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A large peer-to-peer botnet known for its resilience was spotted sniffing out potential victim voice-over-IP (VoIP) servers using an advanced stealth technique of camouflaging its efforts to recruit new bots.The Sality botnet, which was first discovered in 2003 and has been estimated to have hundreds of thousands or more infected machines in its zombie army, scanned IPv4 addresses in February 2011 via a covert scanning method that flew under the radar, according to new research from the University of California-San Diego and the University of Napoli in Italy.

The researchers were able to observe the botnet's activity via UCSD's darknet, called the UCSD Network Telescope, which provides a passive traffic-monitoring system for studying malicious Internet activity. They will present their findings at next month's Internet Measurement Conference 2012 in Boston.

The Sality malware is basically a file infector aimed at Windows machines. It has been used to spread spam, infect Web servers, steal information, and crack passwords, according to Symantec. The new research from UCSD and the University of Napoli reveals yet another use for the botnet: identifying targets for VoIP abuse, including toll fraud or vishing attacks. Sality used some 3 million unique source IP addresses in its scan during the 12-day period the researchers observed the activity and "captured traffic reflecting a previously undocumented largescale stealth scanning behavior (across the entire IPv4 space, we believe)," the researchers wrote in their paper. The malware involved was discovered to be a module of Sality that is used for targeting session initiation protocol (SIP) servers, or VoIP systems.

"The choice of the target IP addresses progresses in reverse-byte-order increments. Moreover, there is a large turnover of bots participating in the scan. The result is that a single network would receive scanning packets 'diluted' over a large period of time -- 12 days in this case -- coming from different sources," UCSD's King says. "This traffic would be very difficult to spot by an intrusion detection system, [for example], because of very low volume and almost each packet coming from a different source IP," he says.
Sality historically hasn't attracted the attention that other botnets have. But according to Symantec, its anti-security software features and advanced method of spreading its payload have made it an efficient and resilient botnet. "Despite being one of the most prevalent threats nowadays, Sality has not received the coverage or attention required to raise awareness and eventually create a momentum to seriously thwart the threat," Symantec wrote in a white paper last year on Sality.

"The malware distributed to these computers include things as 'benign' as spam generators, but also password stealers. In early 2011, one of the programs distributed was geared towards Web credentials theft, with a special emphasis on Facebook and Google Blogger accounts. 

Cross-posted from: Security Dark Reading

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