The KeRanger file-encrypting ransomware program for Mac OS X contains crypto flaws that could allow users to recover their files without paying cybercriminals.
According to researchers from antivirus firm Bitdefender, KeRanger is based on another ransomware program, called Linux.Encoder, that first appeared in November and targeted Linux-based Web servers.
The first three versions of Linux.Encoder had flaws in their cryptographic implementations that allowed the Bitdefender researchers to create tools that could be used to decrypt files affected by the malicious program.
KeRanger was found on March 4 on the official website of the popular Transmission BitTorrent client. The attackers had compromised the server and replaced the installer for Transmission version 2.90 with a malicious one.
The rogue installer was digitally signed with a legitimate Apple developer's certificate that had been issued to a Turkish company. The certificate was revoked by Apple a few days later.
"The infected Mac OS X torrent client update analyzed by Bitdefender Labs looks virtually identical to version 4 of the Linux.Encoder Trojan that has been infecting thousands of Linux servers since the beginning of 2016," the Bitdefender researchers said in blog post Tuesday.
Aside from some compiler-related differences and a new routine designed to locate and encrypt Apple Time Machine backups, all the other functions in the code are identical, said Bogdan Botezatu, senior e-threat analyst at Bitdefender.
According to Botezatu, Linux.Encoder version 4 appeared in early February and has the same crypto flaws as the previous versions, which means that KeRanger's encryption implementation is also broken.
Bitdefender has not yet released a decryption tool for KeRanger-affected files, but will consider developing one if there's enough demand for it.
A representative of the Transmission Project told Reuters that the rogue file was downloaded about 6,500 times. However, the number of OS X users whose files were actually encrypted is likely much lower because the ransomware has a three-day delay built in and it was discovered quickly.
It's a mystery why the attackers went to great lengths to steal a legitimate Apple developer's certificate and break into the website of a trusted software project, only to distribute a flawed ransomware program.
Whatever the reason for this inconsistency, other cybercriminals will most likely try to replicate the attack, and they won't make the same mistakes.