Common Attack Strategies
Attackers targeting a network can leverage combinations of the various security risks and threats to enact their nefarious plans for vulnerable computers. The least sophisticated attacks might corrupt or delete data, potentially requiring a complete reformatting and reload for infected computers. More sophisticated attacks can produce even less desirable results, including placing illegal content on targeted computers, exposing protected data, or even utilizing the compromised computers to levy attacks against secondary targets.
Many hackers make use of the practice of social engineering, which is a psychological scam intended to get users to reveal information or to provide details useful for a successful network attack. Email-borne viruses employ this technique by presenting an innocuously named viral attachment with a From email address matching that of a known associate. Spyware and other security risks can provide attackers with information that can be used to improve social-engineering efforts, such as by allowing a phishing attack to mimic a site the user is known to frequent.
Bots and Botnets
Botnet is a term used by the FBI to describe a group of compromised hosts controlled by a remote attacker, as illustrated in Figure 2-2. Communicating with their creator through Internet Relay Chat (IRC) or other anonymous methods of communication, compromised computers can reside quietly for a lengthy time until given a command to attack a chosen target. These networks can also be used to crack encryption keys and other CPU-demanding tasks, distributing a huge task among tens of thousands of personal computers located around the globe.
Figure 2-2 An idealized example of an IRC-controlled botnet.
Sometimes referred to as zombies compromised computers (bots) are often traded as coin of the realm among hackers seeking access within a particular network. Large botnets are status symbols among some groups, where their originators might fight silent wars against one another using corporate, educational, personal, and even governmental hosts as their playing pieces. Compromised bots in secure networks, such as .gov and .mil sites, can often be traded for thousands of compromised .edu and .com hosts, all traded by the controlling hacker to purchase status, bragging rights, or access to target networks. These transactions occur while the true owners remain unaware that the compromised computers are being bought, sold, and used as a weapon against other networks.
Beyond their value as currency among the various hacker communities, botnets are commonly used in various malicious ways:
Distributed–Denial-of-Service attacks—The most common use of botnets is the massed coordinated attack against a target site or address to saturate the target’s bandwidth or capability to respond to legitimate connections. These attacks have been levied against high-profile sites through the use of thousands of compromised bots scattered around the world. The distributed nature of these attacks makes it more difficult for the target to filter out only the undesirable traffic.
Remote control—Bots provide their controller some measure of control over the compromised computer, allowing the introduction of malicious programs, back doors, spyware, or any of the other security risks previously discussed.
File sharing—Botnets are sometimes used to host contraband files, cracked software titles, audio files, and even entire DVDs that have been ripped and stored on compromised computers with high-bandwidth broadband connections. By replacing valid services on compromised hosts, these bot programs can be configured to serve as HTTP or FTP servers that might appear valid to a cursory audit of the network.
Compromised computers in highly secure or limited-access areas are highly valued by controllers of these botnets, along with computers with high levels of connectivity and large storage capacity. Because of this, commercial targets are commonly identified for attack to compromise servers and other well-connected computers. Educational sites are also commonly targeted because they are generally comprised of large numbers of relatively new computers installed in default configurations, connected to wide-bandwidth Internet backbones, and supported by limited numbers of staff that take publicly posted holidays.
Extending the qualities of a Trojan horse or a back door, root kits replace or modify elements of the operating system to provide an attacker greater control over compromised hosts. These programs can replace or modify the system kernel, system binaries, or other elements of the host’s operating system, often allowing an attacker’s later efforts to pass unnoticed, provided with stealth and cover by the modified system binaries.
Implementations of root kits can replace common user interface functions, allowing an attacker to conceal their implanted services from the Task Manager or to hide files from the explorer interface when a user attempts to check for unexpected files that might reveal the compromise. Root kits can be used to implant a known master password or other mechanism for bypassing the normal protections of the host computer.
Root kits provide the greatest level of control over a compromised host because they target directly the basis for all other applications running on a computer. Attackers who can successfully deploy a root kit can be considered to "own" the compromised computer at a functional level to such an extent that only a full reformat-and-reload can be certain to remove the damage done. Protection strategies are vital to protect against this level of compromise, where backup and recovery strategies might provide the only path back to a functional network environment.