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The Future of Viruses on the Internet

David Chess
Virus Bulletin International Conference in San Francisco, California
October 1997

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Lots of Little Tremors, a Few Big Quakes

There are many different phenomena, both natural and man-made, that come in different sizes. In many cases, the small things come quite often, and the big things only rarely. Small earth tremors are much more common than large earthquakes. Slight rises in the level of a river are much more common than floods. Virus incidents that involve only a few machines are much more common than major outbreaks involving hundreds of systems.

When the distribution of incidents is skewed in this way, we tend to get good at handling the small incidents; they become routine and expected, and our solutions will be efficient and well-tested. Large incidents, since they happen rarely, are handled on a more ad hoc basis; maybe we know who will be on the Crisis Team (assuming that someone's remembered to update the list as people changed jobs), but exactly what the team will do once it's assembled will be determined on the fly to fit the case. That's why the Crisis Team has to contain some of the best people we have available.

Conditions change. If the probability of a large incident increases rapidly, we can be caught unawares, and the number of large incidents can easily overwhelm an ad hoc crisis-based approach that depends on large problems being few and far between. On the other hand, if we can see the change coming, we have a chance to develop methods that can handle the majority of large incidents as smoothly as we already handle the small ones.

How will the continued growth of the Internet change the pattern of virus spread and similar threats? Is our current paradigm of virus containment sufficient for the future?

Viruses and the Internet

At present, computer viruses are a constant low-level annoyance. Every company knows that it ought to have anti-virus software, and that virus protection is a cost of doing business, just like backup and fire insurance. Small virus incidents are common and routine. In organizations that do centralized incident management and reporting and have anti-virus software well deployed, most incidents involve only one or two systems; the virus is caught before it can spread farther than that [1].

When new viruses are discovered, anti-virus software is updated to deal with them on a cycle of weeks or months. Anti-virus vendors generally offer monthly updates, and in a typical corporate environment new updates are installed every one to six months. Because it takes a typical new virus many months, or even a few years, to become widespread, this is reasonable. The recent rise of macro viruses, which can become widespread in just a few months, has put some downward pressure on these time-scales, but not changed their general magnitude. It is still feasible to deal with new viruses through a largely manual process: a customer finding a new virus sends it in to a vendor, the vendor analyses it by hand and returns detection and repair information to the customer, and other customers get the information over the next months, in their regular updates.

The Internet currently plays a comparatively small role in the spread of viruses. No common virus today is network-aware; all of them require help (generally accidental help) from users in order to spread. So the Concept virus spreads over the Net only when someone mails an infected document to someone else, or makes one available on their Web site. Virus authors have taken advantage of the ease of anonymous posting to distribute copies of their viruses via Usenet News, but since the viruses themselves do not make use of Usenet to spread further, this is a one-time "planting" event, not a continual spread. Since all these network transmission methods rely on manual action, manual responses have been adequate to deal with them.

New Systems

There are two major trends in Internet technology that will have an impact on virus spread in the next few years: one is the increasing ubiquity and power of integrated mail systems, and the other is the rise of mobile-program systems.

Integrated mail systems such as Lotus Notes and Microsoft Outlook make it very simple to send anything to anyone, and to work with objects that you receive. They also support application programming interfaces (such as MAPI and the Notes API) that allow programs to send and process mail automatically. To the extent that these systems increase the rate at which people intentionally share programs (including documents with embedded macros), the rise of these systems will increase the rate at which manual virus spread of the kind that we're used to occurs. As these systems, and standards such as MIME, make it easier to send compound objects across the Internet, rather than just within one's local workgroup, the possible range of manual spread also increases. We will consider other implications of these systems in a moment.

Mobile-program systems are systems that are designed to allow programs to move on their own from one system to another. The most widely-hyped examples today are Java and ActiveX. At the moment, this technology is used almost exclusively to allow a program to move from a Web server to a browser client and execute there; but with the integration of Java into Lotus Notes, and ActiveX into Microsoft's mail systems, this is already changing. Unlike traditional mail systems, mobile-program systems are generally designed with some sort of security in mind: some idea that a program that arrives from somewhere else should not always be trusted and obeyed the same way a program launched from the local desktop would.

On the other hand, mobile-program systems are complex, and both Java and ActiveX have been found to have security bugs which allowed untrusted mobile programs to do things they should not have been able to do. There is no reason to think that the last bug has been found; we will continue to see security bugs uncovered in these systems, and it would be foolish to assume that they will continue to be found by the good guys before the bad guys get around to using them. These bugs may be exploited in direct attacks against particular sites, or to manually install traditional viruses on many machines at once. They may also enable entirely new network-aware viruses and worms. (There is no firm theoretical line between a virus and a worm. In general a virus is a fragment of code that embeds itself in some pre-existing file that gets passed around later on, while a worm actively sends itself from place to place; but as the network becomes more powerful and more ubiquitous, this distinction becomes less useful. We will use the term "virus" to refer to any replicating attack.)

New Quakes

What determines how large a virus incident is? Only two factors are relevant: the birth rate (how fast new systems become infected by the virus) and the death rate (how quickly an infected system can be disinfected, and prevented from infecting others).

Figure 1: the number of infected machines over time in a simple simulation. The top curve reflects the course of infection when the birth rate is greater than the death rate. The lower curve, expanded in the detail, shows the virus dying out quickly when the birth rate is less than the death rate.

Figure 1 illustrates the importance of the birth and death rates in a simple simulation of virus spread. When the birth rate is at least as large as the death rate, a virus can become established in a population; when the death rate is higher, the virus quickly dies out. The point where birth and death rates are equal is known as the "epidemic threshold".

Our experience suggests that as long as new systems become infected only by manual transmission of programs from one system to another, the birth rate remains low enough for manual containment to work; judicious use of conventional anti-virus software can keep an organization below the epidemic threshold.

But what happens if network virus spread becomes automatic? We have experience with a few examples.

In 1988, the Internet Worm spread to thousands of systems in a matter of days[2]. It did so by exploiting a number of Unix bugs and features that allowed it to send a copy of itself to another machine, and have that copy begin to execute as soon as it arrived. This is the fastest kind of automatic spread. Note that it is also the paradigm underlying mobile-program systems; this is one reason that security in these systems is so important. The Worm spread so fast that it overwhelmed many of the communication channels by which the experts combating it would normally have cooperated. Eventually, though, the Internet's best hackers got the attack under control, working long hours over many days to finally understand and eliminate the Worm. This is a classic example of ad hoc Crisis Team problem solving, and it worked very well. But it is not an approach that scales well; if attacks like the Internet Worm were to happen once a month, or once a week, much faster and more efficient countermeasures would be needed.

A program bent on spreading itself does not have to execute on arrival in order to succeed. In December of 1987, the CHRISTMA EXEC program spread to thousands of computers connected to EARN, BITNET, and IBM's internal VNET. Unlike the Internet Worm, CHRISTMA could spread only if a recipient received the program (which was disguised as a traditional "Christmas Card" displayer) and intentionally executed it. This meant that the program's birth rate was orders of magnitude lower than the Internet Worm's, but it was obviously fast enough. Again, the problem was solved only by the tireless efforts of the most skilled network experts available, working long hours over the holiday. So a network-aware threat can become widespread very quickly, even if it does rely on user action for one step of its life cycle.

We have seen a similar effect more recently: the ShareFun virus, like the CHRISTMA EXEC, spreads via the mail system when a user intentionally uses an infected object that has come in the mail[3]. Unlike CHRISTMA, which required the user to run a program, ShareFun requires only that the user open an embedded document in Microsoft Word. On the other hand, ShareFun requires an obsolete version of Microsoft Mail to spread, and its implementation is brittle and often fails. But in at least one company that had widely deployed the particular mail system that ShareFun requires, the virus became very widespread very quickly, and required expert intervention to clean up.

Is the threat of network-aware viruses like Sharefun growing? Yes. A competently-coded version of Sharefun that exploited a more modern version of the mail system would have a good chance of becoming very widespread very quickly, and again requiring us to call out the experts to work around the clock to deal with it. A virus six months from now, exploiting a bug in some widely-deployed mobile-program system, might get itself "pushed" to thousands of systems before it was even noticed.

Rising Birth Rates

A number of different network-aware software systems are being deployed widely on intranets and the Internet. To what extent do these particular systems have features that might allow a network-aware virus to spread, and to what extent do they offer protection against that actually happening?

We have examined some of the most popular office and mail systems available for modern personal computers, including Lotus Notes, Microsoft Exchange and Outlook, Netscape Navigator, and Microsoft Internet Explorer (the latter two are usually thought of as Web browsers, but both include mail and news clients as well). We will present some general observations here; we will avoid specific details both because all of these systems are still rapidly evolving (so the details will have changed by the time you read this), and because we don't want to provide any blueprints to the bad guys.

All of these systems allow an embedded program to execute when the user opens an attachment, if only because they all allow the use of Microsoft Word as the viewer for Word documents, and Word documents can contain macros (including viral macros). Most of them allow programs to execute when a piece of mail is simply opened, either because Word can be used as a viewer for the mail itself, or because mail can contain programs in some other language (Java and JavaScript for Navigator and Explorer, Java and LotusScript for Notes, and to some extent ActiveX objects for Outlook). We have not found any features in the current versions of these systems that allow a program embedded in mail to execute immediately on arrival, before the user even opens it, but as CHRISTMA EXEC has shown us, this ability is not necessary for a worm to spread.

There is considerable variation in what the systems allow embedded programs to do and what sort of security is provided, although we are starting to see hints of convergence. Word macros are completely unrestricted, and can do anything that a normal application program can do, including reading, writing, and altering files and sending mail. ActiveX objects also have unrestricted access to the system, at an even lower level. Embedded Java and JavaScript programs, on the other hand, are currently restricted from the most dangerous actions, such as accessing the file system or opening network connections. This is good for security, of course, but it is bad for function; as users come to expect more from their systems, these restrictions are being eased in various ways.

Some versions of Microsoft Word and the Microsoft Mail API provide a rudimentary protection by warning the user that a document contains macros, or a piece of mail contains attachments, and asking for confirmation before proceeding. While this simple protection may be effective in some environments, we anticipate that in environments where innocent macros and attachments are common, it will be widely disabled (the interface makes it simple to tell the system never to present the warning again).

ActiveX and the most recent versions of Java both provide a somewhat richer protection by allowing embedded programs to be digitally signed. This enables the user who receives a program over the net to determine with some certainty who created the program. In ActiveX, the user can use the signature information only to decide whether or not to allow the program to execute at all. Java, as embodied in the most recent Java Development Kit and a forthcoming version of Lotus Notes, will allow users to configure their systems to allow programs signed by different authors to have different degrees of access to the system.

Digital signatures offer a certain amount of protection against malicious "Trojan horses" sent by untrusted parties, since the system will not allow programs from untrusted authors much access to important resources. They offer less protection, however, against viruses and worms. Because replicating attacks tend to spread along existing channels of communication, the people you are most likely to receive viral programs from are roughly the same people you are most likely to have in your trust database. Viruses tend to spread along established communication channels, in the normal course of business.

The lesson so far is that our current virus-containment strategies require ad hoc expert intervention to deal with cases where a network-aware virus actively spreads itself through mail or mobile-program systems. On the other hand, modern systems are making that sort of spread more and more likely. We need to be prepared for a world in which this sort of attack is common, by employing security systems that can deal with them routinely and automatically, reducing them to the level of "little tremors", so that we can free up our experts to deal with whatever Big Quakes are coming down the wire behind them.

Pumping up the Death Rate

If emerging technology is making it possible for viruses to have birth rates far beyond what we are used to, how can we respond? Cold mathematics says that our only possible response is to increase the death rate accordingly. This means detecting infected systems, and preventing them from continuing to spread the virus, very quickly. A monthly update cycle will be far from sufficient. Turnaround times of a week, or even a day, will not be enough to keep a network-aware virus in check.

What will be needed is a system which can detect an anomaly in real time, analyze the threat and produce detection and removal data in hours or minutes, and distribute that data to the original infected machine as well as other machines that may be in the path of the virus. Even automatic protection that protects a single machine is not sufficient; network-aware viruses attack a set of network-connected machines as a whole, as if they were cells in a single body. The effective response to such threats, as we have written elsewhere [4], will be analogous to the response of a biological immune system to attack by disease.

To return to our original analogies, just as buildings in earthquake-prone regions are built with structural stability in mind, and just as our bodies have sophisticated systems for dealing with routine attack by replicating invaders, the network of the future (even the near future) will need to have automatic defenses to handle the rapidly-spreading attacks that are likely to become routine.


  1. Jeffrey O. Kephart and Steve R. White, "Measuring and modeling computer virus prevalence", In Proceedings of the 1993 IEEE Computer Society Symposium on Research in Security and Privacy, May 1993, pages 2-15.
  2. Spafford, E. H., "The Internet Worm Program: An Analysis", ACM Computer Communication Review, 19(1), pp. 17-57, Jan 1989.
  3. For more information on the ShareFun virus, see
  4. For more information on the ShareFun virus, see
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