Botnets pushing up spam loads

Botnet-controlled PCs are responsible for some 83 percent of all spam messages, according to a recent report.

Security firm MessageLabs said in a recent report that despite efforts to shut down service providers notorious for hosting shady operations, spam vendors are continuing to send out huge loads of junk mail through the use of botnet-infected systems.

Researchers estimate that in June spam levels have reached 90 percent of the total global email load, holding steady from levels in May.

Of those spam messages, it is estimated that 83.2 percent were coming from botnets.

The biggest offender by far was the Cutwail botnet. The massive network accounted for some 45 percent of all botnet spam on the web.

Despite the shutdown of one of the botnet's main hosts, Cutwail was able to assemble a network of as many as 2.1 million machines and sending out an estimated 35 percent of all spam emails worldwide.

According to MessageLabs, the majority of Cutwail's infected machines reside in Brazil, the US and Korea, all popular countries for malware and botnet infections.

Second to Cutwail in spam volume was the Mega-D trojan, which logged 9.5 percent of the botnet spam load.

In the third spot was the Grum worm with 6 percent, followed by the Rustock malware with 4 percent.

A number of smaller, unclassified accounted for an additional 10.5 percent of botnet spam attacks.

Researchers noted that webmail systems have also become target for spammers.

"Much of the remainder is sent from compromised mail servers and webmail accounts," read the report.

"Some of the smaller botnets can also control the sending of spam through webmail accounts in such a way as to make it appear as though there is a real person behind the use of each webmail account."