July 21, 2016 By Larry Loeb 2 min read

Petya ransomware has always been known for the peculiar manner in which it encrypts an affected disk. While other threats target files on a one-to-one basis, Petya attempts to encrypt the computer’s master boot blocks.

One for the Good Guys

The first version of Petya needed a reboot before the bad stuff would happen, so security researchers found a vaccine that would stop the machine from rebooting. Point to the good guys.

Petya’s authors took a bit of time to regroup and then responded by adding a module called Mischa as a backup. If Petya couldn’t get running, Mischa would encrypt the victim’s files on a one-to-one basis.

Petya Ransomware Reboots

There’s more bubbling beneath the surface here: Each of those Petya versions was using the Salsa20 algorithm to encrypt the master file table, which would then render the disk inaccessible.

But a few implementation bugs left in the ransomware weakened the intended algorithm. It all stemmed from the fact that the malware’s author implemented Salsa20 incorrectly by generating only 16 bits of a key parameter instead of 32.

This opened a window to potentially recover encrypted data, according to one security researcher who addressed Petya previously on the Malwarebytes blog.

Third Time’s the Charm

The creator realized the mistake and addressed the problem in a new release of Petya, the researcher noted in another Malwarebytes post. This new version seems to feature the proper Salsa20.

According to SecurityWeek, the new version also features a more complex preprocessing algorithm. Petya is reaching maturity. However, Petya’s ransomware-as-a-service page lists the new release as a beta version. What will this self-proclaimed beta end up as?

In the past, Petya used spam emails disguised as job applications as a spear phishing technique. These spam emails included a link that ended up pointing to cloud storage locations. It seems prudent to steer clear of such emails unless you are certain that they come from trusted sources. Trust but verify.

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