The Kraken botnet, believed by many to be the single biggest zombie network until it was dismantled last year, is staging a comeback that has claimed almost 320,000 PCs, a security researcher said.
Since April, this son-of-Kraken botnet has infected an estimated 318,058 machines - about half as big as the original Kraken was at its height in the middle of 2008, according to Paul Royal, a research scientist at the Georgia Tech Information Security Center.
Like its predecessor, the new botnet is a prodigious generator of spam, with a single machine with average bandwidth able to send more than 600,000 junk mails per day.
Curiously, the malware spawning the new zombie network is being spread by a separate botnet that uses the Butterfly framework, a for-hire software kit for infecting Windows PCs. The collaboration between operators of the two networks is generating some head-scratching among researchers.
“We don't know the relationship between the Kraken operators and the installation service operators,” Royal told The Reg. “If they're not the same group then that's really interesting because it suggests that even professional criminal gangs are willing to use these kits, provided that they do their jobs.”
The group performing the mass installations has been dubbed Eboc because that's the user name they use to sign into a copy-protection system designed to curb the pirating of the botnet software according to Pedro Bustamante, a senior research advisor at anti-virus provider Panda Security. To evade detection, they use as many as 1,200 unique malware variants. One widely used strain was flagged by just 50 per cent of AV last week, according to this VirusTotal analysis.
“The Butterfly framework is a very efficient way to infect a lot of users,” Bustamante said. “The Butterfly framework is nothing more than a huge network control mechanism to build huge botnets.”
If Butterfly sounds familiar, it's probably because it was also used to build Mariposa, the 13-million-strong botnet that was shutdown in March by authorities in Spain. Panda Security researchers were instrumental in its downfall, and they've continued to infiltrate Butterfly.
The latest Kraken uses domain names offered by dynamic DNS services to corral its bots into command and control channels. Because the addresses are extensions of legitimate domain names, it prevents them from being shut down by registrars.
“They've created this DNS command-and-control architecture that is in many ways more resilient than attempting to register a second-level domain name with... a shady registrar,” Royal said. “Unfortunately, this gives the operators quite a bit of agility.” ®
This article was updated to correct the number of spam messages sent by a Kraken bot.