Denial of service attack

A denial of service attack is a security threat where attackers bombard a system (usually a web-based system) with fake requests for service and so deny access to authorized users. therefore, the attack compromises the availability of the system. Denial of service attacks are usually  triggered from many separate computers and are therefore sometimes called Distributed Denial of Service (DDOS) attacks.

The motive for DoS attacks is sometimes a dislike of the organization running the system but, more commonly, it is used as a means of blackmail. Sites are threatened with a DoS attack, which will make them unavailable, unless they make a payment to the criminal who is threatening them.

Denial of service attacks are relatively easy to detect and neutralize if they come from a single computer so the most commonly used DoS attack now is a DDOS attack where a large number of computers send tens of thousands of requests to the system that is being attacked. These networks are usually people’s PCs that have been infected with malware that allows these computers to be taken over by a malicious external agent who instructs them to access the web site that is being targeted.

There are a range of techniques that may be used for a DoS attack – Wikipedia has a long article on this.