The Kelihos botnet has a new trick up its sleeve: It references anti-spam services to see if a PC that it's infected has been flagged by anti-spam services.
"The idea here is to use a clean machine to further propagate nastiness," Chris Mannon, a security researcher at Zscaler's ThreatLabZ, said via email. He recently spotted the anti-spam service-referencing capabilities in a variant of the botnet that was first discovered in late July.
Here's how it works: "The attackers are looking at the victim's IP reputation on the Internet as a whole," Mannon said, by checking IP block lists maintained by Barracuda Networks, SpamHaus, Mail-Abuse and Sophos. "If the victim isn't seen in the CBLs (Composite Block Lists) yet, then it may be used as either a proxy C&C [command-and-control server] or spam-bot," he said. If the PC is blocked, however, then the malware effectively focuses its efforts elsewhere.
What might these information sources do to prevent their services from being abused in this manner? "There is not much that can be done beyond throttling suspect traffic coming in at rapid succession," said Mannon. "These services are meant to be used for diagnostics for legitimate security advice."
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