Ransomware Uses Blockchains to Transmit Decryption Keys

Ransomware authors are always looking for new techniques to make their extortion business more efficient, and researchers have discovered a new method that cybercriminals are using to deliver decryption keys: blockchains.

A variant of the CTB-Locker ransomware emerged a couple of months ago, with a focus on encrypting websites, but Sucuri researchers suggest that it hasn’t been as successful as its authors wanted it to be. In fact, they suggest that no website owner affected by the ransomware has paid the ransom and that this is reflected in the fact that the ransom has dropped from 0.8 Bitcoins to only 0.4 BTC.

Unlike CTB-Locker for desktop computers, CTB-Locker for Websites is focused on compromising servers and encrypting the files that normally keep websites online. As it turns out, most website owners affected by this malware were able to restore their files from backups, while those who haven’t yet might have actually abandoned their websites.

What makes CTB-Locker for Websites interesting, however, is the fact that it uses blockchains, which are chains of verified transactions used in the Bitcoin world, to transmit decryption keys. Since blockchains are public, they can be tracked by anyone, and specialized services exist that allow users view Bitcoin blockchains.

CTB-Locker operators are taking advantage of a feature introduced in 2014, when the Bitcoin protocol started allowing for small blocks of arbitrary text (metadata) to be included in the OP_RETURN field. The feature made blockchains applicable to fields unrelated to Bitcoin, and cybercriminals are abusing it, it seems.

According to Sucuri researchers, the ransomware operators create a new Bitcoin wallet with a unique address for each encrypted website, and they publish the address to the ransom demand page. When the victim pays the ransom, the hackers check the transferred sum and the wallet’s blockchain is appended with a new transaction whose OP_RETURN field contains the decryption key.

The OP_RETURN transaction is validated and propagated through distributed nodes of the Bitcoin system, and it also becomes visible in services that provide information on blockchains. This is why cybercriminals advise victims to track their transactions on the blockhain.info site.

The use of blockchain to transmit decryption keys is much more reliable than using payment gates and third-party hacked sites, the researchers note. They also say that the March version of the CTB-Locker reads the keys directly from public, more reliable blockchain information services, making the entire process public and transparent, while also keeping things anonymous and not traceable to real IPs.

“On the other hand, even smart use of a new technology is not a guarantee of success when you porting a tried-and-true business model from one niche to another. The devil is in the details. In the case of website crypto-ransomware, such overlooked details supply easy access to backups for most websites, which allows them to ignore ransom demands,” the researchers say.

Related: “Vaccine” Available for CTB-Locker, Locky, TeslaCrypt

Related: New CTB-Locker Variant Allows Victims to Recover 5 Files for Free