"All the malware we have collected so far is designed to work on Microsoft's Windows operating system. However, there are signs that non-Windows malware does exist," Kaspersky wrote in an expansive white paper (download PDF) released Monday.
Equation -- labeled for its encryption habits -- used exploits, malware and development tools that strongly resembled those attributed to the U.S. National Security Agency (NSA), the country's electronic spying group. Some of the Windows zero-day vulnerabilities leveraged by Equation were identical to ones later used by other notorious espionage-ware, including Stuxnet and Flame.
Kaspersky said that after it "sinkholed" the Equation group's command-and-control (C&C) servers -- meaning that the security firm diverted traffic from those domains to systems it controlled -- it monitored communication between what appeared to be OS X-powered machines and the C&C servers.
The connections marked as Macs originated in China, indicating that the victimized personal computers were located in that country. Kaspersky characterized the number of compromised Macs as a "large pool."
In an interview Tuesday, Vitaly Kamluk, chief malware expert at Kaspersky Lab, said that his company sinkholed only a fraction of the Equation group's C&C servers, and so was unable to estimate the number of hijacked Macs.
Those Mac-to-C&C communiqués resembled the format used by DoubleFantasy, one of several malware modules allegedly deployed by Equation. According to Kaspersky, the cyber-spies used DoubleFantasy to further investigate the targeted computer and/or its user to determine whether the machine, its user and the accessible files were interesting or worthwhile enough to justify further manipulation. If so, other malware was planted on the system.
Even if the computer or user didn't meet that bar, DoubleFantasy implanted a backdoor on the system so it could be accessed later.
The target quality validation done by DoubleFantasy was likely used to limit the deployment of more sophisticated tools to reduce the chance of discovery by victims or, as Kaspersky has claimed, by malware researchers.
Macs that communicated to the sinkholed C&C servers identified themselves as running OS X 10.8, the 2012 operating system better known as Mountain Lion, and either Apple's own Safari or Mozilla's Firefox. "This leads us to believe that a Mac OS X version of DoubleFantasy also exists," said Kaspersky.
Web analytics company StatCounter pegged Macs as an extremely small portion of China's personal computer usage for January: just 1.4% of that of the country at large. By comparison, Windows PCs accounted for at least 96% of the online usage generated in China last month.
Equation may have gone to the effort of creating OS X malware because of Apple's cachet in the People's Republic of China. The very expensive Macs cater to the wealthiest fraction of China's population and are used by some government officials, making them lucrative espionage targets.
Kaspersky suspected that Equation targeted iPhone owners, too.
"We observed that one of the malicious forum injections, in the form of a PHP script, takes special precautions to show a different type of HTML code to iPhone visitors," said Kaspersky, referring to Equation's Web-based attempts to compromise a device. "iPhone visitors are redirected to the exploit server, suggesting the ability to infect iPhones as well."