Attacking Computer Systems and Networks

Although hackers and viruses receive the most attention in the news (due to the volume of these forms of attack), they are not the only methods used to attack computer systems and networks. This chapter addresses many different ways computers and networks are attacked on a daily basis. Each type of attack threatens at least one of the three security requirements: confidentiality, integrity, and availability (the CIA of security). Attacks are thus attempts by unauthorized individuals to access or modify information, to deceive the system so that an unauthorized individual can take over an authorized session, or to disrupt service to authorized users.

From a high-level standpoint, attacks on computer systems and networks can be grouped into two broad categories: attacks on specific software (such as an application or the operating system) and attacks on a specific protocol or service. Attacks on a specific application or operating system are generally possible because of an oversight in the code (and possibly in the testing of that code) or because of a flaw, or bug, in the code (again indicating a lack of thorough testing). Attacks on specific protocols or services are attempts either to take advantage of a specific feature of the protocol or service or use the protocol or service in a manner for which it was not intended. This section discusses various forms of attacks of which security professionals need to be aware.

Denial-of-Service Attacks

Denial-of-service (DoS) attacks can exploit a known vulnerability in a specific application or operating system, or they can attack features (or weaknesses) in specific protocols or services. In a DoS attack, the attacker attempts to deny authorized users access either to specific information or to the computer system or network itself. This can be accomplished by crashing the system-taking it offline-or by sending so many requests that the machine is overwhelmed.

The purpose of a DoS attack can be simply to prevent access to the target system, or the attack can be used in conjunction with other actions to gain unauthorized access to a computer or network. For example, a SYN flooding attack can be used to prevent service to a system temporarily in order to take advantage of a trusted relationship that exists

between that system and another.

SYN flooding is an example of a DoS attack that takes advantage of the way TCP/IP networks were designed to function, and it can be used to illustrate the basic principles of any DoS attack. SYN flooding uses the TCP three-way handshake that establishes a connection between two systems. Under normal circumstances, the first system sends a SYN packet to the system with which it wants to communicate. The second system responds with a SYN/ACK if it is able to accept the request. When the initial system receives the SYN/ACK from the second system, it responds with an ACK packet, and communication can then proceed. This process is shown in Figure 13-1.

In a SYN flooding attack, the attacker sends fake communication requests to the targeted system. Each of these requests will be answered by the target system, which then waits

for the third part of the handshake. Since the requests are fake (a nonexistent IP address is used in the requests, so the target system is responding to a system that doesn't exist), the

target will wait for responses that never come, as shown in Figure 13-2. The target system will drop these connections after a specific time-out period, but if the attacker sends requests faster than the time-out period eliminates them, the system will quickly be filled with requests. The number of connections a system can support is finite, so when more requests come in than can be processed, the system will soon be reserving all its connections for fake requests. At this point, any further requests are simply dropped (ignored), and legitimate users who want to connect to the target system will not be able to do so, because use of the system has been denied to them.

DoS attacks are conducted using a single attacking system. A DoS attack employing multiple attacking systems is known as a distributed denial-of-service (DDoS) attack. The goal of a DDoS attack is also to deny the use of or access to a specific service or system. DDoS attacks were made famous in 2000 with the highly publicized attacks on eBay, CNN, Amazon, and Yahoo!.

In a DDoS attack, service is denied by overwhelming the target with traffic from many different systems. A network of attack agents (sometimes called zombies) is created by the attacker, and upon receiving the attack command from the attacker, the attack agents commence sending a specific type of traffic against the target. If the attack network is large enough, even ordinary web traffic can quickly overwhelm the largest of sites, such as those targeted in 2000.

Creating a DDoS network is no simple task. The attack agents are not willing agents- they are systems that have been compromised and on which the DDoS attack software

has been installed. To compromise these agents, the attacker has to have gained unauthorized access to the system or tricked authorized users to run a program that installed the attack software. The creation of the attack network may in fact be a multistep process in which the attacker first compromises a few systems that are then used as handlers or masters, which in turn compromise other systems. Once the network has been created, the agents wait for an attack message that will include data on the specific target before launching the attack. One important aspect of a DDoS attack is that with just a few messages to the agents, the attacker can have a flood of messages sent against the targeted system. Figure 13-3 illustrates a DDoS network with agents and handlers.

Backdoors and Trapdoors

Backdoors were originally (and sometimes still are) nothing more than methods used by software developers to ensure that they could gain access to an application even if something were to happen in the future to prevent normal access methods. An example would be a hard-coded password that could be used to gain access to the program in the event that administrators forgot their own system password. The obvious problem with this sort of backdoor (also sometimes referred to as a trapdoor) is that, since it is hardcoded, it cannot be removed. Should an attacker learn of the backdoor, all systems running that software would be vulnerable to attack.

The term backdoor is also, and more commonly, used to refer to programs that attackers install after gaining unauthorized access to a system to ensure that they can continue to have unrestricted access to the system, even if their initial access method is discovered and blocked. Backdoors can also be installed by authorized individuals inadvertently, should they run software that contains a Trojan horse. Common backdoors include NetBus and Back Orifice. Both of these, if running on your system, can allow an attacker remote access to your system-access that allows them to perform any function on your system. A variation on the backdoor is the rootkit, and they are established not to gain root access but rather to ensure continued root access.

Null Sessions

Microsoft Windows systems prior to XP and Server 2003 exhibited a vulnerability in their Server Message Block system that allowed users to establish null sessions. A null session is a connection to a Windows interprocess communications share (IPC$). There is good news and bad news associated with this vulnerability. The good news is that Windows XP, Server 2003, and beyond are not susceptible to this vulnerability by default. The bad news is that the millions of previous version machines are vulnerable and patching will not solve the problem. This vulnerability can be used to glean many useful pieces of information from a machine, including user IDs, share names, registry settings, and security settings. A wide range of tools and malware use this vulnerability to achieve their aim.

To harden an affected system from the null session vulnerability requires a bit of work. The seemingly obvious path of upgrading systems to XP and beyond is not a perfect solution, for they too can be tweaked by a malicious user to become susceptible to null sessions. Although there are registry settings to restrict anonymous connections, these will not limit all types; the best method is to limit access to TCP ports 139 and 445 to only trusted users.


The group of protocols that make up the TCP/IP suite was designed to work in a friendly environment where everybody who connected to the network used the protocols as they were designed. The abuse of this friendly assumption is illustrated by network traffic sniffing programs, sometimes referred to as sniffers.

A network sniffer is a software or hardware device that is used to observe traffic as it passes through a network on shared broadcast media. The device can be used to view all traffic, or it can target a specific protocol, service, or even string of characters (looking for logins, for example). Normally, the network device that connects a computer to a network is designed to ignore all traffic that is not destined for that computer. Network sniffers ignore this friendly agreement and observe all traffic on the network, whether destined for that computer or others. A network card that is listening to all network traffic and not just its own is said to be in "promiscuous mode." Some network sniffers are designed not just to observe all traffic but to modify traffic as well.

Network sniffers can be used by network administrators for monitoring network performance. They can be used to perform traffic analysis, for example, to determine what type of traffic is most commonly carried on the network and to determine which segments are most active. They can also be used for network bandwidth analysis and to troubleshoot certain problems (such as duplicate MAC addresses).


Spoofing is nothing more than making data look like it has come from a different source. This is possible in TCP/IP because of the friendly assumptions behind the protocols. When the protocols were developed, it was assumed that individuals who had access to the network layer would be privileged users who could be trusted. When a packet is sent from one system to another, it includes not only the destination IP address and port but the source IP address as well. You are supposed to fill in the source with your own address, but nothing stops you from filling in another system's address. This is one of the several forms of spoofing.

Spoofing E-Mail

In e-mail spoofing, a message is sent with a From address that differs from that of the sending system. This can be easily accomplished in several different ways using several programs. To demonstrate how simple it is to spoof an e-mail address, you can Telnet to port 25 (the port associated with e-mail) on a mail server. From there, you can fill in any address for the From and To sections of the message, whether or not the addresses are yours and whether they actually exist or not.

IP Address Spoofing

IP is designed to work so that the originators of any IP packet include their own IP address in the From portion of the packet. While this is the intent, nothing prevents a system from inserting a different address in the From portion of the packet. This is known as IP address spoofing. An IP address can be spoofed for several reasons. In a specific DoS attack known as a smurf attack, the attacker sends a spoofed packet to the broadcast address for a network, which distributes the packet to all systems on that network. In the smurf attack, the packet sent by the attacker to the broadcast address is an echo request with the From address forged so that it appears that another system (the target system) has made the echo request. The normal response of a system to an echo request is an echo reply, and it is used in the ping utility to let a user know whether a remote system is reachable and is responding. In the smurf attack, the request is sent to all systems on the network, so all will respond with an echo reply to the target system. The attacker has sent one packet and has been able to generate as many as 254 responses aimed at the target. Should the attacker send several of these spoofed requests, or send them to several different networks, the target can quickly become overwhelmed with the volume of echo replies it receives.

Spoofing and Sequence Numbers

How complicated the spoofing is depends heavily on several factors, including whether the traffic is encrypted and where the attacker is located relative to the target. Spoofing

attacks from inside a network, for example, are much easier to perform than attacks from outside of the network, because the inside attacker can observe the traffic to and from the target and can do a better job of formulating the necessary packets.

Formulating the packets is more complicated for external attackers because a sequence number is associated with TCP packets. A sequence number is a 32-bit number established by the host that is incremented for each packet sent. Packets are not guaranteed to be received in order, and the sequence number can be used to help reorder packets as they are received and to refer to packets that may have been lost in transmission.

Man-in-the-Middle Attacks

A man-in-the-middle attack, as the name implies, generally occurs when attackers are able to place themselves in the middle of two other hosts that are communicating. Ideally, this is done by ensuring that all communication going to or from the target host is routed through the attacker's host (which can be accomplished if the attacker can compromise the router for the target host). The attacker can then observe all traffic before relaying it and can actually modify or block traffic. To the target host, it appears that communication is occurring normally, since all expected replies are received.

The amount of information that can be obtained in a man-in-the-middle attack will obviously be limited if the communication is encrypted. Even in this case, however, sensitive information can still be obtained, since knowing what communication is being conducted, and between which individuals, may in fact provide information that is valuable in certain circumstances.

Man-in-the-Middle Attacks on Encrypted Traffic

The term "man-in-the-middle attack" is sometimes used to refer to a more specific type of attack-one in which the encrypted traffic issue is addressed. Public-key encryption, requires the use of two keys: your public key, which anybody can use to encrypt or "lock" your message, and your private key, which only you know and which is used to "unlock" or decrypt a message locked with your public key.

Replay Attacks

A replay attack occurs when the attacker captures a portion of a communication between two parties and retransmits it at a later time. For example, an attacker might replay a series of commands and codes used in a financial transaction to cause the transaction to be conducted multiple times. Generally replay attacks are associated with attempts to circumvent authentication mechanisms, such as the capturing and reuse of a certificate or ticket.

TCP/IP Hijacking

TCP/IP hijacking and session hijacking are terms used to refer to the process of taking control of an already existing session between a client and a server. The advantage to an attacker of hijacking over attempting to penetrate a computer system or network is that the attacker doesn't have to circumvent any authentication mechanisms, since the user has already authenticated and established the session. Once the user has completed the authentication sequence, the attacker can then usurp the session and carry on as if the attacker, and not the user, had authenticated with the system. To prevent the user from noticing anything unusual, the attacker can decide to attack the user's system and perform a DoS attack on it, taking it down so that the user, and the system, will not notice the extra traffic that is taking place.

Hijack attacks generally are used against web and Telnet sessions. Sequence numbers as they apply to spoofing also apply to session hijacking, since the hijacker will need to provide the correct sequence number to continue the appropriate sessions.

Attacks on Encryption

Cryptography is the art of "secret writing," and encryption is the process of transforming plaintext into an unreadable format known as ciphertext using a specific technique or algorithm. Most encryption techniques use some form of key in the encryption process. The key is used in a mathematical process to scramble the original message to arrive at the unreadable ciphertext. Another key (sometimes the same one and sometimes a different one) is used to decrypt or unscramble the ciphertext to re-create the original plaintext. The length of the key often directly relates to the strength of the encryption.

Cryptanalysis is the process of attempting to break a cryptographic system-it is an attack on the specific method used to encrypt the plaintext. Cryptographic systems can be compromised in various ways.

Weak Keys

Certain encryption algorithms may have specific keys that yield poor, or easily decrypted ciphertext. Imagine an encryption algorithm that consisted solely of a single XOR function (an exclusive OR function where two bits are compared and a 1 is returned if either of the original bits, but not both, is a 1), where the key was repeatedly used to XOR with the plaintext. A key where all bits are 0's, for example, would result in ciphertext that is the same as the original plaintext. This would obviously be a weak key for this encryption algorithm. In fact, any key with long strings of 0's would yield portions of the ciphertext that were the same as the plaintext. In this simple example, many keys could be considered weak.

Encryption algorithms used in computer systems and networks are much more complicated than a simple, single XOR function, but some algorithms have still been found to have weak keys that make cryptanalysis easier.

Exhaustive Search of Key Space

Even if the specific algorithm used to encrypt a message is complicated and has not been shown to have weak keys, the key length will still play a significant role in how easy it is to attack the method of encryption. Generally speaking, the longer a key, the harder it will be to attack. Thus, a 40-bit encryption scheme will be easier to attack using a bruteLeading the way in IT testing and certification tools, force technique (which tests all possible keys, one by one) than a 256-bit based scheme. This is easily demonstrated by imagining a scheme that employed a 2-bit key. Even if the resulting ciphertext were completely unreadable, performing a bruteforce attack until one key is found that can decrypt the ciphertext would not take long, since only four keys are possible. Every bit that is added to the length of a key doubles the number of keys that have to be tested in a brute-force attack on the encryption. It is easy to understand why a scheme utilizing a 40-bit key would be much easier to attack than a scheme that utilized a 256-bit key.

The bottom line is simple: an exhaustive search of the keyspace will decrypt the message. The strength of the encryption method is related to the sheer size of the keyspace, which with modern algorithms is large enough to provide significant time constraints when using this method to break an encrypted message. Algorithmic complexity is also an issue with respect to brute force, and you cannot immediately compare different key lengths from different algorithms and assume relative strength.

Indirect Attacks

One of the most common ways of attacking an encryption system is to find weaknesses in mechanisms surrounding the cryptography. Examples include poor random number generators, unprotected key exchanges, keys stored on hard drives without sufficient protection, and other general programmatic errors, such as buffer overflows. In attacks that target these types of weaknesses, it is not the cryptographic algorithm itself that is being attacked, but rather the implementation of that algorithm in the real world.

Address System Attacks

Addresses control many aspects of a computer system. IP addresses can be manipulated, as shown previously, and the other address schemes can be manipulated as well. In the summer of 2008, much was made of a serious domain name system (DNS) vulnerability that required the simultaneous patching of systems by over 80 vendors. This coordinated effort closed a technical loophole in the domain name resolution infrastructure that allowed hijacking and man-in-the-middle attacks on the DNS system worldwide.

Password Guessing

The most common form of authentication is the user ID and password combination.

While it is not inherently a poor mechanism for authentication, the combination can be attacked in several ways. All too often, these attacks yield favorable results for the attacker not as a result of a weakness in the scheme but usually due to the user not following good password procedures.

Poor Password Choices

The least technical of the various password-attack techniques consists of the attacker simply attempting to guess the password of an authorized user of the system or network.

It is surprising how often this simple method works, and the reason it does is because people are notorious for picking poor passwords. Users need to select a password that they can remember, so they create simple passwords, such as their birthday, their mother's maiden name, the name of their spouse or one of their children, or even simply their user ID itself. All it takes is for the attacker to obtain a valid user ID (often a simple matter, because organizations tend to use an individual's names in some combination- first letter of their first name combined with their last name, for example) and a little bit of information about the user before guessing can begin. Organizations sometimes make it even easier for attackers to obtain this sort of information by posting the names of their "management team" and other individuals, sometimes with short biographies, on their web sites.

Even if the person doesn't use some personal detail as her password, she may still get lucky, since many people use a common word for their password. Attackers can obtain lists of common passwords-a number of them exist on the Internet. Words such as "password" and "secret" have often been used as passwords. Names of favorite sports teams also often find their way onto lists of commonly used passwords.

Dictionary Attack

Another method of determining passwords is to use a password-cracking program that uses a list of dictionary of words to try to guess the password. The words can be used by themselves, or two or more smaller words can be combined to form a single possible password. A number of commercial and public-domain password-cracking programs employ a variety of methods to crack passwords, including using variations on the user ID.

The programs often permit the attacker to create various rules that tell the program how to combine words to form new possible passwords. Users commonly substitute certain numbers for specific letters. If the user wanted to use the word secret for a password, for example, the letter e could be replaced with the number 3, yielding s3cr3t. This password will not be found in the dictionary, so a pure dictionary attack would not crack it, but the password is still easy for the user to remember. If a rule were created that tried all words in the dictionary and then tried the same words substituting the number 3 for the letter e, however, the password would be cracked.

Rules can also be defined so that the cracking program will substitute special characters for other characters or combine words. The ability of the attacker to crack passwords is directly related to the method the user employs to create the password in the first place, as well as the dictionary and rules used.

Brute-Force Attack

If the user has selected a password that is not found in a dictionary, even if various numbers or special characters are substituted for letters, the only way the password can be cracked is for an attacker to attempt a brute-force attack, in which the passwordcracking program attempts all possible password combinations.

The length of the password and the size of the set of possible characters in the password will greatly affect the time a brute-force attack will take. A few years ago, this method of attack was very time consuming, since it took considerable time to generate all possible combinations. With the increase in computer speed, however, generating password combinations is much faster, making it more feasible to launch brute-force attacks against certain computer systems and networks.

A brute-force attack on a password can take place at two levels: It can attack a system where the attacker is attempting to guess the password at a login prompt, or it can attack against the list of password hashes contained in a password file. The first attack can be made more difficult if the account locks after a few failed login attempts. The second attack can be thwarted if the password file is securely maintained so that others cannot obtain a copy of it.

Hybrid Attack

A hybrid password attack is a system that combines the preceding methods. Most cracking tools have this option built in, first attempting a dictionary attack, and then moving to brute-force methods.

Birthday Attack

The birthday attack is a special type of brute-force attack that gets its name from something known as the birthday paradox, which states that in a group of at least 23 people, the chance that two individuals will have the same birthday is greater than 50 percent. Mathematically, we can use the equation 1.25k1/2 (with k equaling the size of the set of possible values), and in the birthday paradox, k would be equal to 365 (the number of possible birthdays). This same phenomenon applies to passwords, with k (number of passwords) being quite a bit larger.

Software Exploitation

An attack that takes advantage of bugs or weaknesses in software is referred to as software exploitation. These weaknesses can be the result of poor design, poor testing, or poor coding practices. They can also result from what are sometimes called "features."

An example of this might be a debugging feature, which when used during debugging might allow unauthenticated individuals to execute programs on a system. If this feature remains in the program in when the final version of the software is shipped, it creates a weakness that is just waiting to be exploited.

Buffer Overflow Attack

A common weakness that has often been exploited is a buffer overflow. A buffer overflow occurs when a program is provided more data for input than it was designed to handle. For example, what would happen if a program that asks for a 7- to 10-character phone number instead receives a string of 150 characters? Many programs will provide some error checking to ensure that this will not cause a problem. Some programs, however, cannot handle this error, and the extra characters continue to fill memory, overwriting other portions of the program. This can result in a number of problems, including causing the program to abort or the system to crash. Under certain circumstances, the program can execute a command supplied by the attacker. Buffer overflows typically inherit the level of privilege enjoyed by the program being exploited. This is why programs that use root level access are so dangerous when exploited with a buffer overflow, as the code that will execute does so at root level access.

Malicious Code

Malicious code refers to software that has been designed for some nefarious purpose.

Such software can be designed to cause damage to a system, such as by deleting all files, or it can be designed to create a backdoor in the system to grant access to unauthorized individuals. Generally the installation of malicious code is done so that it is not obvious to the authorized users. Several different types of malicious software can be used, such as viruses, Trojan horses, logic bombs, spyware, and worms, and they differ in the ways they are installed and their purposes.


The best-known type of malicious code is the virus. Much has been written about viruses as a result of several high-profile security events that involved them. A virus is a piece of malicious code that replicates by attaching itself to another piece of executable code. When the other executable code is run, the virus also executes and has the opportunity to infect other files and perform any other nefarious actions it was designed to do. The specific way that a virus infects other files, and the type of files it infects, depends on the type of virus. The first viruses created were of two types-boot sector or program viruses.

Boot Sector Virus A boot sector virus infects the boot sector portion of either a floppy disk or a hard drive (years ago, not all computers had hard drives, and many booted from a floppy). When a computer is first turned on, a small portion of the operating system is initially loaded from hardware. This small operating system then attempts to load the rest of the operating system from a specific location (sector) on either the floppy or the hard drive. A boot sector virus infects this portion of the drive. An example of this type of virus was the Stoned virus, which moved the true Master Boot Record (MBR) from the first to the seventh sector of the first cylinder and replaced the original MBR with the virus code. When the system was turned on, the virus was first executed, which had a one-in-seven chance of displaying a message stating the computer was "stoned"; otherwise, it would not announce itself and would instead attempt to infect other boot sectors. This virus was rather tame in comparison to other viruses of its time, which were often designed to delete the entire hard drive after a period of time in which they would attempt to spread.

Program Virus A second type of virus is the program virus, which attaches itself to executable files-typically files ending in .exe or .com on Windows-based systems. The virus is attached in such a way that it is executed before the program executes. Most program viruses also hide a nefarious purpose, such as deleting the hard drive data, which is triggered by a specific event, such as a date or after a certain number of other files are infected. Like other types of viruses, program viruses are often not detected until after they execute their malicious payload. One method that has been used to detect this sort of virus before it has an opportunity to damage a system is to calculate checksums for commonly used programs or utilities. Should the checksum for an executable ever change, it is quite likely that it is due to a virus infection.

Macro Virus In the late 1990s, another type of virus appeared that now accounts for the majority of viruses. As systems and operating systems became more powerful, the boot sector virus, which once accounted for most reported infections, became less common. Systems no longer commonly booted from floppies, which were the main method for boot sector viruses to spread. Instead, the proliferation of software that included macroprogramming languages resulted in a new breed of virus-the macro virus.

Avoiding Virus Infection Always being cautious about executing programs or opening documents sent to you is a good security practice. "If you don't know where it came from or where it has been, don't open or run it" should be the basic mantra for all computer users. Another security best practice for protecting against virus infection is to install and run an antivirus program. Since these programs are designed to protect against known viruses, it is also important to maintain an up-to-date listing of virus signatures for your antivirus software. Antivirus software vendors provide this information, and administrators should stay on top of the latest updates to the list of known viruses.

Two advances in virus writing have made it more difficult for antivirus software to detect viruses. These advances are the introduction of stealth virus techniques and polymorphic viruses. A stealthy virus employs techniques to help evade being detected by antivirus software that uses checksums or other techniques. Polymorphic viruses also attempt to evade detection, but they do so by changing the virus itself (the virus "evolves"). Because the virus changes, signatures for that virus may no longer be valid, and the virus may escape detection by antivirus software.

Virus Hoaxes Viruses have caused so much damage to systems that many Internet users have become extremely cautious anytime a rumor of a new virus is heard. Many users will not connect to the Internet when they hear about a virus outbreak, just to be sure their machines don't get infected. This has given rise to virus hoaxes, in which word is spread about a new virus and the extreme danger it poses. It may warn users to not read certain files or connect to the Internet.

A good example of a virus hoax was the Good Times virus warning, which has been copied repeatedly and can still be seen in various forms today. It caused widespread panic as users read about this extremely dangerous virus, which could actually cause the processor to overheat (from being put into an "nth complexity infinite binary loop") and be destroyed. Many folks saw through this hoax, but many less experienced users did not, and they passed the warning along to all of their friends.

Trojan Horses

A Trojan horse, or simply Trojan, is a piece of software that appears to do one thing (and may, in fact, actually do that thing) but hides some other functionality. The analogy to the famous story of antiquity is very accurate. In the original case, the object appeared to be a large wooden horse, and in fact it was. At the same time, it hid something much more sinister and dangerous to the occupants of the city of Troy. As long as the horse was left outside the city walls, it could cause no damage to the inhabitants. It had to be taken in by the inhabitants, and it was inside that the hidden purpose was activated. A computer Trojan works in much the same way. Unlike a virus, which reproduces by attaching itself to other files or programs, a Trojan is a standalone program that must be copied and installed by the user-it must be "brought inside" the system by an authorized user. The challenge for the attacker is enticing the user to copy and run the program.

This generally means that the program must be disguised as something that the user would want to run-a special utility or game, for example. Once it has been copied and is inside the system, the Trojan will perform its hidden purpose with the user often still unaware of its true nature.

A good example of a Trojan is Back Orifice (BO), originally created in 1999 and now offered in several versions. BO can be attached to a number of types of programs. Once it is attached, and once an infected file is run, BO will create a way for unauthorized individuals to take over the system remotely, as if they were sitting at the console. BO is designed to work with Windows-based systems. Many Trojans communicate to the outside through a port that the Trojan opens, and this is one of the ways Trojans can be detected.


Spyware is software that "spies" on users, recording and reporting on their activities. Typically installed without user knowledge, spyware can perform a wide range of activities. It can record keystrokes (commonly called keylogging) when the user logs onto specific web sites. It can monitor how a user applies a specific piece of software, that is, monitor attempts to cheat at games. Many spyware uses seem innocuous at first, but the unauthorized monitoring of a system can be abused very easily. In other cases, the spyware is specifically designed to steal information. Many states have passed legislation banning the unapproved installation of software, but spyware can circumvent this issue through complex and confusing end-user license agreements.

Logic Bombs

Logic bombs, unlike viruses and Trojans, are a type of malicious software that is deliberately installed, generally by an authorized user. A logic bomb is a piece of code that sits dormant for a period of time until some event invokes its malicious payload. An example of a logic bomb might be a program that is set to load and run automatically, and that periodically checks an organization's payroll or personnel database for a specific employee. If the employee is not found, the malicious payload executes, deleting vital corporate files.

If the event is a specific date or time, the program will often be referred to as a time bomb. In one famous example of a time bomb, a disgruntled employee left a time bomb in place just prior to being fired from his job. Two weeks later, thousands of client records were deleted. Police were eventually able to track the malicious code to the disgruntled ex-employee, who was prosecuted for his actions. He had hoped that the two weeks that had passed since his dismissal would have caused investigators to assume he could not have been the individual who had caused the deletion of the records.

Logic bombs are difficult to detect because they are often installed by authorized users and, in particular, have been installed by administrators who are also often responsible for security. This demonstrates the need for a separation of duties and a periodic review of all programs and services that are running on a system. It also illustrates the need to maintain an active backup program so that if your organization loses critical files to this sort of malicious code, it loses only transactions that occurred since the most recent backup and no permanent loss of data results.


Rootkits are a form of malware that is specifically designed to modify the operation of the operating system in some fashion to facilitate nonstandard functionality. The history of rootkits goes back to the beginning of the UNIX operating system, where they were sets of modified administrative tools. Originally designed to allow a program to take greater control over operating system function when it fails or becomes unresponsive, the technique has evolved and is used in a variety of ways. One high-profile case occurred at Sony BMG Corporation, when rootkit technology was used to provide copy protection technology on some of the company's CDs. Two major issues led to this being a complete debacle for Sony: first, the software modified systems without the user's approval; and second, the software opened a security hole on Windows-based systems, creating an exploitable vulnerability at the rootkit level. This led the Sony case to be labeled as malware, which is the most common use of rootkits.

A rootkit can do many things-in fact; it can do virtually anything that the operating system does. Rootkits modify the operating system kernel and supporting functions, changing the nature of the system's operation. Rootkits are designed to avoid, either by subversion or evasion, the security functions of the operating system to avoid detection. Rootkits act as a form of malware that can change thread priorities to boost an application's performance, perform keylogging, act as a sniffer, hide other files from other applications, or create backdoors in the authentication system. The use of rootkit functionality to hide other processes and files enables an attacker to use a portion of a computer without the user or other applications knowing what is happening. This hides exploit code from antivirus and antispyware programs, acting as a cloak of invisibility.


It was once easy to distinguish between a worm and a virus. Recently, with the introduction of new breeds of sophisticated malicious code, the distinction has blurred.

Worms are pieces of code that attempt to penetrate networks and computer systems. Once a penetration occurs, the worm will create a new copy of itself on the penetrated system.

Reproduction of a worm thus does not rely on the attachment of the virus to another piece of code or to a file, which is the definition of a virus. Viruses were generally thought of as a system-based problem, and worms were network- based. If the malicious code is sent throughout a network, it may subsequently be called a worm. The important distinction, however, is whether the code has to attach itself to something else (a virus) or if it can "survive" on its own (a worm).

Some recent examples of worms that have had high profiles include the Sobig worm of 2003, the SQL Slammer worm of 2003, the 2001 attacks of Code Red and Nimba, and the 2005 Zotob worm that took down CNN Live. Nimba was particularly impressive in that it used five different methods to spread; via e-mail, via open network shares, from browsing infected web sites, using directory traversal vulnerability of Microsoft IIS 4.0/5.0, and most impressively through the use of backdoors left by Code Red II and sadmind worms.

Application-Level Attacks

Attacks against a system can occur at the network level, at the operating system level, at the application level, or at the user level (social engineering). Early attack patterns were against the network, but most of today's attacks are aimed at the applications. This is primarily because this is where the objective of most attacks resides; in the infamous words of bank robber Willie Sutton, "because that's where the money is." In fact, many of today's attacks on systems are combinations of using vulnerabilities in networks, operating systems, and applications, all means to an end to obtain the desired objective of an attack, which is usually some form of data.

War-Dialing and War-Driving

War-dialing is the term used to describe an attacker's attempt to discover unprotected modem connections to computer systems and networks. The term's origin is the 1983 movie War Games, in which the star has his machine systematically call a sequence of phone numbers in an attempt to find a computer connected to a modem. In the case of the movie, the intent was to find a machine with games the attacker could play, though obviously an attacker could have other purposes once access is obtained. War-dialing is surprisingly successful, mostly because of rogue modems-unauthorized modems attached to computers on a network by authorized users. Generally the reason for attaching the modem is not malicious-an individual may simply want to be able to go home and then connect to the organization's network to continue working.

The problem, however, is that if a user can connect, so can an attacker. If the authorized user has not implemented any security protection, this means of access could be totally open. This is often the case. Most organizations enact strict policies against connecting unauthorized modems, but it is difficult to enforce this kind of policy. Recently, new technology has been developed to address this common backdoor into corporate networks. Telephone firewalls have been created, which block any unauthorized modem connections into an organization. These devices make it impossible for an unauthorized modem connection to be established and can also enforce strict access policies on any authorized modems.

The term war-driving has been used to refer to the activity in which attackers wander throughout an area (often in a car) with a computer with wireless capability, searching for wireless networks they can access. Some security measures can limit an attacker's ability to succeed at this activity, but, just as in war-dialing, the individuals who set up the wireless networks don't always activate these security mechanisms.

Social Engineering

Social engineering relies on lies and misrepresentation, which an attacker uses to trick an authorized user into providing information or access the attacker would not normally be entitled to. The attacker might, for example, contact a system administrator pretending to be an authorized user, asking to have a password reset. Another common ploy is to pose as a representative from a vendor needing temporary access to perform some emergency maintenance. Social engineering also applies to physical access. Simple techniques include impersonating pizza or flower delivery personnel to gain physical access to a facility.

Attackers know that, due to poor security practices, if they can gain physical access to an office, the chances are good that, given a little unsupervised time, a user ID and password pair might be found on a notepad or sticky note. Unsupervised access might not even be required, depending on the quality of the security practices of the organization.