OS X Ransomware KeRanger Based on Linux.Encoder

KeRanger, a threat that is believed to be the first fully-functional OS X ransomware, is actually a Mac version of the Linux ransomware known as Linux.Encoder, according to researchers.

The existence of KeRanger came to light earlier this month after cybercriminals attempted to distribute the malware using the official installer for the BitTorrent client Transmission. The application’s developers told Reuters that the malicious version of the installer was downloaded roughly 6,500 times from their website.

According to Palo Alto Networks, once it infects a system, KeRanger waits for three days before contacting its command and control (C&C) server over the Tor network. The malware then encrypts files, including backups, and instructs victims to pay 1 Bitcoin (roughly $400) to get the key needed to decrypt them.

Researchers at security firm Bitdefender analyzed the malware and determined that it’s nearly identical to version 4 of Linux.Encoder, a piece of ransomware designed to encrypt files on Linux systems.

Linux.Encoder, which is based on the open-source “educational” ransomware Hidden Tear developed by Turkish security enthusiast Utku Sen, has infected thousands of devices over the past months. Since the ransomware code released by Utku Sen includes an intentional encryption backdoor, experts have been able to develop tools that allow users to recover their files without paying the ransom.

All four versions of Linux.Encoder have been cracked by researchers. Both Bitdefender and Dr. Web, the Russian security firm that first reported on the threat, developed tools that allow victims to recover their files.

“The encryption functions [in Linux.Encoder and KeRanger] are identical and have same names: encrypt_file, recursive_task, currentTimestamp and createDaemon to only mention a few. The encryption routine is identical to the one employed in Linux.Encoder,” said Catalin Cosoi, chief security strategist at Bitdefender.

Bitdefender told SecurityWeek that while the tools it has developed to help Linux.Encoder victims recover their files don’t work on OS X, the file recovery method for KeRanger is the same.

Malware such as KeRanger should normally be blocked by the Gatekeeper security feature in Apple’s operating system. However, attackers signed the malicious Transmission installer with a valid digital certificate issued by Apple to a Turkish company, which has allowed them to bypass Gatekeeper.

Linux.Encoder and KeRanger are not the only pieces of ransomware based on the Hidden Tear source code — Ransom_Cryptear.B was also developed using the same code.