Ransomware authors are using the bitcoin blockchain, which serves as the cryptocurrency’s public transaction ledger, to deliver decryption keys to victims.
The technique, which removes the burden of maintaining a reliable website-based infrastructure for cybercriminals, was observed in a recent version of the CTB-Locker ransomware that targets Web servers.
CTB-Locker has targeted Windows computers for a long time, but a PHP-based variant capable of infecting websites first appeared in February, marking an interesting evolution of this ransomware threat.
The decryption routine in the original PHP-based CTB-Locker version involved a script called access.php that served as a gateway to the attackers’ back-end server. This gateway script was hosted on multiple hacked websites and was necessary to obtain the decryption key after victims made a payment.
The approach was not reliable, according to researchers from Web security firm Sucuri, because those hacked websites could be cleaned by their owners. Constantly updating the list of gateways used by the malware was probably a hassle for the criminals.
Because of that, CTB-Locker’s creators came up with a new approach: using the bitcoin blockchain itself to deliver the decryption keys. This new behavior was observed by Sucuri’s researchers in a CTB-Locker version released in March.
The technique relies on a