Biggest spam botnet grows by 300 per cent

Rustock, a ‘botnet’ of infected PCs has grown by 300 per cent to become the largest spam-sending network in the world, according to a new report from security firm MessageLabs.

Botnets work by infecting individual machines and using them as relays to mass-mail spam around the globe. The use of botnets is increasing, with almost 84 per cent of all spam now sent by one of the illegal networks.

Although the number of emails sent each day by individual machines in Rustock's network has fallen by 65 per cent, it's more than made up for by an increase in the number of infected computers – known as 'zombies' – which has risen to between 1.6 and 2.4 million.

The increase establishes the botnet as the world's biggest individual spammer, sending 43.4 billion unsolicited messages a day – nearly a third of global botnet spam.

Rustock overtakes previous top dog Cutwail, which was drastically hit by the closure of Latvian ISP Real Host, losing the ability to update many infected machines.

The report offers little comfort for Microsoft, with nearly 92 per cent of all infected computers running Windows.

MessageLabs also reports that the geographical spread of botnets has changed over the last year, with India, the USA and Brazil becoming the three countries with the greatest number of infected computers.