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Security from A to Z: Botnet

These armies of zombie PCs are used by cybercriminals for sending spam. Part of a series on hot security topics.

A botnet--also known as a "zombie network" or "zombie army"--is a collection of Internet-connected PCs that have been compromised by infection from malicious software.

That compromise means they can be controlled remotely by a malicious outsider, often without the PC owner's knowledge. Computers not protected by adequate antivirus and firewall software are at greatest risk of being corralled into a botnet.

Armies of zombie PCs are used by cybercriminals for sending spam or viruses or committing denial-of-service attacks. Capacity on botnets is rented out to criminal gangs or individuals for as little as $100 for a couple of hours by their creator--often a very commercially motivated virus writer.

A zombie army was used in a high profile distributed denial-of-service attack against Akamai Technologies this year, affecting the Web sites of some of its big-name tech clients.

Natasha Lomas reported for in London.