The world's first fully functional OS X ransomware, KeRanger, is really a Mac version of the Linux Encoder Trojan, according to new research from Romanian security software firm Bitdefender.
The infected OS X torrent update carrying KeRanger looks virtually identical to version 4 of the Linux Encoder Trojan that has already infected thousands of Linux servers this year.
KeRanger spread via an infected version of an otherwise legitimate open source BitTorrent application, Transmission. The tainted version (2.90) was available for download between March 4 and March 5, 2016 and came signed with a legitimate developer certificate.
Apple's OS X ships with a security feature called Gatekeeper, allowing users to restrict which sources they can install applications from in order to minimize the likelihood of deploying a malicious app. The default setting allows users to install applications from the Mac App Store or applications that are digitally signed by a developer.
By using a developer certificate to sign their wares, the crooks behind KeRanger were able to circumvent Apple's GateKeeper control. Apple has since revoked the misused certificate, which was issued to a Turkish firm, so the immediate panic is over.
However, similar attacks along the same lines might easily re-appear, so merely disallowing unsigned software from running on Macs is no defense.
KeRanger isn't the first Mac malware with the capability to circumvent Gatekeeper. For example, three years ago the same trick was used in a trojan (KitM.A) found on computers belonging to Angolan civil rights activists, Bitdefender reports.
"Once the infected installer is executed, the Trojan connects to the command and control centers via TOR and retrieves an encryption key," explained Catalin Cosoi, chief security strategist at Bitdefender.
"After encryption finishes, the KeRanger ransomware creates a file called README_FOR_DECRYPT.txt, which holds the information on how the victim should pay the ransom. The encryption functions are identical to those deployed by the Linux Encoder Trojan and have the same names."
Six months ago, only Windows and Android smartphone users needed to worry about ransomware, but that has changed more recently so that Linux server admins and even Mac users need to be wary of potential attack. Windows remains the target of the greatest number of different ransomware strains and the main locus of the problem, as it is for other types of malware.
The developers behind the Linux Encoder malware have either expanded to OS X or have licensed their code to a cybercrime group specializing in OS X attacks, according to Bitdefender. ®