The previously undiscovered malware represents a missing puzzle piece tied to “Turla,” a so-called advanced persistent threat (APT) disclosed in August by Kaspersky Lab and Symantec. For at least four years, the campaign targeted government institutions, embassies, military, education, research, and pharmaceutical companies in more than 45 countries.

The unknown attackers—who are probably backed by a nation-state, according to Symantec—were known to have infected several hundred Windows-based computers by exploiting a variety of vulnerabilities, at least two of which were zero-day bugs. The malware was notable for its use of a rootkit that made it extremely hard to detect.

Now researchers from Moscow-based Kaspersky Lab have detected Linux-based malware used in the same campaign. Turla was already ranked as one of the top-tier APTs, in the same league as the recently disclosed Regin for instance. The discovery of the Linux component suggests it is bigger than previously thought and may presage the discovery of still more infected systems.

“The [Turla] operations are being carried out in broader environments than we previously knew,” Kaspersky Lab expert Kurt Baumgartner told Ars. “All the other stuff we’ve seen from Turla has been windows based. This piece of the puzzle shows us that they do not limit themselves.”

Magic Numbers

Like its Windows counterparts, the Linux trojan is extremely stealthy. It can’t be detected using the common netstat command. To conceal itself, the backdoor sits dormant until attackers send it unusually crafted packets that contain “magic numbers” in their sequence numbers. The malware may have sat unnoticed on at least one victim computer for years, although Kaspersky Lab researchers still have not confirmed that suspicion.

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