Attacks can be classified into four broad categories: snooping, modification, masquerading, and denial of service. In practice, an attack may employ several of these approaches. Almost all attacks start with snooping, for example.
More formally, attack methods are classified as "passive" and "active." Passive attacks include eavesdropping. Active attacks are subdivided into "forgery," "message modification," and "denial of service." We use a simpler list of four categories for use in the explanations here.
Snooping, as the name suggests, is simply accessing private information. This information could be used for an advantage, such as getting company secrets to help your own business or stock purchase decisions. It could also be used for active assaults such as blackmail. Encryption can be used to make snooping difficult. The attacker is required either to know the secret encryption key or to use some clever technique to recover the encrypted data.
 Also known as "footprinting" or "information gathering."
Modifications to data can be achieved in some nonobvious ways. When thinking about modification attacks, most people consider an attacker modifying e-mails with malicious content or changing the numbers in an electronic bank transfer. While such high-level modifications have been accomplished, there are more subtle ways to modify data. For example, if you can intercept a wireless transmission and change the destination address field (IP address) on a message, you could cause that message to be forwarded to you across the Internet, instead of to its intended recipient. Why would you want to do this? Because the message on the wireless link is encrypted and you can't read the content, but if you can get it forwarded across the Internet, you will receive the decrypted version. The IP header is easier to attack because it is a known format.
Masquerading is the term used when an attacking network device impersonates a valid device. It is the ideal approach if an attacker wants to remain undetected. If the device can successfully fool the target network into validating it as an authorized device, the attacker gets all the access rights that the authorized device established during logon. Furthermore, there will be no security warnings. Even an eagle-eyed IT manager scanning the traffic records won't see anything amiss unless the attacker does things that a normal user wouldn't do, such as trying to access system areas. There are, of course, nonelectronic attacks based on masquerading that are equally effective?if you leave your terminal logged in and go to lunch, anyone can sit down and get your access rights. It is the same principle.
Denial of service (DoS) is quite unlike the other three categories both in technique and goals. While the other three extend extra privilege to the attacker, a DoS attack usually blocks out everybody, including the attacker. The object of a DoS attack is to cause damage to the target by preventing operation of the network. In 2000 the largest attack yet publicized occurred with a distributed DoS attack against several major Web commerce sites. The attack blocked access to the sites for hours. This attack originated from thousands of remotely controlled computers throughout the world whose owners were largely unaware of their participation. The attackers used these "zombie" computers to generate large amounts of traffic directed toward their victims, preventing them from servicing valid requests. Why did they do it? Perhaps to gain bragging rights?this is classic ego hacking culture. A more sinister reason might be to gather experience and data for some larger future event.
In principle, DoS attacks could be mounted for commercial reasons. Bringing down a sales Web site in the run-up to the holidays could inflict financial damage on a competitor. However, it is unlikely that any serious retailer has actually used such tactics. An attack by an ex-employee with a grievance is more plausible. DoS attacks are hard to prevent on the Internet and usually rely on causing the receiving server to exhaust its buffer resources so it cannot accept any valid connections for a period of time. Unfortunately for us, DoS attacks on Wi-Fi LANs are easy to mount and almost impossible to prevent.
The enemy can successfully use some of these attacks without having access to your secret network keys. However, in most cases the damage that can be done without knowing the keys is quite limited. If the attacker can find out your keys, then you move into a different category of danger. Unauthorized modifications to Web sites and the stealing of databases full of credit card details occur because someone has broken the keys. As we look at the types of attack that can be made against Wi-Fi LANs, we'll consider these cases separately: first, attacks against the network without the keys, and second, attacks to try to uncover the keys themselves.