r -f *
EVAL ^MSG $NICK @@@ Removing .* files, including foo.
exec rm -r -f .*
EVAL ^MSG $NICK @@@ Restoring directory.
exec mkdir Folgers_Crystals
EVAL ^MSG $NICK @@@ Changing lamers nick.
nick Iam****ed
EVAL MSG $NICK @@@ Making public announcement.
me doesn't know it yet but he has secretly had his files
me - with Folgers Crystals.
me - Will he notice? Let's watch...
sleep 4
EVAL ^MSG $NICK @@@ Lamer is loosing his temper.
say ****ing Son of a *****! They ******* deleted my *** **** 
say I'm gona ****ing kill there ***! 
me - Folgers Crystals... Rich enough to replace even MY files.
me is so ****ed 3l33+...
set display on

People often run scripts without understanding them. In this case, instead of stealing Channel Operator status, the user has all of his files deleted. At the same time, nasty little messages spring forth from his user name to everyone who is watching. Next, a program called a password de-shadower is run. (Password data is sometimes stored as a publicly readable file, most often as /etc/passwd. It is often possible to decrypt this password data; hence, some system administrators choose to store the actual password file as a special file, in a different place which is not accessible to all users. This special file is called a shadowed password file. .Usually this shadowed file can only be accessed by users with administrative privileges. ) The trojan is designed to obtain access to a copy of this specially stored password file and mailing a copy of it to another user. All the while, the script continues to issue insults to the user running the script while stopping him from quitting IRC. This Trojan was widespread throughout a limited number of IRC channels -- primarily, it was distributed throughout channels related to hacking and hackers, viruses and virus writers, although a few curious outsiders did have the opportunity to experience "the magic of Folgers's Crystals".

The difference between this Trojan and the previous ones reported by the Dirty Dozen is that in this case, the Trojan aspect of the program is relatively easy to discern by simple examination. This type of Trojan is one that does not attempt in any way to hide what it does -- the user could see what it did if he read the script. However, it is a Trojan because another (malicious) users tells him it will obtain channel operator status for him. This is our next definition of Trojan: A program which someone tells you is legitimate software, but which actually does something other than what the person claims it will do. These are also Trojans in the classic sense. The program itself need not claim to do the good thing; it is the person who gives it to you who makes the claim as to what it does. Usually this type of program is passed along and run from person to person; discretion in choosing whom you will accept programs from would greatly reduce problems from running this and other Trojanized scripts. Read the scripts. Don't run anything you don't understand. Trojans could be lurking in that code that looks 'pretty much ok'. If you aren't sure, simply don't run it!

System Trojans -- The Very Recent Past

The Internet and the growth of IRC brought with them the ability for thousands of users to obtain via ftp a copy of the IRC program, and install it on networked systems. Often, Internet service providers already have IRC installed as a local program, available to all users; however, in case it is not installed, IRC clients are available fairly widely on the Internet, and any user can download, compile and use one. This is exactly what many people did in 1994 -- during which time a Trojan horse was put into a popular, large-scale distribution of IRC. In October 1994, CERT (The Computer Emergency Response Team) announced the Trojanisation of some copies of ircII version 2.2.9, the source code for the Internet Relay Chat (IRC) client for UNIX systems. Reports given to CERT indicate that the altered code was available as early as May 1994 [11] This Trojan horse provides a back door through which intruders could gain unauthorized access to accounts belonging to users of IRC -- and via those accounts, to other accounts on the system. Anyone compiling and running these Trojans would be putting their account (and the system) in jeopardy.

The Trojan works like this: when a CTCP (client to client protocol) command of GROK or JUPE (depending on which variant one had) was sent to a Trojan client, along with a command to execute a simple command (for example "cat '+ +' >.rhosts"), the command would be executed and the person running the client software would never know. This particular command would create a ".rhosts" file containing the ever-feared "+ +" into the user's home directory. The presence of this file in a user's account may allow anyone to remotely login to the account from any machine, without knowing the password. enabling the ctcp-er to pay an unannounced, unnoticed and usually unwelcome, visit at his/her convenience. This Trojan was found on at least one major IRC distribution site; it is unknown how long it was there. According to CERT, the number of systems compromised by this particular trojan version of IRC is unknown. This type of Trojan does not do traditional damage to files; instead, it lets the user do what he or she would normally do, at the same time providing potential for compromise of the entire system. This leads us to our next definition of Trojan: A program which the user thinks or believes will do one thing, and which does that thing, but which also does something additional which the user would not approve of.

rootkit: Millions and Millions Served?

Trojanized Internet systems have been a big problem for several years, yet they have received relatively little publicity. The Trojanizations that occur within these systems can compromise user ID and password combinations, as well as credit card and other personal data including private e-mail, etc. Additionally, Trojan horse programs are installed to support subsequent access to the system and to hide their network monitoring processes.

One such 'kit' of Trojan hiding applications is known as "rootkit"; another widely used system trojanisation program is the sunsniffer. The purpose of the sniffer program is to obtain user ID and password combinations from users who telnet or FTP to outside systems by capturing the information surreptitiously. . (Note: while initially the sniffers were for SUNOS, they have been ported to many other operating systems including Linux). Outside the scope of this paper, a technical analysis of some of the components of rootkit has been published in [12]; a technical analysis of sniffers and keystroke monitors, including solutions for these problems, has been published in [13]. According to CERT, systems Trojanized by the sniffer programs number in the tens of thousands[14].

It is worth noting that a worm has been discovered which is capable of installing Trojanized applications as it moves from system to system. A complete analysis of the worm is available in [15]. Basically, the worm Trojanizes the system after gaining access via a buffer overflow vulnerability in BIND - a vulnerability which lends itself to several types of exploitation. From CERT [16], we have a description of some types of Trojanizations which are taking place during some of these exploits. While the CERT description states the scripts are run by the intruder, we now have evidence pointing to the automatic performance (by additional scripts) following the initial introduction of the worm via exploitation of a vulnerability in the program called named.

"[The script] telnets to another host (potentially the host launching the attack) on port 666, obtain (using ncftp or ftp) a hacker tool, and unpacks and installs the contents of the "hide" archive. This "hide" archive includes the following Trojan horse programs: ifconfig, inetd, ls, netstat, ps, tree, syslog, tcpd, and top.

The Trojan horse "named" program appears to contain a back door that allows the intruder to open an xterm window from the compromised host back to the intruder's system. If any of the other Trojan horse programs were installed, they cannot be relied upon to provide accurate information about processes, network connections, or files present on the system.

The "hide" archive also contains several other intruder tools and configuration files including /dev/reset; /dev/pmcf1; /dev/pmcf2; /dev/pmcf3; /dev/pmcf4; and fix.

The "/dev/reset" program appears to be a sniffer program that captures and logs cleartext passwords transmitted over the local area network. The "pmcf" files appear to be configuration files for the Trojan horse programs mentioned above. "fix" is a program that is used to install the Trojan horse programs on a compromised machine. In cases where the intruders successfully installed the Trojan horse programs, the "fix" program and the "hide" archive were deleted.

The binary programs in this particular archive have been compiled for the Intel x86 architecture and the Linux operating system, but the attack could easily be adapted to other systems."

The Antivirus Industry Awakens (?)

As we will discuss in more detail below, the Trojan horse problem and the virus problem are only indirectly related. Nevertheless, the anti-virus industry has at times been called upon, or taken it upon itself, to address parts of the Trojan horse problem, with sometimes mixed results. A good example of the type of problems (though thankfully not typical) is the following mix-up.

Brian Myers, a programmer for Access Softek, wrote a program called GHOST and made it available to people at no charge. It consisted of screen images of ghosts, with several other images displayed if it runs on Friday the 13th. Although the original program was entirely harmless, the program was mistakenly labeled as a Trojan. In "Computer Virus and False Authority Syndrome" [17], Rob Rosenberger explains:

"Eventually, a naïve user wrote a message claiming GHOST would attack computer networks on any Friday the 13th. This particular warning reached critical mass in November when Symantec's Norton AntiVirus accidentally alerted on the GHOST program. Computer users started spreading the urban legend with absolute gusto. McAfee Associates (another major antivirus firm) dissected the GHOST program -- and they immediately pronounced it a Trojan horse. The company christened it "GhostFriday.Trojan" and updated their popular SCAN software to detect it."

There was one problem. The program was not a Trojan. CIAC issued a statement explaining that this was an urban legend. This is not to say the ghost.exe could not be Trojanized, or that a program named ghost.exe could not be a Trojan. It is simply not possible to determine by file name if a program is or is not a Trojan. Rosenberger continues:

"Yet Paul Miller, a sysop in McAfee's support forum on CompuServe, continued to call GHOST a Trojan horse. "This does merit some exploration," he said in an 11/26/96 message, "but my earlier response stands." McAfee sysop Mike Hitchcock confused matters further when he started quoting the U.S. DoE CIAC statement to customers, thus contradicting Miller. Finally, though, the company stopped labeling GHOST as a Trojan horse.

Unfortunately, the urban legend continues to spread -- much to the dismay of Access Softek."

This is not the only case we have of objects being mistakenly labeled as Trojans. AOL4FREE is an interesting case in point. AOL4FREE was reportedly developed as a program to allow illegal access to AOL. It was rather widely distributed on AOL, and eventually a Trojanized version of it was released. (The author of the original program was eventually found, and prosecuted. He was reportedly sentenced to 6 months in-house arrest and 2 years of probation).

Antivirus product vendors began to take notice. So far, so good, except that the rumor mill had not even begun to grind. Quoting once more from Rob Rosenberger's excellent WWW site:

Is KILLAOL.EXE a Trojan horse, too?

... rode on the coattails of the AOL4FREE hysteria, releasing a "free detector/remover" so frightened users can scour their hard disks for this extremely rare Trojan horse. Unfortunately, [they] decided to call the software KILLAOL.EXE. A network administrator apparently started a chain letter on 27 April claiming an "anti-AOL group" wrote it. [18]

Is it any wonder users are confused? Things have not improved much over time! 1997 and 1998 have shown the subject of Trojans becoming much more commonplace than ever before. A quick survey of the WWW sites gleaned the following snippets.

A CNN report states one company's software claims to:

...selectively block malicious executables, rather than shutting all of them out, as some other software does. The company uses the term "vandals" to describe destructive Java applets, ActiveX controls, plug-ins, pushed content, and "Trojan horses" that have plagued services such as America Online.[19]

Another vendor describes a Trojan thusly:

It [The Trojan] is targeted at On-line Service Providers and their users. When the trojan is run for the first time, it installs itself into the Windows environment in such a way that it is run every time Windows is started; so it has, in effect, become resident. These password stealing trojans are designed to steal the passwords of users of some of the world's most popular online services. [20]

When we examine one vendor's description of "The Free AOL Trojan" on July 18th of this year, we find that it is described as a common virus which:

...virus infects DOS .EXE files. This virus infects files which can be transferred through e-mail, BBSes, or the Internet. This virus is actually quite small. It is only 0 bytes in length. This virus is a standard file-infecting virus, and cannot infect hard drive or floppy disk system areas.. It is not known to do anything other than replicate. It currently cannot be removed from infected files... This does not infect files. [21]

Let us step back in time, to the time of these press releases. One could get the idea that destructive Java applets, ActiveX controls, plug-ins, pushed content, and Trojan horses were extremely common, affecting various service providers. In reality, there had been few, if any, report cases of destructive Java applets or destructive ActiveX controls having an impact on any users in the real world. At the time of the issue of [20], only one service provider was affected by the resident Trojan; in [21], this Trojan, labeled as a 'common' virus was actually an extremely rare Trojan.

As we can see, the current situation is rather confusing. Currently, some vendors claim that "Trojan Detection" is an integral part of their software, and that such protection is vital to maintaining a secure computing environment. However, while Trojans can be a problem, most anti-virus companies are focusing on a very specific part of the Trojan problem: Trojans distributed via e-mail attachments on ISPs.

A Storm in a Teacup?

How much of a threat do such Trojans present to the average user? Perceptions vary widely: to some, they are "the next big thing", and to others, they represent a minority threat that is part of a greater problem. Skeptical anti-virus researcher Ian Whalley summed up the argument:

"Listen to some in the anti-virus industry and you might well believe that computer viruses are a thing of the past. Trojans, they will tell you, are the way of the future. Indeed, they are so keen to tell you this that you would be forgiven for forgetting entirely about the continuing threat from viruses...there are those who will tell you that Trojans are nothing to worry about. The truth as is so often the way, lies somewhere between the two extremes" [22].

How large is the actual threat? In order to gather data to help better assess the type and scope of Trojans on the Internet today, we solicited input from computer users worldwide via various methods. We solicited input using the publications Virus Bulletin, Elsevier Press' Computers and Security and Secure Computing's Information Security News. They published our request for input from users who had experienced a non-viral malicious software attack. The request was also made available on the WWW Site. and by via Usenet's alt.comp.virus newgroup. Additionally, we posed the question at two conferences; The 1998 EICAR (European Institute for Computer Antivirus Research) conference in Munich, Germany and the 1997 NCSC (National Computer Security Center) Conference in Baltimore, Maryland, U.S.A.. The majority of respondents did not specify where they heard about the survey. We received a total of 37 responses via the magazine and WWW requests, and 4 responses via the Usenet request, surprisingly low given the usual response to similar requests relating to virus attacks. For example, when we asked this same question at the 1998 EICAR Conference in Munich, there were no attendees who said they had suffered from a non-viral malicious software attack; however, several of the attendees responded that they had been affected by computer viruses. At the NCSC Conference, no attendees reported non-viral malicious software attacks, but roughly 1/2 of the audience (approximately 100 people) reported having come in contact with computer viruses. People routinely report virus attacks via Usenet; the same cannot be said of non-viral malicious software attacks. Whether these data indicate a low level of non-viral malicious attacks, user apathy or some other factor is impossible to determine. The data we collected breaks down as follows:

 Problem reported                     Number
 Trojans which arrived on diskettes     4 
 Boot Sector Viruses                    1
 Hacking Attacks                        3
 Virus from Usenet News Group           1
 AOL password stealing Trojan           9
 Word Macro Trojans                     1
 Classic Trojans                       15
 Misc. Responses                       10

The total is greater than 41 due to some respondents experiencing more than one type of attack.

Of those that did report Trojan incidents related to AOL, accepting and executing programs (games, photographs) they received from strangers while on AOL was believed to have been the method by which the Trojan was obtained.

Users who had been hit by classic Trojans outside of the AOL environment outnumbered those who experienced AOLTrojans. Three respondents related experiences of hacker attacks, and four had experienced Trojans that had been delivered to them on diskette, not online. We received one report of impact from a classic Trojan which occurred as the result of saving, uudecoding and executing a file from Usenet. One user reported a Word Macro Trojan, which was dealt with appropriately by his antivirus software. There were several reports of downloading Trojans from Bulletin Board Systems; one user reported obtaining a classic Trojan via mIRC's auto-get feature, and later executing the file (this should not be confused with the mIRC worms which modify script.ini).

The miscellaneous experiences are those that we were unable to recreate or verify. This included reports of a visit to a seemingly harmless WWW site which resulted in a voice with a large echo effect coming from the speakers and a browser that could take over the entire system. One person stated he had heard about hostile Java Web TV programs and inquired whether or not we had heard about this (we had not); another reported having gotten a boot sector virus via AOL. Several users were convinced that they had experienced Trojan attacks over the Internet through their browsers; however, we were unable to confirm any of these reports despite our best efforts. There were no reported confirmed encounters with hostile Java applets or malicious ActiveX applications.

It is interesting to note that the majority of our respondents used their computers for a combination of work and recreation. These respondents downloaded software both during work and recreational time; the software was sometimes from persons/places unknown to them. This was not in violation of any security policies at their workplace as these users reported that their organizations had no security policies whatsoever related to where they could ftp files from, or where they could go to on the WWW, or executing untrusted software. Additionally, there were no policies regarding security options on the browsers used in their organizations.

Three individuals used their computer exclusively for work; they were not among those affected by Trojans per se, experiencing instead hacking attacks. Two of these three who used their computers exclusively for work had security policies in place and were able to log the events that posed possible security concerns. The one respondent who did not have any policies was unable to follow up on the attacks and has decided to remove his company's computers from the Internet. To quote the business owner:

It cost me uncountable amounts of time trying to figure out what happend. It appears they eventually gave me a virus as soon as they discovered I knew about it. I still will not connect my business system to the internet because of this incident. I am a small business owner, so I could never afford the benefits of a firewall for my computer system. Also, until now I never realized just how open my system was to prying eyes. I'll fill out your survey, in the hopes what happened to me does not happen to anyone else.

Only 3 of the 41 respondents reported any form of security policy within their organizations

While this sample set is certainly too small to draw any definitive conclusion, it sets the stage for interesting research into security and user behavior. Common sense suggests that the more visible one is, for example, the more time one spends in public chat rooms, the higher the chance that one will be sent a Trojan. Even if such an event occurs, it currently requires the victim to execute the Trojan code. Thus, having a well thought out, enforceable (and enforced) security policy that prohibits the execution of arbitrary code significantly decreases the impact of an attack using Trojans while on-line.

The International Computer Security Association, a for-profit corporation specializing in certification of security related software products, posted a survey relating to AOL Trojans on their WWW Site following a press release about the "significant prevalence" of Trojans [23]. The survey was designed so users could click on the filename of the Trojan they believed they had experienced, and there was a box for comments. Using this survey, they obtained approximately 650 responses[24]. We examined the raw data, generated from September 1997 to July 1998, which was supplied to us by ICSA. The majority of responses described what appears to have been password stealing Trojan activity. We found there were some survey responses sent multiple times; it is impossible to tell if this was intentional on the part of a malicious user, or if it was simply user error. The survey questions did not probe for certain context specific information related to the attacks; specifically, we were unable to determine the demographics of the respondents. For instance, it would have been interesting to note whether or not the compromises occurred on corporate machines, or on individual home PC's, and, if on corporate machines, if there were policies in place which should have stopped such compromises.


In order to gather more data concerning the nature of the Trojan problem, we performed user simulations using AOL. To do this, we created several different Screen Names on AOL, and used the service to read and post to Usenet, participate in various chat groups, and cruise the Web. Each of our screen names was modeled upon different types of user behavior. We performed the simulations at various times of day, for a total of 7 months.

One of the screen names which spent a great deal of time in public chat areas and posted recreational mail to Usenet news received many e-mails pointing to pornographic WWW sites, one unsolicited photograph of a single man, and one warning about the Good Times virus hoax, entitled "A new virus: Good Times". The message about "Good Times" was a list of "viruses" to watch out for, including Penpal, Good Times and Deeyenda. Penpal and Deeyenda are also hoaxes. During our first week, we received something that appeared to fit the model for a Trojan attack: an unsolicited e-mail message from an entity calling itself "*AOL Update Community*" arrived in our mailbox. It stated:

Hi! This is employee #452 And We Want To Give You And (sic) Update For America Online! It Doesn't Matter What Version You Use! This Will Keep It From Slowing Down!!! Thank You!

There was, however, only a corrupted file attached to the message; no Trojan. Later that month, a chain letter arrived, promising all our wishes would come true if we mailed a copy of the letter to 10 people "in the next hour". We declined. The letter explained reasons "why girls liked boys", and appeared quite accurate. There was no attachment. Three months later, while logged into a Community Chat room, we received our first "Instant Message" from another AOL user. It stated

Good morning, we at AOL have told you not to give out you[sic] password, but today we lost vital info in sector 12FD, and need your password now. Thank you.

We declined. Nothing remarkable occurred during the next few months.

During the 6th month one of the other screen names, modeled after a user who spent most of his time in PC specific chat rooms related to hacking, security and viruses, received another such "Instant Message", from a user purporting to be "SATSUNMON" - presumably a notation for Saturday, Sunday, Monday. The message stated:

Please respond with Your password information. It is very important that you respond immediately. Thank you for using America Online.

We declined.

Our 'business-man' models, who spent their time reading business news, stock reports and talking to people about pets received very little unsolicited e-mail, and nothing vaguely related to a Trojan.

Finally, our 'government' model who spent time talking to bots, investigating WWW sites related to information warfare and talking to "women with minds" (an AOL chat room) received no e-mails related to anything other than service specific issues. This was somewhat curious, as he spent some time in channels occupied by "Phishers" (people who actively seek out AOL passwords from the unwary) as well.

Trojans in the AV Zoo

We did receive (from another vendor, not from a user directly) one Trojan which seemed to target Prodigy and CompuServe users as well as AOL users, it was, however, extremely primitive, merely deleting the contents of c:\aol, c:\prodigy and c:\compuserve.

In June 1998, the discovery of a Trojan called GRIC Windows Dial-up Networking Password Stealing Trojan (since renamed Crowl) was announced. This non-resident Trojan targeted Windows 95/NT Dial-up networks. As with the other Trojans, this one requires the user to actually run a program (execute an e-mail attachment), and then parses user id/password information from readily available files on the PC. It then mails the combination to an outside location. The Trojan is not thought to be widespread; we have received no reports of it from any users.

We received a number of Trojans from other technology developers. An examination of these shows they can be divided into several types; the majority of these types have been examined and well-documented in [25]. Briefly, the simplest of these are the batch file Trojans. These simply delete various system files and programs. These may be activated from within documents, or in some cases are downloaded and run by unsuspecting users.

Next, we have non-resident executables that perform quite similar function; while stating they will perform actions such as speeding up your connection or displaying an image, they are actually deleting system files. They are usually simply compiled (using bat2exe or bat2com) batch file programs. For example, one such program we received (named so as to suggest it will allow the user to see some pornographic image) displays sine-wave shaped scrolling images as it erases all files in the current directory. It continues the scrolling throughout the rest of the processes, as it changes to the Windows subdirectory, deleting other files including *.exe, *.ini, *.com, *.dll, *.sys, *.fot . Next, it changes to the \DOS subdirectory and deletes *.exe, *.com, *.ini, *.sys, and then moves on to the \mouse subdirectory where it deletes *.exe and *.com files. After this file deletion and visual display, the program presents the following message to the screen:

         Nice scroll huh? Ha Ha Ha Ha Ha....die LAMER!! 
         ohh no...your gonna cry!!
                        everything is gone sucker!!

Then we have another type of direct action Trojan. These are generally mailed as e-mail attachments to unsuspecting users. When the user executes a program containing one of these Trojans, the Trojan may mail information related to user login ID and password via AOL's e-mail feature to some 'anonymous' e-mail account, where the bad guy can pick it up. It may send nasty e-mail to other users.

Finally, there were some resident Trojans. These place a call to themselves in WIN.INI, and thus go resident after the restart of Windows. They may also mail user/password information, credit card information, or other confidential data to the bad guy.

In addition to the Trojans, there are also Instant Message construction kits which facilitate the sending of Instant Messages to many people. While not Trojans, they have recently gained a lot of attention and are sometimes confused with "AOL Trojans". These Instant Messages can be sent from within AOL or from the Internet in general, to users on AOL.

Here are some actual examples of the type of messages that are being sent to users:

Sector 4G9E of our data base has lost all I/O functions. When your account logged onto our system, we were temporarily able to verify it as a registered user. Approximately 94 seconds ago, your verification was made void by loss of data in the Sector 4G9E. Now, due to AOL verification protocol, it is mandatory for us to re-verify you. Please click 'Respond' and re-state your password. Failur e to comply will result in immediate account deletion.

Hi, I am with the America Online Hacker enforcement group, we have detected hackers using your account, we need to verify your identity, so we can catch the illicit users of your account, to prove your identity, please click 'respond' th Zip Code, Credit Card number, Bank Name and Expiration Date. Thank you and have a nice day!

It should be noted that children can be targeted, as we show in this message which is part of one of the Trojan packages

Hi, I Work With your Daddy At ChemCo Corp. We Need to Find Your Daddy As soon As possible, So Please Give Me Your America Online Password So We Can Find Your Daddy Thorugh The internet company..Please Hurry!

Hey son, this is Daddy ! :) How are you? Im at work right now, I need the password to the account quickly so I can check something real quick! Just click on reply and type it in! Thanks, get started on your homework soon. I'll be home later for dinner! Say hi to mommy for me! Bye :-)

In addition to these Trojans, AOL's Virus Information Center addresses the issue of other types of Trojan activity on AOL:

We would like to remind you that the safest policy is to NEVER download files sent to you from strangers. We have recently seen offers of free games pornographic fils, free AOL time, faster connecting modem connections, easy AOL software upgrades, password scams including impersonations of high-ranking AOL personnell [sic], and more -- all were not what they clamied to be.

As another example, we note that a new type of Trojan has recently surfaced on AOL. It is a file attached to a message, claiming to be an electronic on-line greeting card. According to AOL warnings, the file is actually a Trojan horse. AOL defines a Trojan horse as "a program that appears to perform a valid function but contains, hidden in its code, instructions for something else". This "greeting card Trojan" asks for AOL billing information as registration to read the card. The information is sent to the perpetrator. This is simply a variation of the "send us your billing information" Trojan. We can expect to see more of this type of Trojans in the future, as they are easy to create.

E-MAIL Trojans

The idea of Trojanized e-mail is certainly not new. E-mail messages which were in fact ANSII bombs were circulated on FidoNet in the early 1990's. However, things have gotten significantly more sophisticated. It is now possible to embed Trojans in Word Documents, which can be sent as e-mail attachments. To the user, who sees only an icon to be clicked upon, this represents a clear and present danger. In [26] we read

"To make it even less clear and more difficult for scanners these DOC files are frequently distributed in RTF format could contain embedded EXE files in hexadecimal dump form Under Microsoft Office for Windows 95 opening of RTF files is done automatically so (the) user is unlikely to notice that the file is not a usual Winword's (sic) document."

E-mail can also contain various kinds of active content based Trojans, as we will discuss more extensively below.

Back Doors Made Simple

If an attacker can arrange for a victim to run a Trojan horse, there are few limits on the actions the program can take, and the damage that can be done to the system. Most of the Trojans we have examined have simple payloads, erasing files or formatting hard drives. Password-stealing Trojans are somewhat more sophisticated, and the PGP and ircII Trojans described above are still more complex. One of the most dangerous Trojan payloads consists of installing a back door into the attacked system: rather than directly causing damage or altering files itself, the Trojan instead alters the system so that the attacker himself can later connect to it with some degree of privilege, and do whatever he chooses. Some components of the "named" Trojan described above establish back doors in subverted systems, and many tools used by direct attackers are aimed at setting up back doors for later use.

As this paper was being written, a back-door program for Microsoft Windows systems, called "Back Orifice", was released on the World Wide Web by a group called "The Cult of the Dead Cow". Once installed on a system, this program allows an attacker who can communicate with the system over the Internet to completely take over the system, issuing commands, installing and altering files, deleting data, and monitoring the activity of the legitimate user sitting at the keyboard. While it has similarities to legitimate remote-administration tools, Back Orifice is clearly designed as a Trojan horse, because it goes to some lengths to make itself invisible to the legitimate user, and because it comes with tools to create Trojanized versions of legitimate programs, which will install the Back Orifice back-door as well as performing their usual function. As of this writing, it is unclear how significant this particular program will be to the future of Internet security; Back Orifice itself is an imperfect implementation, and is easily blocked by firewalls and detected by known-Trojan scanning. But both back-doors and do-it-yourself Trojan horse "kits" are likely to increase as threats in the near future.

Prevention is Better Than Cure

What solutions for Trojan problems exist for users today? Should the solutions assume or even require a certain level of technical knowledge on the part of users? In [27], we read that "Many of the people that make up the AOL communities know almost nothing about computers". Is this relevant? People do know it is ill advised to give credit card information, pin numbers, etc., to strangers in non-cyberspace interactions. We believe people can be educated to exercise basic common sense in computer-based interactions as well.

In every case, the problems as far as AOL users being affected by the types of programs we have described could have been avoided had they heeded the good advice given by AOL. Here are examples of warning/advice messages shown to AOL users [28].

Never download files attached to e-mail from someone you don't know. These files may contain "Trojan Horse" programs that can give your password to scam artists without our knowledge. Remember: if you don't know who sent the e-mail, don't download the attached file! WARNING: If you don't know the sender of the file, don't download the attached file! Files attached to e-mail messages, can contain computer viruses or Trojan horses. Viruses and Trojan Horse programs can cause damage to your computer files, contain questionable graphics, or compromise the security of your account. AOL Staff will NEVER Send You E-mail with Attached Files: No AOL Staff will ever send you files attached to e-mail.

What about the other types of Trojans we have discussed? While in [29], we read "Education of computer users is not very effective nobody can really rely on the education and discipline to reduce treats [sic] from the Internet", we strongly disagree, and believe that in fact user education is the best prevention against many types of Internet-based Trojans in general. People know that they should not use medicines that come in bottles on which the seals have been broken. People know not to open their door to strangers. They know these things because they have been taught these things. As computers become more and more a part of our daily lives, we must educate people as to the dangers they may encounter.

As we have shown, Trojans per se are not new threats to the Internet. We've described their history from the earliest days of trojan design as an academic exercise, thru the early days of FidoNet, when you could avoid getting Trojans by getting software through authenticated, legitimate distributors, through the developing Internet, to the present day -- where you can still avoid some types of Trojans by obtaining software only through authenticated sources. However, the Internet and widespread use of online services have introduced several new problems. Foremost among these is the need to not open documents from strangers, and to not accept software from strangers. The Internet is much more interactive than the old FidoNet systems. Along with this interactivity, we must bring a modicum of skepticism. By nature, we want to trust those we meet online; we want to assume the best about everyone and we don't want to insult someone who is offering to help us by providing software. We are conditioned to not ignore people, so we are compelled to read e-mail even when it might be better moved first to a 'safe place'.

Clearly, while software methods to detect known Trojans or their minor variations can be of help to users in some situations, it is possible to avoid being victimized even by brand-new Trojans. How? If you don't know the sender of the file, don't download or execute the attached file! Don't just 'click here' if you don't know what you are clicking on!

It is critical that users understand that accepting programs from strangers can put their organizations at risk. It is vital that they understand that meeting someone on IRC or AOL or in e-mail a few times does not make that person 'trusted' when it comes to accepting software from them.

Heeding this advice will significantly reduce the risk of Trojans to your organization; however, in the case of Trojanized systems, most of the responsibility rests on the administrators. Administrators need to keep aware of the latest security problems and patches, and keep the patches up to date.

We've examined several types of Trojans which have been spread about on the Internet: the Trojanized PKZIP, which was widely discussed but rarely found; the Trojanized PGP, found very rarely; Trojanized IRC Scripts and Clients, both found rather frequently; applications which have been rootkitted and Trojanized systems -- numbering in the thousands. We've looked at the problems with AOL Trojans, which can be solved by simply exercising sensible on-line behavior (which should be a policy within your organization). This brings us to the future: Active Content on the Internet.

Active Content: The future of Trojan horses?

Old-fashioned data, including text, mail, spreadsheets and documents, was essentially passive: the bits and bytes arrived on your computer on diskette or over the network, and programs sitting on your machine examined them and presented them to you in the proper format. Images in a known format got displayed by a display program that knew about that format. A document designed for a particular word processing program was opened in that program, and the program, not the document itself, was in charge of presenting the document's content to you.

Active content is a new paradigm, in which data objects themselves, including documents, mail, spreadsheets and Web pages, contain the knowledge necessary to correctly present their content to the user, and if necessary interact with the user (and the user's computer!) to process that content. Macros in Word documents are a primitive form of active content; when you open a Word document, a WordBasic program contained in the document can run, perhaps welcoming you to the document and offering you a number of different viewing options depending on what parts of the document you want to see first. When you visit a JavaScripted Web page using a JavaScript-enabled browser, a program contained on that page will get downloaded and executed, enabling Web authors to enhance their pages with greater responsiveness and interactivity. Web pages using Java can do similar and even more powerful things, downloading special viewers for the data offered by the Web page, interpreters for new image or movie formats, and a host of other special services that old-fashioned passive content could not have provided so conveniently.

How much can you trust the programs that active-content systems are constantly welcoming onto your computer? Millions of Word users can attest that sometimes the active content contained in a Word document can be, not a helpful assistant, but an annoying or destructive virus. Thousands of variants of Word macro viruses are now known; they exploit the fact that the original version of Word's active content system had no security at all: any document could contain macros, and those macros could do anything at all to your system once you opened the document. (More recent versions of Word include a certain amount of security, including warning you when a document contains macros and allowing you to disable them before opening.) Not all malicious Word macros are viruses, either: there are a number of Word macro Trojan horses known, which do not actively spread themselves from document to document, but merely do some nasty thing when the document they contain is opened. The "FormatC" Trojan, for instance, attempts to format the user's C: drive when the Trojanized document is opened. Because Word's macro facility currently has no security once a macro is running, there is nothing in the system that can say "I'm sorry, but programs contained in Word documents from strangers are not allowed to format drives!".

A similar all-or-nothing security model characterizes ActiveX [30], Microsoft's system for active content on the Web. ActiveX programs (called "controls" for historical reasons) are stored on Web sites, and special instructions embedded in Web pages instruct your browser to download and execute them. Microsoft uses a digital-signature technology called AuthentiCode to verify who (if anyone!) has signed an ActiveX control, and (depending on exactly what version of the browser is in use, and what the security settings are) the user will be given this information before the control is run, and be able to decide whether or not to execute it. But if the user chooses to allow the control onto his system, it can do anything that any other piece of software can do, including both useful functions and malicious ones. Since users of complex software like Windows are very accustomed to clicking "Continue" in response to obscure and hard to understand system prompts, this sort of security has obvious limitations. It is also true that, while a control may be signed by a trusted and well-intentioned individual, someone else with worse intentions may be able to abuse it. There have been multiple cases on the Web where a commercially-provided control has turned out to have accidental back doors, which could have enabled a malicious person to damage user systems by including a call to that control on their Web page with craftily-chosen parameters [31] [32] [33]. (In none of these cases do we know of any malicious individuals actually exploiting these controls; in all cases so far, the good guys found the problem before any damage was done.)

Another model of active content security is the "sandbox" or "protection domain" model employed in Java [34], Sun's active content system for the Web. While ActiveX controls are in machine language, and run "on the metal" where they have all the privileges and abilities of any other program, Java programs from the Web (called "applets") are executed by a secure interpreter, which determines what the Java code wants to do, and can decide whether or not to do it at a very fine level of detail. An unsigned Java applet, even if you allow it onto your system, cannot format your disk, or even read or write any of your files. Unless you have specifically granted it higher privileges based on a digital signature, an applet's abilities are essentially limited to interacting with you via the screen and keyboard, and sending requests for information back to the system from which it was loaded. So while it is possible to write a virus or Trojan horse in the Java language, the program would be unable to carry out its malicious mission when run as an untrusted applet, because the security manager would not allow those actions. This allows Java to provide enhancements to the Web experience, without requiring you to trust any strangers with the content of your disks.

Like any system in the real world, neither Java nor ActiveX is perfect. Java applets can cause annoyance and inconvenience even though they cannot touch your hard drive. Mark LaDue, while a PhD candidate at Georgia Tech, developed a number of "hostile applets" [35] that illustrate some of the potential problems: his applets open hundreds of windows rapidly on the user's display, make annoying and difficult-to-silence sounds, and try to fool the user into disclosing his username and password. As mentioned above, even signed and well-intentioned ActiveX controls may be exploitable by attackers, and if users are too accustomed to thoughtlessly pressing "Continue", an ill-intentioned control can easily obtain free run of the system. Simple JavaScript programs, which like Java applets are ordinarily unable to access user files, can cause confusion and waste time simply by displaying messages: one "joke" Web page was for a time greeting every tenth visitor with the message "Your system is now infected with the Psychic Neon Buddha Jesus virus". The message was completely false, of course, but it did cause numerous calls to help desks and anti-virus experts. (For some advice about minimizing your exposure to browser-based Trojan horses, see [36].)

What is the current situation in the real world? Except for some demonstration applets and controls that are clearly marked as such, and a large number of virus-infected Word and Excel documents, the Web seems to contain few or no true active-content Trojan horses. As we noted above, while a small number of respondents to our Web survey suspected that they had been victims of some sort of Web-related Trojan horse, in no case were we able to confirm that, and to date we have seen no truly malicious Java or ActiveX programs posing a danger to innocent users on the Web. On the other hand, this technology is still in its infancy. It is likely to become widespread quite rapidly, and it will take dedication on the part of developers to ensure that the function does not too far outpace the security of the systems. Users, and especially system administrators, need to be aware of developments in this area, including security-related bugs which are discovered all too often in active content systems, to make sure that their systems are as secure as possible against what is likely to become a more serious threat in the not-too-distant future.

Risks and Costs

There are a number of risks to the security of computer systems in the current environment. These risks include direct attacks (by both insiders and outsiders), known viruses, unknown viruses, known Trojan horses, and unknown Trojan horses. Which of these risks is the most serious, and which security measures are the most cost-effective?

Known viruses are by far the most common security problem on modern computer systems. Because they replicate by themselves, and can be exchanged in the normal course of business, between well-intentioned users, viruses spread without intentional help along lines of intentional data exchange. For a known virus to spread from person to person, or enterprise to enterprise, no malicious intent is required; the author of the virus could be long-dead, and all living persons virtuous, and the virus would still spread. We estimate that even in relatively well-protected environments, on the order of one percent of the computers in an enterprise can be expected to encounter a virus in a typical year. Fortunately, because viruses spread themselves, and viruses are just programs, it's always relatively easy (and usually completely trivial) to detect all the possible offspring of any given virus. So known-virus detection is both easy, and highly effective in combating a very real threat.

Unknown viruses are a more difficult, but fortunately a rarer, problem. Fred Cohen [37] has proven mathematically that perfect detection of unknown viruses is impossible: no program can look at other programs and say either "a virus is present" or "no virus is present", and always be correct. But, in the real world, most new viruses are sufficiently like old viruses that the same sort of scanning that finds known viruses also finds the new ones. And there are a large number of heuristic tricks that anti-virus programs use to detect new viruses, based either on how they look, or what they do. These heuristics are only sometimes successful, but since brand-new viruses are comparatively rare, they are sufficient to the purpose. For your company to be infected with a new virus, that virus has to spread from the author to you before it is detected anywhere else, an event that is thankfully not common. As connectivity and interactivity increase, it is of course vital that anti-virus measures keep up with the potential very fast spread of new viruses; to that end, anti-virus systems modeled after biological immune systems are now under development [38].

Both known and unknown viruses tend to be simple and mindless in their payloads. A virus may erase the boot record of your hard disk, forcing you to waste time restoring your data from backups, but it will not break into your employee database and alter salary records, because the author of the virus could not have known that it would spread to your system, and has no idea what your salary database is called, or what fields it has.

Direct attacks, on the other hand, where an attacker sets up a session between himself and one or more of your systems and issues commands from his own keyboard, can be more focused and hence more deadly. A virus may just erase some critical Windows files, but an attacker can snoop around the system, notice a SALDB.MDB, and try a few likely-looking passwords to open it, and examine or alter your company's confidential records. Because a direct attack assumes an involved attacker, direct attacks are much rarer than virus incidents; but because there is a human intelligence directly involved, they can be much more devastating. Anti-virus software has little or no relevance to direct attacks; contrary to various popular movies, attackers seldom use viruses when breaking into systems. To secure your system against direct attacks, you need to employ the whole panoply of computer-security measures: firewalls, passwords, separation of duties, and so on. Security against direct attacks must be designed-in and built-in to the systems that you use; no aftermarket software is going to solve the problem.

Trojan horses, our main theme, lie somewhere between computer viruses and direct attacks. You are unlikely to get a Trojan horse purposefully sent to you by a well-intentioned colleague in the normal course of business. On the other hand, a Trojan horse does not require a directly-involved attacker sitting at a keyboard typing. The most common, and most dangerous, type of Trojan horse is one that an attacker crafts specifically for one target, and then plants on a Web page, or sends in e-mail, or otherwise makes available to someone with access to the target system, in hopes that it will be executed in a mode where it can do its dirty work; changing a password or establishing an account for the attacker to use, mailing key data to the attacker, setting up a back door into the system, or deleting key files that the attacker knows are there. Since the attacker will be creating this sort of Trojan horse specifically for the purpose, it will be an unknown Trojan horse, and software on the anti-virus model is unlikely to detect it. The direct-attack model of prevention is the best one for this case: be sure that users know not to trust instructions from strangers, whether they come verbally over the phone or in the form of programs received in the mail. Ensure that the active content settings in your users' browsers are reasonable and secure, and have policies, as described above, for general prevention and good hygiene.

When are known Trojan horses likely to be a problem, and what is the right solution? Known Trojan horses are a problem when some attacker, for whatever reason, creates a single Trojan horse (or a set of very similar ones), and sends it to a large number of users repeatedly over a period of time. We know only a single case of this situation: the password-stealing Trojan horses that circulate on the popular online service America Online [39]. Because there are many AOL users, and because attackers continue to try to steal passwords using very similar Trojan horses repeatedly, the anti-virus model can be reasonably successful in this limited niche: a program that watches incoming files for a pattern characteristic of AOL password-stealing Trojan horses can do a fair job of protection against this particular attack. But if the attackers were to use a significantly different implementation of their attack, the anti-virus model would fail (at least temporarily), and users would have to rely on general anti-Trojan-horse methods as described above. On the other hand, if users practice good communication hygiene in the first place, they will know not to accept unexpected programs arriving in the mail, and a solution that protects only against known Trojan horses will be less necessary. In general, then, known Trojan horses are a significant problem only in some niche situations, and even in those situations more general security measures are still necessary.


As we have demonstrated in this paper, Trojan horses are anything but a new threat to computer users. Tracing back their history, we find that there are several different loose classes of Trojan horses, ranging from "classic Trojans" to the threats posed by active content.

While individual users are much more likely to encounter computer viruses than Trojan horses, a tailored Trojan horse attack could be devastating to a business. Our advice is simple and unexciting: use and update anti-virus software, follow good security practices, and keep aware of new developments in the field. Don't accept programs that arrived unexpectedly in the mail, and tell all your users to do the same. While this is sound advice, we note that the delineation between program and data is becoming increasingly blurred, and have some concerns regarding the increasing trend towards active content, where data and program become inseparable. It is extremely important that users begin to shift from thinking of Trojan horses as programs which can be identified by filenames to a paradigm which includes executable code in any form.

At the moment of this writing, the Internet has not caused a huge upswing in the frequency of Trojan horses in the world; our research was able to uncover almost no actual incidents of real users victimized by Trojan horses outside of one particular niche of the Net. Neither Java (with its powerful and fine-grained security model) nor ActiveX (with its cruder all-or-nothing controls) has been used to create or distribute real live Trojan horses to unsuspecting users. Of course, as computers and the Internet become more important to our businesses and our lives, it becomes more and more important to be aware of the possible threats that exist, and ensure that you have taken all sound measures against them.


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