Denial of Service

Denial of service attacks are essentially brute force attacks aimed at completely overwhelming a server, making it too busy to respond to requests from genuine users. A denial of service attack come in many forms and may overwhelm  the processor, hard drive, RAM or use all available bandwidth of a server. This may result in a web application being made unavailable or being slower then normal, for genuine users.

Most successful large scale denial of service attacks are in fact distributed denial of service attacks, which is a denial of service attack perpetrated by more then one individual or by bots.

Bots or bot nets are often used in distributed denial of service attacks. Bots are programs that perform simple tasks over and over again (such as sending a request to a server over and over again). A person performing denial of service attacks may have one bot or many bots (also known as a bot-net). Each bot of a bot-net is often on a separate "zombie" computer and the owner of the computer is often unaware that the bot is there. A bot-net can have thousands of bots or only a few. In recent years their has been a move to more smaller bot-nets, to "fly under the radar" and avoid detection.

Distributed denial of service attacks are often used by 'hacktivists' to target websites for political reasons. A famous example of this is the distributed denial of service attack by Anonymous members that was aimed at the Visa, MasterCard, PayPal and Amazon websites. The reason for the attack was because Anonymous believed these organizations were attempting to censor WikiLeaks.