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9.8. Intentional and Accidental Interactions

Computer virus researchers have observed a set of interesting behaviors that are the result of intentional and accidental interactions between various kinds of malicious code. This section describes common interactions.

9.8.1. Cooperation

Some computer viruses accidentally cooperate with other malicious code. For example, computer worms might get infected with a standard file infector virus as they pass through already infected nodes. It is common to encounter multiple infections on top of in-the-wild computer worms. It is not uncommon to find three or even more different viruses on the top of a worm carrier. This can help both the network worm and the standard file infector virus in various ways.

A worm can take advantage of the infection of an unknown file infector virus. If the file infector virus is unknown to antivirus products, the computer worm body might not be detectable. For example, in some cases the worm body will be embedded deep inside the virus code, leaving little chance for the antivirus program to find it. See Figure 9.19 for an illustration.

Figure 9.19. The accidental interaction of a worm infected with a file infector virus.

In Step A, the computer becomes infected with a worm. In Step B, the worm successfully penetrates a new remote system. That computer, however, is already infected with a virus that infects exactly the same type of files in which the worm propagates itself. Thus the file infector virus attaches itself to the worm. In Step C, the multiple infection arrives at a new computer. When the worm is executed, the file infector virus runs (in most cases, it will run before the worm's code) and infect other objects.

In Step D, the combination arrives at a system protected by antivirus software that knows the file infector virus on top of the computer worm carrierbut does not know the computer worm underneath. If the antivirus can disinfect the file infector virus, it might create a file object that is not exactly the same as the original worm. For example, the binary file of the worm might get larger or smaller, and important fields in its header might also change. (Of course, an antivirus is just one possible agent that might interact with other malicious programs.)

Thus a "mutant" worm will have the chance to propagate itself further and infect a new system in Step E. In practice, no antivirus software would consider the Step E case a variant of the original worm. However, antivirus programs need to address this issue. For example, the MD5 checksums of the "mutant" worm body are clearly different, and if the antivirus or content-filtering software uses such checksums to detect the original worm, it will fail to detect the "mutant" worm.

An intentional variation of such cooperation appeared in the "symbiosis project" of GriYo. He released the mass-mailing worm, W32/Cholera, in infected form. The worm was infected with the polymorphic W32/CTX virus and became a carrier. The result was a quick succession of both W32/Cholera and W32/CTX infections worldwide. As a result, W32/CTX was reported on the Wildlist.

File infector viruses such as W32/Funlove often infect other worms and often multiple times. Such file infector viruses occasionally disappear from the top 50 charts but suddenly attack again during major computer worm outbreaks. This kind of activity was seen increasingly when the W32/Beagle worm appeared in the wild. As discussed earlier, some variants of Beagle send a password-protected attachment to recipients. Because the worm creates the archive (ZIP) file on the local system, the executable file of the worm can easily get infected by viruses such as Funlove before the executable gets packed. Thus a virus like Funlove also enjoys being password-protected36. Because several antivirus softwares had problems detecting the password-protected attachments reliably, "traveler viruses" could take advantage of this accidental cooperation.

A form of cooperation also exists with the previously mentioned W32/Borm creation, which infects BackOrifice-compromised systems. W32/Borm does not attempt to kill Back Orifice; it simply takes advantage of compromised systems to propagate. Similarly, the aforementioned Mydoom had a backdoor that was utilized by the Doomjuice worm to spread itself.

With macro and script viruses, "body snatching" attacks often occur. Two or more script or macro viruses might form a new creation as they accidentally propagate each other's code.

9.8.2. Competition

Competition between malicious codes was also experienced among computer viruses. Several viruses attack other viruses and disinfect them from the systems that they have compromised. An example of this is the Den_Zuko boot virus37, which disinfects the Brain virus. These viruses are often called "benefical viruses" or "antivirus" viruses.

Antiworm computer worms started to become more popular in 2001 with the appearance of the CodeRed worm and the counterattacking, CodeGreen. (However, antiworm worms had been experienced previously on other platforms, such as Linux.)

Because IIS could be exploited more than once, CodeGreen could easily attack CodeRed-infected systems. The worm sent a similarly malformed GET request to the remote target nodes to CodeRed, which had in front the message shown in Listing 9.9.

Listing 9.9. The Front of a CodeGreen GET Request
GET /default.ida?Code_Green_<I_like_the_colour-_-><AntiCodeRed-

The worm also carried the following messages shown in Listing 9.10.

Listing 9.10. Other Messages of the CodeGreen Worm
HexXer's CodeGreen V1.0 beta CodeGreen has entered your system
it tried to patch your system and
to remove CodeRedII's backdoors

You may uninstall the patch via
SystemPanel/Sofware: Windows 2000 Hotfix [Q300972]

get details at "".
visit "

CodeGreen removed the CodeRed infections from systems and also removed the backdoor components of other CodeRed variants. Furthermore, it downloaded and installed patches to close the vulnerability.

Similar attacks against W32/Blaster worm were experienced when the W32/Welchia worm began its antiworm hunt against Blaster, which started the "worm wars" (as I decided to call it after Core Wars).

Another enthralling example is the W32/Sasser worm. Sasser targeted an LSASS vulnerability that was previously exploited by variants of Gaobot worm. Thus Gaobot's author was not impressed because Gaobot needed to compete with Sasser for the same targets. Consequently, the W32/Gaobot.AJS38 worm was developed with a vampire attack. I decided to call this kind of attack a "vampire" based on the Core War vampire attack. Vampire warriors can steal their enemies' souls (see Chapter 1, "Introduction to the Games of Nature," for details).

Gaobot.AJS is a vampire because it attacks Sasser when the two worms are on the same machine. Instead of simply killing Sasser, Gaobot.AJS modifies Sasser's code in a very tricky way. As a result of the modification, Sasser can still scan for new targets and even exploit them successfully. However, when Sasser connects to its shellcode on the compromised system to instruct it to download and execute a copy of Sasser via FTP, the code modifications of Gaobot.AJS will get control. In turn, Gaobot.AJS sends commands to Sasser's shellcode on the remote machine and instructs it to download a copy of Gaobot.AJS's code instead of Sasser's. Furthermore, Gaobot closes the connection to the remote machine so Sasser cannot propagate but is used as a Gaobot propagation agent in a parasitic manner.

Another gripping example is the W32/Dabber worm, which appeared right after Sasser. As mentioned, Sasser's shellcode is instructed to download a copy of Sasser via FTP. On the attacker system, Sasser implements a crude FTP server. However, this routine of Sasser had a simple buffer overflow vulnerability that could be exploited. (Indeed, worms can have their own vulnerabilities!) Dabber was released to exploit Sasser's vulnerability to propagate itself. It scans for targets that were compromised by Sasser and attempts to connect to Sasser's vulnerable "FTP server" to exploit it successfully.

It is expected that competitions between malicious programs will become more and more common in the future.

9.8.3. The Future: A Simple Worm Communication Protocol?

Although increased competition among malicious programs is likely, it also makes sense for attackers to invest in cooperating techniques. For example, computer worms could use a special protocol such as simple worm communication protocol (SWCP) to exchange information, as well as plug-ins ("genes") among different families of computer worms that support SWCP. Computer worms could swap payloads, exchange information about systems to attack, or even collect e-mail addresses and share them with the other worms that occasionally communicate using SWCP. I highly anticipate that such techniques will appear in the very near future.

Of course, communication can have other forms. For instance, viruses could "reproduce sexually"39 to cross their genomes to produce offspring, which can evolve or devolve. The closest currently known example of accidentally "sexually reproducing" computer viruses can be found in macro viruses which occasionally swap, or snatch their macros ("genes") as discussed in Chapter 3, "Malicious Code Environments,". However, specifically written binary viruses could possibly demonstrate similar behavior that would lead to further evolution in computer viruses on their own.

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