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8.6. DoS (Denial of Service) Attacks

In the past, researchers commonly believed that targeted attacks against selected computers or organizations were not possible using computer viruses. But modern operating systems in the networked world give attackers with political motivations the potential to carry out successful attacks against targeted businesses, such as financial institutions.

There have been several successful DoS attacks in the past, some caused by computer worms. Most of these attacks were not targeted against a particular organization. However, computer worms flood the network with data to such an extent that the side effects of propagation develop into a DoS attack. The W32/Slammer worm was an example of such an accidental DoS attack. The worm was small, and it could propagate itself aggressively on the network. During the outbreak, Internet devices such as routers were heavily overloaded. The result was very unhealthy Internet communications, with as high as 90% packet loss between particular locations. It was difficult to use e-mail during the attack because the network was so slow all around the world. In addition, the speed with which W32/Slammer put packets on the network resulted in ATM failures, canceled airline flights, and election interference8.

On August 14, 2003, several analysts speculated that the W32/Blaster worm was responsible for the major blackout in the United States and Canada. Blaster worm was on the loose for three days by that time. Well, official reports quickly denied these claims.

Indeed, it is believed that Blaster worm was not the primary cause of the blackout. However, Blaster was a potential contributor by slowing down communication systems between electricity control operation centers. Thus the network operators did not have data in time to control the electric systems to avoid further power surges. Clearly, reports indicated that the electricity control center had "computer problems," which pretty much sound like a worm infection9. Eventually, the east coast experienced power loss, including the area of New York City.

In the end, Blaster worm infections happened so rapidly that vulnerable systems could not be connected to the compromised networks (unless they were protected by a personal firewall) to download the security patches because the worm hit the compromised machine almost immediately. Blaster attempted to attack the Windows Update Web site; however, the attacker failed to pick the right target, so the attack was not successful against the real site. Evidently, a successful attack against the Windows Update site would make it even harder to patch vulnerable systems because the updates would be more difficult to download. Someone could argue, however, that it was hard enough to download the update because vulnerable systems could easily get infected before the patches could be downloaded and installed on them.

On July 16, 2001, the Chinese W32/CodeRed worm attempted to execute a targeted DoS attack against (with the IP address by connecting continuously to the site. In response to the attack, the IP address was quickly changed. However, the worm carried another payload that targeted systems using U.S. English codepage (0x409).

In such a case, the worm installed a hook routine on the TcpSockSend() function of the INFOCOMM.DLL module of Microsoft IIS. The worm's hook routine did not let an infected system access any HTML content. Instead, the worm displayed the page shown in Figure 8.7 for all Web access.

Figure 8.7. The activation routine of the CodeRed worm.

Probably the most infamous worm on Linux operating systems was Linux/Slapper. (Detailed technical information about the previously discussed worms, as well as many others, is available in Chapters 9 and 10, "Exploits, Vulnerabilities, and Buffer Overflow Attacks".) For the sake of completeness, though, it is interesting to mention here that Slapper was designed to build a peer-to-peer network of compromised systems to execute DDoS (distributed denial of service) attacks. This allowed the attacker to connect to one infected node and control all infected "zombie" systems connected to that node from a single location by sending commands to all at the same time. Each copy of the worm carried a command interface that the attacker could use to execute various types of DoS attacks, including several flooding techniques. Although a few unconnected attack networks were found, the largest network consisted of nearly 20,000 zombie systems waiting for the attacker's commands.

Many other types of DoS attacks were developed in worms. For example, 911 attacks against the phone system are a common payload of computer viruses. (911 is the emergency services phone number in the U.S.) The Neat worm (as discussed in Chapter 3, "Malicious Code Environments"), on Microsoft WebTV systems, was an example of such an attack. The worm simply reconfigured the WebTV system to call 911 instead of the default ISP phone number.

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