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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The new CBL-lookup functionality is contained in a file named "rasta01.exe" -- an apparent reference to the Rastafari African-based spiritual ideology. On a potentially related note, the Malware Must Die blog has been chronicling ongoing attempts to sinkhole the C&C servers tied to the latest variant of Kelihos, and found that some related domain names were registered using the Internet.bs registrar in the Bahamas.

Whoever's behind the latest version of Kelihos, the new functionality demonstrates the ever-advancing botnet state of the art. But compared with other botnets, just how advanced is Kelihos? "I wouldn't call it 'advanced' in the advanced persistent threat sense of the word, but this is a sophisticated threat that has adapted to use P2P [peer-to-peer] techniques to stay a step ahead of security researchers," said Mannon. "By decentralizing its command-and-control servers through P2P communication, Kelihos is much harder to take down." While the anti-spam lookup service capability is unusual, it's not the first time that developers have tapped free information sources to make their malware infections and C&C networks more effective. "For example, many malware threats, such as BiFrose, have utilized Web tools such as http://www.whatismyip.com/ before," said Mannon.

The Kelihos botnet has proven persistent despite repeated attempts to eradicate it. The latest enhancements to the botnet were first spotted in late July, according to anti-spyware firm Lavasoft.

"The Kelihos botnet is still in operation despite all the takedowns in the past. This is attributable to a sustainable P2P architecture," said Lavasoft. As of late July, it said that 47% of the infections associated with this Kelihos variant are located in the Ukraine, followed by Russia (15%), Taiwan (14%), India (7%), Bulgaria (5%) and Romania (4%).

The latest version of the malware can spread via USB drives, create HTTP requests using a number of dummy user agents, log keystrokes and steal stored passwords from 59 different applications, including Bromium, Chrome, Firefox, Internet Explorer and MyFTP.

What's the best way for users to avoid a Kelihos infection? "Follow better browsing methods," said Mannon. "For example, NoScript and a few other tools impede JavaScript/Flash from running automatically," which helps block the majority of Kelihos-delivery mechanisms.