Researchers from Russia-based Kaspersky Lab have uncovered a gang of hackers for hire who specialize in surgical strikes that quickly infiltrate suppliers to Western companies, steal highly sensitive data, and then vanish.

Icefog, as the group of "cyber mercenaries" has been dubbed, is made up of six to 10 members who are able to infect both Windows and Mac computers with advanced malware that's extremely hard to detect, Kaspersky researchers revealed in a report published Wednesday.

That's a tiny membership compared with other gangs engaged in advanced persistent threat (APT) attacks that siphon gigabytes or even terabytes of sensitive data out of corporations, defense contractors, and government agencies. The so-called APT1 group that has hacked more than 100 large companies, for instance, has as many as 100 members, a rosterinfected by exploiting vulnerabilities in Oracle's Java browser plug-in, Microsoft's Office applications, and other software that hadn't been updated with recent security patches. There's no evidence the group used so-called zero-day attacks, which exploit previously unknown security bugs.

Most of the companies the Icefog gang has penetrated are located in South Korea and Japan. It's possible that the ultimate goal of the intrusions was to appropriate data belonging to organizations located in the US and other western countries that do business with the victims.

In some ways, the Icefog gang is the hacking equivalent of a highly skilled cat burglar who spends weeks or months learning where to find the diamonds and fine art in a targeted penthouse so he can break in, immediately steal them, and make a quick