Viruses and Worms

This chapter is from the book

Maximum Security, 3rd Edition

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This chapter is from the book

This chapter is from the book 

Maximum Security, 3rd Edition

Learn More Buy

Who Writes Viruses, and Why?

There are certain stereotypes associated with virus writing. On the whole, they're rarely useful. Most virus writers try, for obvious reasons, to preserve their anonymity, so testing the truth of these images is somewhat problematic. Some virus writers do discuss and display their craft and their angst in more-or-less public forums such as the newsgroup alt.comp.virus. These do seem to tend to be young males, and some research indicates that mostly they "age out" and leave the field as they acquire girlfriends and a life.

However, it's unsafe to assume that the "virus writers" who dominate such newsgroups are always who they say they are, let alone that they are as talented as they claim to be, or necessarily serious representatives of all virus writers. Indeed, it's possible that this group represents a constituency of wannabes rather than a group of real, competent virus writers. Certainly many successful viruses seem to have been written by focused loners with no particular affiliations, rather than by groups.

It's also noticeable that many of the most vociferous individuals quoted and feted by the media, law enforcement agencies, politicians, and others are not widely respected among their peers. Of course, the same is true of other types of computer vandals, not to mention many self-styled security and/or virus experts.

Some virus writers have responded to the very few serious attempts at research in this area. However, quantitative research is not realistically possible, and the research that has been done leans to the ethnographic. That is, rather than try to establish numerical data with large samples, researchers in this field have tended to rely on qualitative data, using interviews with very small samples (that is, just a handful of virus authors).

The acknowledged authority in this area is Sarah Gordon, who has written extensively in this area and in related ethical areas. Her papers for the Fourth and Sixth Virus Bulletin Conferences on The Generic Virus Writer are particularly relevant. A number of her papers, including both Generic Virus Writer papers, can be found at http://www.badguys.org/papers.htm.

A second widespread stereotypical notion is that people who write anti-virus software also write viruses, in an attempt to drum up business for their products. I can't say with absolute certainty that no vendor or researcher has ever written a virus, released a virus, or even paid a bounty for samples of original viruses. However, it's hard to comprehend why any anti-virus professional would see a need to stray toward "the Dark Side" at this stage of the game. There are more than enough amateurs producing viruses. In Generic Virus Writer II, Gordon notes that older security professionals, especially systems administrators and such, make their own contribution to the virus glut through (probably well-meant) experimentation. However, despite the eagerness of virus writers to implicate "the enemy" in the problem, there is no conspiracy between systems administrators and vendors to keep vendor profits high. Or if there is, no one has offered me a percentage.

There are probably as many reasons for writing viruses as there are virus writers, although the reasons cited by virus writers (actual or wannabe) don't always stand up to closer analysis. Some appear fascinated by the concept of a self-replicating and/or self-modifying program, and are curious to see how far their creations spread. Indeed, some apologists suggest that virus writing is a legitimate means of research into artificial life forms, or even artificial intelligence. (However, the adaptive behavior displayed by even the most sophisticated viruses is usually rather restricted.)

Many virus authors seem to enjoy matching wits with the anti-virus establishment. Indeed, some viruses go straight from the creator to his favorite anti-virus company without any attempt to spread it through the general population. Others, however, are more concerned with inspiring the admiration of their peers, rather than gaining the attention of the anti-virus professionals. Others don't make a hard and fast distinction between writing viral and anti-viral software, and might write both. This isn't normally the case in the anti-virus industry, and those who've used their experience on both sides of the barbed wire to support of their search for a job in the industry have usually been sadly disappointed. In fact, development teams in the industry have practical as well as ethical reasons for preferring to employ programmers whose experience is in other areas. It saves them having to clean ill-founded technical preconceptions out of the newcomer's head.

There are, of course, many viruses that are intended to cause widespread damage, although deliberate destruction is the goal far less often than most people seem to believe. (Often, virus damage comes from thoughtlessness or sheer incompetence on the virus writer's part.) Some virus writers argue that computer users who don't have the technical savvy to protect themselves deserve everything they get. On the other hand, some virus writers also claim that they have no personal involvement in virus dissemination, and are not responsible for the use made of their code by others. In other words, the distributors are the problem, not the authors. This would be more convincing if such authors never made their creations available as source code and/or binaries on Web sites, in e-zines, and other locations. Then, viruses would be less easily available to anyone who asks, or trawls Vx (Virus eXchange) Web sites.

How Are Viruses Created?

Some people seem to believe that computer viruses appear spontaneously in the same way that biological viruses seem to do. This isn't quite as silly as it sounds. Completely new viruses don't just pop out of the primeval soup without warning. However it's not uncommon for a new variant (not necessarily a viable virus in terms of replication and the capability to infect) to be born without direct human intervention. For instance, a macro virus consisting of a fixed number of modules might mutate by losing some of its constituent macros or gaining unconnected (not necessarily viral) macros. WM/Cap, for example, mutated into many hundreds of variants of the original virus. However, someone had to write the original version.

It's not impossible that an operating environment might come into general use in which a viral program could be created from scratch without direct human intervention, but it doesn't seem to have happened yet.

Most virus writers (and a high percentage of the rest of the world) have an exaggerated view of the ability needed to produce a working virus. Undoubtedly, some virus writers produce technically competent code: many more don't. Furthermore, as we've seen, many viruses are one-trick ponies. They might do the replication trick well or not so well, but replication, even when done efficiently, represents a somewhat limited functionality, compared to that of a compiler or business application.

Older viruses were often written in assembly language. In fact, it's difficult to write some types of virus in a high-level language, even with the help of an inline assembler. This is an advantage, from the viewpoint of virus victims, in that it takes a certain level of programming expertise to create even a weak virus (or even to modify an existing virus so as to create a variant). Many variants are, in fact, simply existing viruses with a slight change that doesn't affect functionality (such as modification to unimportant embedded text). Such a change might require no programming at all.

Some virus writers and their admirers still regard proficiency in assembly language as the hallmark of programming excellence. (This is actually in sharp contrast to the professional programmer, whose choice of tool, given a choice, is liable to be somewhat more pragmatic.) However, the current is, by and large, flowing the other way.

As virus technology developed, some virus programmers turned their attention to creating kits to allow a wannabe virus author to "develop" other viruses without programming. That is, using virus generators to produce virus code. This has not, however, necessarily resulted in an increase in the total number of viruses "in the wild."

Kit viruses are often not actually viable (that is, they don't replicate), and are frequently detectable generically. A new kit virus might be identifiable as having been generated by a particular generator, simply by family resemblance. Thus, kit viruses have tended to contribute to the "glut" problem (the sheer weight in numbers), rather than to the "in-the-wild" problem (see next section).

Certainly, assembly language is not necessarily the language of choice among the current generation of virus writers. Interpreted macro languages (especially Visual Basic for Applications) are generally harder to use than kits, but much easier than assembler. Furthermore, disk space and main memory are no longer expensive, and grossly bloated files are less conspicuous in a Windows environment. Thus, it's become more practical (as well as easier) to write viruses and worms in C++ or Delphi.

What Does "In the Wild" Really Mean?

A virus is deemed to be "in the wild" when it has escaped or been released into the general population. The general population refers to computing environments outside the development environment where the virus was created and tested, or the collections of anti-virus vendors, researchers, and collectors. Viruses in these environments are typically (hopefully) processed under controlled circumstances, where no danger is posed to the surrounding communities. However, when a virus escapes a controlled environment, it might be said to be "in the wild" (often expressed adjectivally in the anti-virus community as In-the-Wild or ItW). Note, however, that in the anti-virus community, the fact that a virus is available on a vx (Virus eXchange) bulletin board or Web site does not make it In-the-Wild. Because access to such resources and exchange of viruses is voluntary, this counts as a controlled environment.

In his conference paper Counting Viruses (Virus Bulletin 1999), Paul Ducklin makes the distinction very clearly:

"For a virus to be considered In the Wild, it must be spreading as a result of normal day-to-day operations on and between the computers of unsuspecting users. This means viruses which merely exist but are not spreading are not considered 'In the Wild'."

In fact, the definition used by the WildList Organization is far stricter. For a virus to be on the WildList, the nearest thing to an industry standard metric for "In-the-Wildness," it must be reported by two or more of the virus professionals who report to the WildList Organization. Furthermore, these reports must be accompanied by replicated samples. (Viruses that are reported by only one reporter are put into the supplementary list.) Clearly, this strictness means that the WildList can't represent all the ItW viruses at a given time, but does represent viruses that are genuinely "out there." Such data are often more useful than absolute numbers to the organizations and individuals using the WildList as a basis for testing and research. (Note that the WildList indicates a virus's presence "out there," but not the total number of virus incidents in which a single virus is implicated. Thus the list only provides a very rough guide to prevalence.)

What matters most for our purposes, however, is the disparity between the number of In-the-Wild viruses at one time (a few hundred according to the prevailing WildList) and the total number of viruses in existence. (At the time of writing between 50,000 and 60,000, depending on how you measure.)

How Do Viruses Work?

A virus is, conceptually, a simple program. In its simplest form, a direct action virus can be modeled in terms of an algorithm like this:

begin
  Look for (one or more infectable objects)
    If (none found)
    then 
      exit
    else (infect object or objects).
    endif
end

They don't remain in memory, but execute all their code at once, and then hand over control to the host program.

Many viruses go memory resident (install themselves into memory) after the host program is executed, so that they can infect objects accessed after the infected application has been closed.

The term hybrid is sometimes used for viruses that stay active as long as the host program is running. It is also (perhaps with more justification) applied to viruses that are both direct action and memory resident.

In fact, all viable boot sector infectors are memory resident—they have to be. Otherwise, their code can only be executed during the boot process, which rather limits their opportunities to infect other boot sectors. We consider boot sector viruses in detail later on in this chapter.

Of course, all but the most incompetent viruses are a little better error-trapped than this, and at least check that the infectable object hasn't already been infected. You'll notice that I've also skated over the infect object subroutine. We'll come to infection mechanisms when we discuss the main virus types later in this section.

Some viruses, of course, do more than just replicate. We sometimes describe viruses as having up to three components: an infective routine, a payload, and a trigger. The previous models demonstrate an infective routine, although it could be said that finding an infectable object is the trigger for the infective routine. However, we more often think of the trigger as being the condition that has to exist before the payload (or warhead) can be executed. The payload can, in principle, be any operation that any other program can perform. In real life, however, it tends to be something flippant and irritating, like visual or audio effects, or else downright destructive. So now our model looks more like this:

begin
  (Go resident)
  if (infectable object exists)
  then
    if (object is not already infected) 
    then 
    (infect object)
    endif
  endif
  if (trigger condition exists)
  then
    (deliver payload)
  endif
end

The trigger condition might, for instance, be the execution of a file, or a particular date or time. The combination of a trigger and a malicious payload is sometimes called a logic bomb.

Viruses can be classified conveniently (but by no means definitively) into five main classes: Boot Sector Infectors (BSIs); file infectors; multipartite viruses; macro viruses; and scripting viruses.

Boot Sector Infectors (BSIs)

These PC-specific viruses infect the Master Boot Record and/or DOS Boot Record. At one time, these viruses accounted for the majority of reported incidents, but now they constitute a dwindling proportion of the total number of threats found in the wild, and new BSIs are something of a rarity. This might reflect the fact that people now increasingly use email and networks rather than floppy disks to exchange files. The fact that these are harder to write than macro viruses and scripting viruses (or even file viruses) is also relevant.

When a modern PC boots up, it goes through a process called Power On Self Test (POST). This stage of the boot process includes checking hardware components. Some of its information comes from information stored in CMOS, especially information relating to disk and memory type and configuration. If the CMOS settings don't match the actual drive geometry, the machine will not be able to find system areas and files where they should be, and will fail to finish the boot process.

The Master Boot Record (MBR), sometimes known as the Partition Sector, is found only on hard disks, where it is always the first physical sector. It contains essential information about the disk, giving the starting address of the partition(s) into which it is divided. On diskettes, which can't be partitioned and don't contain an MBR, the first physical sector is the boot record or DBR. On hard drives, the boot record is the first sector on a partition. The boot record contains a program whose job is to check that the disk is bootable and, if so, to hand over control to the operating system.

By default, if there is a bootable floppy present, most PCs will boot from drive A, the first floppy drive, rather than from drive C, the first hard drive. This is actually an unfortunate default because this is the normal entry point for a boot sector virus. If the PC attempts to boot from a floppy with an infected boot sector (even if the floppy doesn't contain the necessary files to load an operating system and therefore can't complete the boot process), the infected floppy will infect the hard drive. Characteristically (although not invariably), once the hard drive is infected, the virus will infect all write-enabled floppies.

The majority of boot sector viruses also contain some provision for storing the original boot sector code elsewhere on the drive. There is a good reason for this. It isn't because the virus programmer kindly intends to eventually return the MBR to its original state, although retaining a copy of the original boot sector can make disinfecting the virus easier. Rather, it is because he has to. Typically, a virus will keep a copy of the original boot record and offer it whenever other processes request it. This not only enables the system to boot in the first place, but also makes it harder to detect the virus without anti-virus software that specifically recognizes it. However, some viruses simply replace the normal boot sector code with code of their own.

Some BSIs (Form is a particularly well-known and widespread example) only infect the boot record, even on hard disks. This creates particular problems with Windows NT and Windows 2000, and will usually prevent the system from booting at all. Thus a largely innocuous virus has suddenly become a major nuisance in some environments.

File Viruses (Parasitic Viruses)

File viruses infect executable files. Historically, most file viruses have not been particularly successful in terms of their epidemiology (that is, at spreading). Many thousands have been written, but the number actually seen in the wild has been comparatively small compared to BSIs and, more recently, macro viruses. Nonetheless, those that have survived in the wild have often spread surprisingly well—CIH, for example. Some of the most prevalent contemporary file viruses, however, are more commonly described as worms, as considered later in this chapter.

After a virus infects an executable file by direct attachment, that file, when executed, will infect other files. Fast infectors go for instant gratification. Each time the infection routine is executed, it infects a whole directory, all folders on the current path, a whole volume/disk, even all currently mounted volumes. Even file infectors that infect only one or two files at a time can spread quickly across systems and networks in a modern environment, where multiple binary executables are opened and closed many times over a single session. Every time you open an application, at least one executable file is loaded. Some applications will open several files at startup, whereas others periodically open multiple files when performing a particular operation.

Sparse infectors forgo the temptation to infect as many files as possible, usually in an attempt to make themselves less conspicuous. They may not infect every time the virus is executed, but only under very specific conditions, even when an infectable object is there to infect.

Multipartite Viruses

File and boot viruses are the most common example of multipartite viruses, viruses that use more than one infection mechanism. In this case, both boot-sectors and binary executable files might be infected and used as the means of disseminating the virus. However, it's likely that there will be an increase in multipartite viruses consisting of other combinations of virus types.

Macro Viruses

Macro viruses infect macro programming environments rather than specific operating systems and hardware. Microsoft Office applications are by far the most exploited environment. These can be regarded as a special case of file virus, in that they appear to infect data files rather than binary executables. However, this way of looking at the process might actually confuse the issue. Macros are essentially a means of modifying the application environment, rather than (or as well as) the data file. Indeed, in the case of Microsoft Office applications that support macro programming languages (Visual Basic for Applications and, in earlier versions, WordBasic and AccessBasic), the macro language cannot be unbound from the application's own command infrastructure. Macro viruses usually infect the global template, and often modify commands within the application's menu system. Macro viruses are particularly successful against Microsoft applications because they allow executable code (macros) to exist in the same file as data. Applications that segregate macros and data into different files are less susceptible to this kind of attack.

Script Viruses

Script is rather an imprecise term, but in this context normally (currently) refers to VBScript and other malware that can be embedded in HTML scripts and executed by HTML-aware email clients through the Windows Scripting Host. Many of the viruses that use this entry point are often better characterized as worms, and are therefore treated under that heading later in the chapter. VBscript and Jscript are more virus friendly than JavaScript (for instance), primarily because they have many of the file I/O capabilities of other variations on the Visual Basic theme. Extant JavaScript malware usually takes advantage of an easily patched vulnerability in Internet Explorer.

This view of script viruses is rather restrictive. A broader definition might include HyperCard infectors, batch file infectors, UNIX shell script infectors, and many more. However, these are of less practical importance, currently.

Memetic Viruses

There is a further class of "viruses," which is unique, in that it comprises viruses that don't exist as computer code. The term meme seems to have been coined originally by Richard Dawkins, whose paper Viruses of the Mind draws on computer virology as well as on the natural sciences. A meme is a unit of cultural transmission, of replication by imitation, much as a gene is a unit of inheritance (a rather imprecise unit, perhaps). The memes we are most concerned with in this chapter are those sometimes known as metaviruses. A metavirus is itself a virus (what Dawkins calls a "virus of the mind, not a computer virus"), but purports to deal with other viruses (which are computer viruses). These viruses don't happen to exist. In other words, they are virus hoaxes. Virus hoaxes are not only a subclass of memes in general, but a subset of a particular type of meme, the chain letter. However, the virus hoax is particularly relevant to this chapter, because the administrator who manages virus incidents will usually also be the person who has to respond to plagues of virus hoaxes. The same might not be true of other hoaxes and chain letters.

The most commonly encountered hoaxes are derived from the infamous Good Times hoax of the mid-1990s. They conform to a pattern something like this:

[THIS WARNING WAS CONFIRMED BY SYMANTEC AND MCAFEE THIS MORNING.] IF YOU RECEIVE EMAIL WITH THE SUBJECT <GREEN EGGS AND HAM> DO NOT OPEN IT, BUT DELETE IT IMMEDIATELY!!! IT CONTAINS A VIRUS THAT JUST BY OPENING THE MESSAGE TRASHES HARD DRIVES AND CAUSES MOUSE-MATS TO SPONTANEOUSLY COMBUST. MICROSOFT, AOL, IBM, FCC, NASA, CND, AND KKK HAVE ALL SAID THAT THIS IS A VERY DANGEROUS VIRUS !!! AND THERE IS NO REMEDY FOR IT AS YET. PLEASE FORWARD THIS TO ALL YOUR FRIENDS, RELATIVES, COLLEAGUES, AND ANYONE ELSE WHOSE EMAIL ADDRESS YOU HAVE HANDY SO THAT THIS DISASTER CAN BE AVERTED.

By the way, as far as I know there is no Green Eggs and Ham virus or hoax. I've just done what many real hoaxers have done and pulled a silly title out of thin air (or in this case my daughter's bookshelf). In fact, the infuriating aspect of this problem is that most hoaxers are abominably lazy and unoriginal, and the subject of the email which carries the supposed virus is often the only bit of the hoax that varies between two variants.

This sort of hoax only continues to work because masses of people with little technical knowledge of computers (let alone computer viruses) join the Internet community for the first time every day. Each one is at high risk of passing on such a hoax because they don't know any better. Of course, a hoax can be much more subtle than this, but I'm not here to tell you how to write a hoax that might fool even an expert.

Here are a few of the features that would alert the experienced hoax watcher to the unreliability of the Green Eggs and Ham alert:

• Uppercase is used throughout and the message carries clusters of exclamation marks for emphasis. This doesn't, of course, prove anything about the accuracy of the alert. Nevertheless, it's been observed many times that use of uppercase, liberal exclamation marks, and poor spelling, grammar and style characterize most of the common hoaxes. On no account, however, should you assume that an alert is accurate simply because it doesn't have these characteristics.

• The reference to McAfee and Symantec doesn't give contact or reference information. It's just there to add credibility to the hoax. There's no real indication of when it was written, either. There are hoaxes circulating the Internet right now, saying that IBM announced something "yesterday," that have been around for years. The "yesterday" is just there to give a false impression of urgency.

• It's true that some email viruses/worms arrive with a characteristic subject header. However, there are many others that don't, and it makes more sense to avoid executing any attachment than to try to remember which silly header goes with which virus. In fact, administrators trying to block particular viruses by filtering mail on subject alone and using inappropriate criteria are responsible for a whole subclass of indirect Denial of Service (DoS) attacks in and on themselves.

• It makes sense to be cautious about email, but just opening a message can only infect your system if you have certain mail programs (Outlook, primarily) set with incautious defaults. Most mailers don't execute code just by viewing the message. An alert that says that this will happen but doesn't specify any particular mailer, should be regarded with suspicion.

• It's implied that the malicious code works on any hardware. This is pretty suspicious. What's more, a payload that triggered as soon as you opened the message/attachment would be pretty ineffective at spreading. You might think the mouse-mat payload is a bit over the top. Actually, real hoaxes are often as ridiculous as this (although they often conceal their improbability behind technobabble).

• Of all the organizations listed, only IBM has any real expertise in viruses. The others are only listed to impress you.

• It's claimed that there is no "remedy" for the virus. Anti-virus vendors can usually supply fixes for new viruses in hours, even minutes. Of course, the effects of some viruses might be impossible to reverse, but data recovery firms can perform near-miracles sometimes.

• A virus that trashes your system as soon as you execute it is unlikely to travel very far. What is being described here sounds more like a destructive Trojan, and they don't generally spread well through email.

• The warning urges you to forward the mail to everyone you know. That makes it a chain letter. Reputable and knowledgeable organizations don't send alerts that way, although clueless ones sometimes do.

How Do Worms Work?

The 1988 Morris Worm (the Internet Worm) and its siblings, such as WANK and CHRISTMA EXEC, usually targeted heavy-duty mainframe and minicomputer hardware, mail, and operating systems. More recent threats have been aimed primarily at PCs, and, in one highly publicized incident (the AutoStart worm), Apple Macs. However, they might have the incidental effect of bringing down mail servers through the sheer weight of traffic they generate. Some of these have been variously classified by different researchers and vendors as viruses, as worms, as virus/worm hybrids, and occasionally as Trojan horses.

Today's worms and email viruses tend to be fast burners. They have the potential to spread globally before anti-virus vendors have time to analyze them and to distribute means of detection and disinfection. Some of the malware commonly referred to as worms are actually specialized viruses that infect only one file. This doesn't mean, of course, that a virus like Lehigh, which infects only COMMAND.COM, can sensibly be defined as a worm.

Universally accepted classifications of worms don't exist, but Carey Nachenberg, in a paper for the 1999 Virus Bulletin Conference, proposed a classification scheme along the following lines:

• Email Worms, unsurprisingly, spread via email.

• Arbitrary Protocol Worms spread via protocols not based on email (IRC/DCC, FTP, TCP/IP sockets).

As well as proposing classification by transport mechanism, Nachenberg also proposed classification by launching mechanism:

• Self-launching Worms such as the 1988 Internet Worm require no interaction with the computer user to spread: They exploit some vulnerability of the host environment, rather than in some way tricking the user into executing the infective code. However, KAK and the rather rarer BubbleBoy are examples of self-launching worms. By exploiting a bug in the Windows environment, they can execute without user intervention.

For information on dealing with this problem by applying a patch, see http://support.microsoft.com/support/kb/articles/Q262/1/65.ASP.

• User-launched Worms interact with the user. They need to use social engineering techniques to persuade the victim to open/execute an attachment before the worm can subvert the environment so as to launch itself onto the next group of hosts. Many of today's VBScript worms fall into this or the Hybrid-launch category.

In fact, some of the worms we've seen to date are probably better classified as Hybrid-launch Worms (by Nachenberg's classification scheme) or multipartite (in terms of conventional virus terminology) because they use both self-launching and user-launched mechanisms.

Virus Characteristics

The following characteristics are not necessarily restricted to particular virus/worm classifications, but are of some importance if only because of the way the terms stealth and polymorphism are so often misused:

• Stealth. Almost all viruses include a degree of stealth, that is, they attempt to conceal their presence in order to maximize their chances of spreading. There have been viruses that asked permission before infecting, but this courtesy has not been rewarded by wide dissemination. Conspicuous payloads tend to be avoided, or are delivered fairly irregularly. Stealth viruses use any of a number of techniques to conceal the fact that an object has been infected. For example, when the operating system calls for certain information, the stealth virus responds with an image of the environment as it was before the virus infected it. In other words, when the infection first takes place, the virus records information necessary to later fool the operating system.

This also has implications for anti-virus tools that work by detecting that something has changed rather than by detecting and identifying known viruses. To be effective, such tools must use generic anti-stealth techniques. Of course, it isn't possible to guarantee that such techniques will work against a virus that has not yet been discovered. However, virus scanners that detect known viruses are at an advantage in this respect, because vendors will normally compensate for a new spoofing technique when they add detection for the virus that employs it. The trick employed by some BSIs of displaying an image of the original boot sector as if it was still where it belonged is a classic stealth technique. File viruses characteristically (but not invariably) increase the length of an infected file, and can spoof the operating system or a anti-virus scanner by subverting system calls so that the file's attributes before infection, are reported, including file length, time and datestamp, and CRC checksum.

• Polymorphism. Polymorphic viruses are adored by virus authors and feared by nearly everyone else. This is partly because of an over-estimation of the impact of the polymorphic threat. Non-polymorphic viruses usually infect by attaching a more-or-less identical copy of themselves to a new host object. Polymorphic viruses attach an evolved copy of themselves, so that the shape of the virus changes from one infection to another. Early polymorphic viruses used techniques such as changing the order of instructions, introducing noise bytes and dummy instructions, and varying the instructions used to perform a specific function. A more sophisticated approach is to use variable encryption, drastically reducing the amount of static (unchanging) code available to the anti-virus programmer to use to extract a pattern by which the virus can be identified. You might imagine (as many people do) that this makes polymorphism a formidable technology to counter. Indeed, the emergence of polymorphic viruses and plug-in mutation engines (enabling almost any virus author to include variable encryption in his own work without reinventing the wheel) contributed to the disappearance of some of earlier anti-virus packages. However, although polymorphic viruses are popular with virus authors demonstrating their skills, they have been less well represented in the field than in the collections of anti-virus researchers, certification laboratories, comparative testers, and others who need as complete a collection as possible. Anti-virus scanning technology has also moved on, and simple signature scanning for a fixed character string doesn't play a large part in the operation of a modern scanner.

The classifications of viral malware described earlier do not cover the entire range of objects detected by anti-virus software. Some vendors are quick to point out that what they sell is anti-virus software, not anti-malware software. Nonetheless, nowadays most commercial products detect some Trojan horses (see Chapter 18) and other objects that barely qualify as malware, let alone viruses. Such objects include intended (non-functioning) viruses, joke programs, DDoS programs (Distributed Denial of Service), even garbage files that are known to be present in poorly maintained virus collections likely to be used by product reviewers.

It might be noticeable that this chapter has been largely PC-centric. Certainly, there are more viruses that infect PC platforms (DOS and all flavors of Windows) than any other operating system. Native Macintosh viruses are far fewer. In fact, there are probably more native viruses on systems such as Atari and Amiga that have never had the same popularity (in corporate environments, at least). However, the fact that Apple Macintoshes share with Windows a degree of vulnerability to Microsoft Office macro viruses makes them the other main virus-friendly environment today.

It should not be assumed, however, that other platforms don't have virus problems. Access controls can be imposed on unprivileged accounts in UNIX (including Linux), NT, NetWare, and other platforms to restrict infection flow. However, they can't prevent unprivileged users from sharing files, if only by email. Nor can they prevent a privileged user inadvertently spreading infection. Even systems that don't support any known native viruses (servers or workstations) can carry infected objects between infectable hosts, a process sometimes known as heterogeneous virus transmission. It's as important to scan network file servers, Intranet, and other Web servers, regardless of their native operating system. In fact, an increasing number of products detect viruses associated with other operating environments. Thus some Mac products detect PC viruses, and vice versa.

Clearly, viruses do represent a risk on the Internet. That risk is higher for those running DOS, any variant of Windows, or certain macro-capable applications, especially the Microsoft Office applications suite. Mostly this is a matter of market share. Most virus writers target PCs and Windows because that's what they have access to. However, there are other factors that increase the risk: for example, PC hardware architecture, Microsoft's rosy view of the lack of need for security on single-user systems, and the dangers of having macro code and data in the same file. There are some tools to help keep systems safe from virus attacks listed later in this chapter. Be aware, though, that the only way to guarantee safety is by obeying Richards' Laws of Data Security—don't buy a computer, and, if you do buy one, don't turn it on. (A tip of the hat to Robert Slade for bringing that one to my attention.) Anti-virus software is mostly reactive: It responds to a perceived threat, and works most effectively against threats it can identify with precision (that is, known viruses). The best defense against unknown viruses is often to work in an environment that doesn't provide a host to particular classes of threat. Sadly, however, this is often not an option, particularly in some corporate environments where Microsoft products are considered obligatory.