Denial of service

Denial of service is an attack on a computer server intended to prevent it from functioning. The usual method is to overload the server with a mass of generally ill-formed requests to trigger an abnormal and paralysing response. The attack very often uses a multitude of "zombie PCs" working together, all infected by backdoors or Trojan horses and remotely controllable by a hacker.
It is also possible to remotely block routers by exploiting weaknesses in their software.