In a Denial of Service (DoS) attack a machine or network resource is made unavailable to its intended users by temporarily or indefinitely disrupting services of a host connected to the Internet. In a Distributed Denial of Service (DDoS) attack is an attempt to make an online service unavailable by overwhelming it with traffic from multiple sources. Revenge, blackmail and activism can motivate these attacks.
In European countries, committing denial-of-service attacks may lead to arrest. In the case of revenge or blackmail the case is treated as a criminal case. As an unusual exception, the UK has outlawed denial-of-service attacks and set a maximum penalty of 10 years in prison with the Police and Justice Act 2006, which amended Section 3 of the Computer Misuse Act 1990. DoS attacks are made easy by ignoring vulnerabilities, but ignoring vulnerabilities in general seems to be common.