THE ART OF COMPUTER VIRUS RESEARCH AND DEFENSE [Electronic resources] نسخه متنی

7.7. Virus Construction Kits

Virus writers continuously try to simplify the creation of virus code. Because most viruses were written in Assembly language, writing them remained out of reach for many kids. This inspired virus writers to create generators that can be used by just about anyone who can use a computer.

Virus construction kits evolved over the years as viruses targeted new platforms. Construction kits and virus mutators were built to generate a wide variety of malicious code, including DOS COM and EXE viruses; 16-bit Windows executable viruses; BATCH and Visual Basic Script viruses; Word, PowerPoint, and Excel viruses; mIRC worms; and so on. Recently, even PE virus code can be generated by such kits.

Virus construction kits are a major concern of antivirus producers. It is impossible to predict whether or not virus writers will use a particular kit. Thus the virus researchers need to spend time with even the most primitive kits to see what kinds of viruses they can produce and to design detection and repair capabilities against all possible cases. To add to this problem, many kits generate source code instead of binary. Novice attackers can change the source code further, beyond the capabilities of the kit itself, so it is not always possible to develop perfectly adequate protection.

To make the job of antivirus scanners more difficult, kits deploy armoring techniques such as encryption, antidebugging, antiemulation, and anti-behavior blocking techniques. Furthermore, some kits can mutate the code of the viruses similarly to metamorphic techniques.

7.7.1. VCS (Virus Construction Set)

The first virus generator was VCS, written in 1990. The authors of the kit were members of the Verband Deutscher Virenliebhaber (the Association of German Virus Lovers).

VCS was a rather simple kit. All viruses that the kit can generate are 1,077 bytes and saved to VIRUS.COM on the disk. The user's only options are to specify the name of a text file with a message and to set a generation number to display that message. The VCS viruses can only infect DOS COM files. The payload of the virus is to kill AUTOEXEC.BAT and CONFIG.SYS files and display the user's message.

VCS viruses are simple but encrypted. The only remarkable feature of VCS viruses is that they can check whether the FluShot behavior blocker is loaded in memory and avoid infecting if it is.

7.7.2. GenVir

In 19901991, a kit called GenVir was released as a shareware in France by J. Struss. The original intention was to "test" antivirus product capabilities with a tool that could generate newly replicating viruses. Very few viruses were ever created with GenVir. In 1993, a new version of GenVir was released to support newer versions of DOS.

7.7.3. VCL (Virus Creation Laboratory)

In 1992, the VCL virus construction kit was written in the U.S. (see Figure 7.11). The author of this kit was a member of the NuKE virus-writer team who called himself Nowhere Man.

VCL looked rather advanced at the time because it supported a Borland C++ 3.0based IDE with mouse support. Instead of creating binary files, VCL creates Assembly source code of viruses. These sources need to be complied and linked afterwards by the attacker to make them live viruses. VCL supports a large selection of payloads, as well as encryption and various infection strategies. Not surprisingly, not all viruses that VCL generates are functional. Nevertheless, the viruses look rather different because of the generator options, and thus their detection is far less straightforward than VCS. Several VCL viruses managed to become widespread. For the first time, VCL indicated that even beginner attackers could manage to cause problems in the field.

7.7.4. PS-MPC (Phalcon-Skism Mass-Produced Code Generator)

Because virus-writing groups are in competition, we did not have to wait long before another group announced its kit. PS-MPC was created in 1992 in the United States, by the virus writer, Dark Angel.

PS-MPC does not have the fancy user interface of VCL, which actually makes it a much more dangerous kit. Because PS-MPC is script-driven, it can be much simpler to generate hundreds of copycat viruses. PS-MPC is also a source generator, just like VCL. However, the PS-MPC viruses are much more functional than VCL's. Not surprisingly, virus writers utilized this particular kit to generate more than 15,000 PS-MPC variants and upload them to several antivirus companies' FTP sites.

Although the initial versions of PS-MPC could only create direct-action viruses, other versions were released that could create memory resident viruses as well. Furthermore, later versions support infections of EXE files also.

PS-MPC is one of the reasons I decided to introduce kits in this chapter. The creator of PS-MPC realized that to be successful, generators must create different-looking viruses. To achieve this, PS-MPC uses a generator that effectively works as a code-morphing engine. As a result, the viruses that PS-MPC generates are not polymorphic, but their decryption routines and structures change in variants.

PS-MPC was quickly followed by G2, a second-generation kit. G2 adds antidebug/antiemulation features to PS-MPC, with even more decryptor morphing.

7.7.5. NGVCK (Next Generation Virus Creation Kit)

NGVCK was introduced in 2001 by the virus writer, SnakeByte. NGVCK (see Figure 7.12) is a Win32 application written in Visual Basic. This kit is very similar to VCL in many ways, but it can generate 32-bit viruses that infect PE files. There are well over 30 versions of NGVCK alone.

NGVCK has a rather advanced Assembly source-morphing engine. The viruses that it creates have random function ordering, junk code insertion, and user-defined encryption methods. It also attacks the SoftICE debugger on Windows 95 systems. Later versions of the kit also have support to infect files using different techniques.

The viruses that NGVCK creates are automatically morphed, so every single time somebody uses the kit, a new minor variant of the virus is generated. The code generation uses principles described in Chapter 6, "Basic Self-Protection Strategies." Unfortunately, it is much simpler to develop source morphing than real code-morphing engines, so such techniques are likely to be widely adopted by the attackers in the future.

7.7.6. Other Kits and Mutators

Several other copycat generators have been used by amateur virus writers. Amateurs have many choiceswell over 150 kits and code mutators are available, and many of them have been used to produce working viruses. In 1996 such tools became extremely popular. In particular, we have received new viruses from English schools that were generated by the IVP (Instant Virus Production Kit). IVP was written by Admiral Bailey of the YAM (Youngsters Against McAfee) group in Turbo Pascal. It supports EXE and COM infection, as well as encryption and code mutationand kids loved it because of its Trojan payloads.

One of the most infamous examples of a virus generatorbased attack was known to the general public as the "Anna Kornikova" virus outbreak. This worm was created by the VBSWG kit, and the kit's user, a 20-year-old Dutch man, admittedly did not know how to write a program. Nevertheless, the mass-mailing ability of the VBS worm, combined with social engineering, worked effectively. Many users were eager to see a recent picture of Anna Kornikovabut executed the script instead.

Table 7.1 lists some other common generators.

Evidently, in the future we can expect the tendency of virus construction kits to continue toward networked viruses, worms in particular. There are already a few kits that can be driven via a CGI script over a Web interface. This allows the attacker to avoid releasing the complete code of the virus generator, allowing fewer opportunities for antivirus vendors to test the capabilities of such tools.

7.7.7. How to Test a Virus Construction Tool?

The use of virus construction kits is not purely a technical question; it is also an ethical question. As Alan Solomon has argued, it is ethical for the antivirus researcher to use kits as long as the samples are stored on a dirty PC only and destroyed as soon as the researcher has developed a detection solution for them. Thus the generated samples shall not be stored anywhere, not even for further testing. This method is the only approach widely accepted as ethical among computer virus researchers.