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9.2. The Generic Structure of Computer Worms

Each computer worm has a few essential components, such as the target locator and the infection propagator modules, and a couple of other nonessential modules, such as the remote control, update interface, life-cycle manager, and payload routines.

9.2.1. Target Locator

To spread rapidly on the network, the worm needs to be able to find new targets. Most worms search your system to discover e-mail addresses and simply send copies of themselves to such addresses. This is convenient for attackers because corporations typically need to allow e-mail messages across the corporate firewalls, thereby allowing an easy penetration point for the worm.

Many worms deploy techniques to scan the network for nodes on the IP level and even "fingerprint" the remote system to check whether such a system might be vulnerable.

9.2.2. Infection Propagator

A very important component of the worm is the strategy the worm uses to transfer itself to a new node and get control on the remote system. Most worms assume that you have a certain kind of system, such as a Windows machine, and send you a worm compatible with such systems. For example, the author of the worm can use any script language, document format, and binary or in-memory injected code (or a combination of these) to attack your system. Typically, the attacker tricks the recipient into executing the worm based on social engineering techniques. However, more and more worms deploy several exploit modules to execute the worm automatically on the vulnerable remote system without the user's help. Exploitation of vulnerabilities is the subject of Chapter 10, "Exploits, Vulnerabilities, and Buffer Overflow Attacks."


Some mini-worms such as W32/Witty and W32/Slammer appear to combine the target locator (network scan) and infection propagator in a single function call. However, they still support distinct features: the generation of random IP addresses and the propagation of the worm body to new targets.

9.2.3. Remote Control and Update Interface

Another important component of a worm is remote control using a communication module. Without such a module, the worm's author cannot control the worm network by sending control messages to the worm copies. Such remote control can allow the attacker to use the worm as a DDoS (distributed denial of service) tool7 on the zombie network against several unknown targets.

An update or plug-in interface is an important feature of advanced worms to update the worm's code on an already-compromised system. A common problem for the attacker is that after a system is compromised with a particular exploit, it often cannot be exploited again with the same one. Such a problem helps the attacker to avoid multiple infections of the same node, which could result in a crash. However, the intruder can find many other ways to avoid multiple infections.

The attacker is interested in changing the behavior of the worm and even sending new infection strategies to as many compromised nodes as possible. The quick introduction of new infection vectors is especially dangerous. For example, the intruder can use a single exploit during the first 24 hours of the outbreak and then introduce a set of others via the worm's update interface.

9.2.4. Life-Cycle Manager

Some worm writers prefer to run a version of a computer worm for a preset period of time. For instance, the W32/Welchia.A worm "committed suicide" in early 2004, and then the B variant of Welchia was released in late February of 2004 to run for three more months. On the other hand, many worms have bugs in their life-cycle manager component and continue to run without ever stopping. Furthermore, we often encounter variants of computer worms that were patched by others to give the worm "endless" life.

Consider the statistics collected on an individual Welchia honeypot administered by Frederic Perriot between August 2003 and February 2004, shown in Figure 9.1. The sudden drop of Welchia is related to its life-cycle manager, which triggers the worm's self-killing routine.

Figure 9.1. The suicide of Welchia worm.

The cumulative number of distinct Welchia attacking systems was around 30,000 when the worm started to kill itself when observed on a particular DSL network (see Figure 9.2).

Figure 9.2. The cumulative number of Welchia attackers.

9.2.5. Payload

Another optional but common component of a computer worm is the payload (activation routine). In many cases, computer worms do not contain any payload. An increasingly popular payload is a DoS attack against a particular Web site. However, a common side effect of computer worms is accidental DoS attacks as a result of overloaded networks, especially overloaded network routers8. However, other interesting side effects have also been observed, such as accidental attacks on network printers.

Computer worms also can utilize the compromised systems as a "super computer." For example, W32/Opaserv9 attempts to break a DES-like10 secret key11 by sharing the attack among the infected nodes, similarly to the SETI network. (In fact, some computer worms, such as W32/Hyd, download and install SETI to compromised systems. The W32/Bymer worm is an example of a DNETC [Distributed Network Client] installation to compromised systems.) Such attacks were first predicted in 198912.

Another interesting tendency is the planned interaction between two computer worms as a payload. Several antiworms have been released with the intention of killing other computer worms and installing patches against the vulnerabilities they exploited. Examples include Linux/Lion versus Linux/Cheese and W32/CodeRed versus W32/CodeGreen. In this chapter, I will also discuss other kinds of interactions between malicious programs.

Recently it is becoming popular to install an SMTP (Simple Mail Transfer Protocol) spam relay server as the payload of a worm. Spammers compromise systems on a large scale using worms such as W32/Bobax and then using the SMTP relay server created by the worm to spam messages from the "zombie" systems.

9.2.6. Self-Tracking

Many computer virus authors are interested in seeing how many machines the virus can infect. Alternatively, they want to allow others to track the path of the virus infections. Several viruses, such as W97M/Groov.A13, upload the IP information of the infected system to an FTP site.

Computer worms typically send the attacker an e-mail message with information about the infected computer to track their spread. The Morris worm deployed a self-tracking module that attempted to send a UDP datagram to the host at after approximately every 15 infections, but this routine was bogus, and it never sent any information14. A few other examples of self-tracking are mentioned later on in this chapter.

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