Top three botnets sending 21 billion spam messages a day

The top three botnets — or herds of compromised computers controlled by hackers — are sending as many as 21 billion spam messages a day, according to Symantec’s MessageLabs Intelligence report.

The top botnets, also known as zombies, have been identified as Donbot, Cutwail, and Mega-D and they’re vying for the top spam title. All three are responsible for distributing 15 to 20 percent of all global spam, the company said. Symantec and McAfee estimate now that spam is 90 – 92 percent of all email. Many of those messages are intercepted by Internet service providers, but you can see what a drag on the whole Internet it has become.

It’s quite possible that Twitter has been hit by a botnet attack today, as the company said it was defending itself against a denial of service attack that brought down the service this morning.

Botnets are herds of compromised computers that are used to do the bidding of criminal hackers. They can be used in distributed denial of service attacks, where each bot repeatedly sends requests to a web site, overwhelming it with traffic. Defending against the attack often requires a company to methodically block the IP addresses of the bots.

Hackers round up their herds of bots by compromising machines that have no antivirus protection. The users may notice that their machines slow down, but there is often no other clue that the machine has become a drone doing the bidding of remote masters.

Cutwail was taken out briefly last weekend when a Latvian Internet service provider, Real Host, was taken offline. Cutwail then restored itself and was back in operation on Monday. Spam messages hit a peak of 108 billion on July 28. Much of the spam relates to discounted (and often fake) medicines.

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