The massive Flashback botnet of Mac machines originated from hacked and malware-rigged WordPress blog sites, researchers revealed today.

There were between 30,000 to 100,000 WordPress sites infected in late February and early March, 85 percent of which are in the U.S., said Vicente Diaz, senior security analyst for Kaspersky Lab, in a press briefing today.

Kaspersky Lab researchers say the infected WordPress blog sites were rigged with code that silently redirected visitors to a malicious server. "When the connection was made to the malicious server, that server would determine which OS was running and serve exploits accordingly," says Roel Schouwenberg, senior researcher for Kaspersky. It was a pay-per-install scheme to spread malware, including the Flashback Trojan.

Most researchers say a gradual decline in machines infected by the Trojan is still under way: As of yesterday, there were about 140,000 infected Macs still out there, according to Symantec, and Kaspersky says it sees only about 30,629 Flashback-infected bots in its sinkhole.

Still on the horizon, too, is the possibility of a Flashback comeback, with the command-and-control servers sending their bots updates. "We are watching the command-and-control domains used to control this botnet for any updates ... We haven't seen any new updates being delivered," says Liam O Murchum, manager of operations for Symantec Security Response. "Flashback generates new domains every day, which shows us the attackers have probably written malicious code before. They are aware that their botnet could be taken down with a single domain, so they generate a new one every day."

Flashback may be the largest known botnet made up of Apple Macintosh computers, and the outbreak of infections, mainly in the U.S., appears to have ushered in the beginning of the end of the age of innocence for Mac users. While attacks on the Mac aren't new, this one was high-profile and widespread.

Word spread rapidly earlier this month that a massive botnet of Mac OS X machines was building, and it reached more than 700,000 machines before antivirus vendors including Kaspersky Lab, F-Secure, and Symantec issued their own detection and removal tools. Apple issued an update to patch for the exploited vulnerability over the weekend. The Flashback malware exploits a known vulnerability in Java that had been patched by Oracle.

Apple did not respond to a press inquiry for this article. Its updates for OS X Lion and Mac OS X v10.6 patch the Java implementation hole and remove Flashback, and Apple also provided an update for OS X Lion that removes Flashback from Macs that don't run Java.

Kaspersky's Diaz says the biggest drop in infections came after Apple's update. Apple has been under fire for the late response to the flaw.

Symantec, meanwhile, hasn't seen infected WordPress sites used in the Flashback attack per se, but has seen URL redirections to the Flashback Trojan domains for spreading the malware. "We won't know how it was infected in the first place -- whether it was WordPress [or something else], but it was redirecting to the exploit pages distributing Flashback," Murchu says.

Murchu says the attackers behind Flashback demonstrated the same skills as those who target Windows -- an indication that the shift to the Mac OS X is just another business decision by cybercriminals looking for more ways to make money as the Mac becomes more mainstream.

"The techniques they used were like those who are skilled, or how an experienced virus-writer would use. Clearly, they had previously written malware before and more than likely for Windows," Murchu says. "They know how this works, injecting malware, redirecting users, and making money off that. They have transferred [that knowledge] over to the Mac."

A Mac APT
But, wait, there's more: Both Kaspersky and Symantec have been studying targeted attack malware called SabPub written for the Mac OS X.

Costin Raiu, director of global researach and analysis for Kaspersky Lab, says Kaspersky found a single email targeting a pro-Tibetan supporter that contained two attachments that come rigged with the SabPub backdoor Trojan. Raiu says the attacks occurred in February, and that it's related to the LuckyCat APT campaign that targeted entities in Japan and India.

"The C&C server which was used by the SabPub malware is no longer active: It was shut down at the beginning of the week. Hence, infected users will have a 'dormant' malware on their machines, which awaits for C&C to come back to life," Raiu says.

"I presume that is because of the high number of attacks against Tibetan activists. Some of them probably switched to Macs as a way of escaping the attacks and mitigating the risks. The LuckyCat gang probably noticed this shift and decided to implement attacks than can also affect such targets," he says.

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