Stalking the Stealth Viruses
By Christopher O'Malley

Like computer users around the world, Greg Buckley heard the warnings about the dreaded Michelangelo "virus," a malicious software program designed to wipe out the contents of an infected PC's hard disk on March 6, the artist's birthday. But unlike most, Buckley, a plumbing contractor in Boynton Beach, Fla., was a victim. He turned on his PC that fateful Friday to find all of his accounting files gone. But if 1992 was the year Michelangelo pushed computer viruses into the spotlight, 1993 may be the year viruses go under cover. A new generation of highly sophisticated "stealth" viruses has begun circulating among PCs, spreading with little or no evidence of their presence. More ominous still, some of these viruses are mutating strains that alter their makeup as they spread, posing a more insidious that than fixed viruses such as Michelangelo, Stoned, Jerusalem. By definition, computer viruses are clandestine creatures. These programs conceal their instructions inside other software programs, secretly attaching themselves to other files and floppy disks or lodging in the special start-up area of a disk known as the boot sector. Once a program or disk has become "infected" with one of these invisible stowaways, it executes the virus's instructions -- some destructive, some merely annoying--without prompting or warning. The virus spreads when an infected program is copied to another computer or an infected disk is read during start-up.

Traditionally, a virus leaves telltale signs of its presence. Many viruses noticeably increase the size of files or reduce the amount of available memory, symptoms that can provide early warnings of an infection. Other viruses make changes to critical start-up areas of a disk that can e inspected for unwanted alterations, such as the "boot record" and "partition table" on IBM PC-compatible systems. And most viruses bear a kind of digital signature--a unique string of software codes that can be easily detected by so-called anti- virus scanning programs, which seek out and remove viruses.

Unlike conventional viruses, however, the newer stealth strains employ a variety of cloaking techniques to make themselves invisible to both the human eye and the electronic scrutiny of anti-virus programs. The longer these viruses remain undetected, the further they can spread and the more damage they can do.

While their camouflaging methods vary, stealth viruses- -almost all of which have been detected since 1990--employ two basic techniques to avoid detection: getting "under" the operating system and subverting a computer's disk operations to conceal the presence of a virus, and digitally encrypting the virus itself to thwart scanners looking for a familiar signature or pattern.

Disk deceptions are the most common stealth ploys. The 4096 virus, also known as Frodo or Hundred Years virus, is one prominent example. This virus, which originated in Israel, infects program files (typically files ending with the extension .EXE or .COM) on IBM-compatible PCs. In the process, the virus adds 4,096 bytes to the length of each file. But that increase never shows up on a computer screen; the virus stores the original file-size data at the end of the infected file and summons it whenever the DIR (directory) command requests a list of file, so all the files appear to be in the original lengths. The 4096 virus also has a number of built-in defenses against disk mapping and debugging programs, making those tools virtually useless in detecting it.

These defenses buy the 4096 virus time to slowly and surreptitiously weave a web of improper links among program and data files, damaging both. The virus also has a trigger date: On or after Sept. 22 of any year (the birthday of Frodo, a character in the Lord_of_the_Rings books), the 4096 virus will cause system crashes. A number of other new file- type viruses, including a Bulgarian strain called Dir-2, a German virus called Whale, and a virus of unknown origin known as Crazy Imp, play similar tricks on the DOS file system.

Many viruses that infect the boot, or start-up, areas of a DOS disk have also taken on stealthy sophistication. These boot-type viruses infect the hidden programs a PC reads when it's turned on or restarted, loading themselves into memory before anything else can take place. Ordinarily, a disk-editing or anti-virus program can simply inspect these special disk areas and remove a virus caught nesting there. But some of the latest boot viruses can fool these programs into thinking all is well when the start-up areas are in fact corrupted.

The Joshi virus, probably the most widespread of all stealth viruses to date, infects the boot sector of floppy disks and the partition table of hard disks. But when a program attempts to read these areas, Joshi intercepts the probe and directs it to a copy of the original boot sector or partition table stored on another part of the disk. Joshi, developed in India, is not a particularly malicious virus-- every Jan. 5 it displays the message TYPE HAPPY BIRTHDAY JOSHI on the screen and freezes the computer until the user obliges. But some stealth boot viruses, such as NoInt from Canada (a stealth variation of the Stoned virus), can cause file damage or loss as they maneuver around the operating system.

The most formidable virus threat today comes from new "super stealth" strains that combine these hide-and-seek games with complex encryption schemes that conceal, and even dynamically alter, the viral code itself. The technology of digital encryption is not new--businesses often use encryption to protect sensitive data--but it takes on a darker dimension when applied to viruses. Essentially, the encryption process uses a mathematical algorithm and a random-number generator to disguise bytes of binary code (ones and zeros) as a new series of numbers. Most computers have timer or counter chips that can easily generate random numbers, so a virus program simply needs an encryption algorithm and the instructions to use it. Once encrypted, a virus is extremely difficult to detect because its unique signature has been wiped away.

The recently discovered Tequila virus, traced to Switzerland, is one of the more common super-stealth strains. Tequila is both a file- and boot-type infector that hides any increases in file sizes and uses a sophisticated encryption method to avoid being detected or disassembled. The Telecom and Holocaust viruses, both from Spain, are similarly encrypted. Tequila and Holocaust can cause errors and possibly damage files; Telecom, which also goes by the name Spanish Telecom-2, will eventually overwrite hard-disk files.

The latest virus-encrypting technology is the Dark Avenger Mutating Engine, a byproduct of the notorious Dark Avenger author (or authors) from Bulgaria (see the Bulgarian Connection). The Mutating Engine is not a virus itself but an elaborate encryption program that can be attached to almost any virus to create a polymorphic virus--a virus that can take many forms. Polymorphic viruses are not only encrypted, they change their encryption frequently so as to never look the same twice. The Mutating Engine is an incredible efficient encoder, allowing no more than three bytes (out of hundreds of thousands) to remain constant from one encryption to the next. It also tosses in meaningless instructions to further ensure a unique result.

Though the Dark Avenger Mutating Engine was the first documented earlier this year, several polymorphic viruses have already been developed with it. None of these Bulgarian viruses are terribly destructive: Pogue plays music between 8 a.m. and 9 a.m., Fear can cause a computer to freeze up which an infected program is run, and Dedicated simply keeps cloning itself. But the potential for more lethal polymorphic viruses is intimidating, and the trend toward nastier viruses is growing.

The Michelangelo virus is a testament to this negative trend. The Stoned virus from which it evolved is believed to be the most prevalent virus in the United States and is certainly on of the most innocuous, flashing YOUR PC IS NOW STONED on the screen when you boot an infected disk. But if Stoned is typical of the prankish mentality behind many viruses of the 1980s, Michelangelo and its wanton destruction may be the prototype for the 1990s.

The Casino virus, discovered about the same time last year as Michelangelo, may best embody this growing mean- spiritedness among virus writers. Triggered on the 15th of January, April, or August, Casino informs the victim that it has destroyed the file allocation table, which is used by DOS to track the location of files on a disk. But the virus has saved a copy of the file in memory, and it offers "a last chance to restore your precious data" by playing a slot machine game. If you lose, it wipes out the file allocation table, making it impossible to access any if the files on the disk. A variant called Casino-B destroys the file table regardless of your luck.

"We're starting to see the angry young men come out now," says Robert Bales, executive director of the National Computer Security Association in Mechanicsburg, Pa. NO one knows for sure who writes computer viruses or why, but the sophistication of today's virus suggest the authors are talented young programmers with time on their hands and a taste for mischief. Viruses have originated in almost every country, including the United States, but many of the most common and dangerous ones come from Eastern Europe, particularly Bulgaria. In many of these countries are scores of well-trained computer programmers who have few opportunities to practice their art commercially. In addition, software piracy is rampant in many cases, allowing viruses to spread rapidly. Anti-virus forces are fighting back with innovations of their own. Many anti-virus packages, including Symantec's Norton Anti-Virus and Central Point's Anti-Virus, not only scan for known viruses, but also employ "sentry" programs that stay in memory and watch for suspicious activity, such as changes to files and boot areas, to try to catch unknown and stealth viruses at work. The programs usually intercept the virus and repair infected files.

A few anti-virus programs, including Certus International's Novi and Fifth Generation Systems' Untouchable, claim to be so good at monitoring a system for viral activity that no upgrades are necessary, as they often are for simple scanners. It's the sheer quantity of new viruses being developed all the time that necessitates these upgrades: The number of unique viruses soared from fewer than a dozen in 1986 to more than 1,000 by early 1992, and that number is expected to double by year's end, according to Patricia Hoffman, an independent virus researcher who maintains a database of all known computer viruses.

Scanning for virus signatures is still the backbone of nearly all anti-virus products; adapting the scanning techniques to snare encrypted and polymorphic viruses is not the impossible task it might seem. All encrypted viruses, even polymorphic ones, must leave a tiny portion of themselves--the "kernel" program that decodes the scrambled virus--unencrypted so the virus can still operate. At least one scanning program, a popular shareware program called Scan from McAfee Associates, already can detect the presence of viruses cloaked by the Dark Avenger Mutating Engine.

A number of creative hardware devices are helping stem the viral tide as well. ThunderByte PC Immunizer, from Glynn International of Brookline, Mass., is a $249 circuit board that plugs into an IBM-compatible PC and attaches via a cable to the system's hard-disk controller board. This connection physically prevents viruses from modifying program files or start-up areas of a hard disk. Virus Trap, a $295 board from JAS Technology of Warrenton, Va., offers similar protections. To reduce the risk of viruses entering a PC in the first place, Corporate Management Group of Austin, Texas, sells a series of floppy-disk-drive locks that the company characterizes as "chastity belts" for computers.

In their current form, however, anti-virus software and hardware products are flawed solutions, for no other reason than the fact that PC users must make the effort to buy and use them. Incororating anti-virus technology into the operating system may be more effective. That would make the safeguards automatic and eventually extend them to almost everyone who uses a PC. Microsoft, which supplies the DOS operating system used by nearly all IBM-compatible PCs, says it is working on adding some file-system and boot-sector protections to a future version of DOS and to the next major release of its Windows graphical user interface.

But the operating system approach has its disadvantages. To accommodate anti-virus features, the operating system will probably use more system memory, and keeping anti-virus features up to date might mean more frequent upgrades of the operating system. Making virus protection the domain of an operating system like MS-DOS could also have the effect of painting a bull's-eye for virus writers. "Frankly, we're a little concerned it would look like we're throwing down the gauntlet," says Mack McCauley, manager of DOS and Windows development at Microsoft.

Nonetheless, beta versions of DOS 6.0, expected to be completed by early 1993, reportedly include Central Point's anti-virus software. Many PC vendors now include anti-virus software with their systems as well.

A more comprehensive and permanent answer to the virus dilemma may lie in redesigning PC hardware to close some of the openings that make a PC so vulnerable to rogue programs in the first place. Today's PCs allow any type of software-- including viruses--to run unchecked, with unfettered access to critical disk and memory information. This hands-off approach is one element of the "open architecture" that has helped increase competition and cut the price of PCs. But it's also an open invitation to virus writers.

At George Washington University in Washington, D.C., Lance Hoffman, a professor of electrical engineering and computer science, and graduate student Paul Clark have proposed a new start-up procedure for PCs that would use solid-state memory cards instead of disks. This plan would eliminate the risk of boot-type viruses and make it easier to cleanse a computer that's been infected with file-type viruses.

Others have suggested that hardware makers design systems that would physically prevent viruses from writing to boot sectors or program files. Virus expert Richard Levin, author of The_Computer_Virus_Handbook and an anti-virus program called Checkup, is promoting a built-in mechanism that would allow you to selectively protect boot sectors and program files by turning a keyed switch located in the front of the computer. (The key lock currently found on the faceplate of some PCs is simply a keyboard lock.) His system would still allow free access to data files. "PC users are entitled to security, and it should come with the computer," says Levin. "I think it's despicable that we're preying on people's misfortunes when we could stop this problem tomorrow if we really wanted to."

American Megatrends, a leading maker of PC motherboards, has taken a step in that direction. AMI recently modified its basic input-output system (BIOS)--software on a programmable memory chip that directs traffic between the operating system and the hardware--to prevent boot-sector viruses like Michelangelo from writing to the boot sector of the disk. Award Software is doing the same with its BIOS software, and Phoenix Technologies plans to add the boot- protect feature in a future version.

But PC security can be a hard sell. Hardware makers say there's still little demand for security features and that PC users may balk at a hardware solution that forces them to change their ways. Dell Computer Corp. in Austin, Texas, for example, is considering adding virus checkers to the read- only memories of some of its forthcoming systems, but the company doesn't foresee any fundamental hardware changes in the near future. "The reality is that many users are willing to take the risk of getting a virus rather than have something that interferes with their day-to-day use of the computer," says Michael O'Dell, vice president of product development at Dell.

Potential danger

The risk of encountering a virus is still remote. Only about one PC in 1,000 contracts a virus every three months, according to an ongoing study being conducted by the High Integrity Computing Lab at IBM's Thomas J. Watson research center in Hawthorne, N.Y. In the grander scheme of security issues, viruses still take a backseat to human errors and equipment failures.

The potential for damage posed by computer viruses, however, is unparralleled. The proliferation of computer networks and dial-up information services makes sharing software--and spreading viruses--much easier. The new emphasis on stealth technology makes it more likely than ever that a virus could quietly spread across a large number of computers and wreak havoc on the machines that manage many aspects of daily life, from a corporation's quarterly budgets to a hospital's records.

There's a less apparent danger as well. "Viruses threaten to rattle the underlying confidence people now have in computers," says Peter Tippett, president of Certus International, a Cleveland-based publisher of anti-virus software. "And if people stop relying on computers, that's everybody's problem."