Botnet Spotted Silently Scanning IPv4 Address Space For Vulnerable VoIP

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.

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 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.

"They were probably trying to brute-force SIP servers to create accounts to be used for free calls, anonymous calls, VoIP fraud, etc.," says UCSD's Alberto Dainotti, who, along with fellow UCSD researchers Alistair King and Kimberly Claffy and University of Napoli Federico II's Ferdinando Papale and Antonio Pescape, conducted the Sality research and authored the report (PDF).

But even more compelling is the way the botnet tried to hide its recruitment efforts, using "reverse-byte order scanning," which results in a low number of packets per day, for instance. And 1 million of the around 3 million IPs the researchers watched in the scanning activity transmitted only one probe and then dropped out of the scanning activity -- yet another method of laying low.

"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.

It's not that this stealth-scanning technique is unique, but it's the first time such an event has been both empirically observed and documented, Dainotti says. "Some experts believe this behavior has been adopted by other botnets. However, we are not aware of any data confirming any event similar to this one."

David Piscitello, senior security technologist for ICANN, says this indeed appears to be the first time researchers have identified a botnet that employs this scanning method using reverse-byte sequential increments of target IP addresses. "The botnet employs sophisticated 'orchestration' techniques to avoid detection," Piscitello says. "Simply put, the botnet operator divvied up the scans across nearly 3 million bots to scan the full IPv4 address space using a scanning pattern that maximizes coverage and overlap, but is unlikely to be detected by current automation."

[ Notorious 3322.org domain operator has agreed to cooperate with Microsoft and Chinese Computer Emergency Response Team to block malicious traffic and malware in the domain. See Microsoft Hands Off Nitol Botnet Sinkhole Operation To Chinese CERT. ]

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. Tomorrow, the operators of the botnet could decide to steal banking information," the report (PDF) warned.

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