The program is dubbed Satana -- meaning "Satan" in Italian and Romanian -- and, according to researchers from security firm Malwarebytes, it is functional but still under development.
Satana is the second ransomware threat affecting the MBR and seems inspired by another program, Petya, that appeared in March.
The MBR code is stored in the first sectors of a hard disk drive, contains information about the disk's partitions and launches the operating system's boot loader. Without a proper MBR, computers don't know which partitions contain the OS and how to start it.
There are significant differences between Satana and Petya. For example, Petya replaces the MBR in order to launch a custom bootloader that then encrypts the system's master file table (MFT) -- a special file on NTFS partitions that contains information about all other files, like names, sizes and mappings to the hard disk sectors.
Satana doesn't encrypt the MFT. It just replaces the MBR with its own code and stores an encrypted version of the original boot record so it can restore it later if the victim pays the ransom. This leaves the computer unbootable, but can be fixed more easily than if the MFT had also been encrypted.
In May, Petya was combined with a separate ransomware program, called Mischa, which exhibits a more traditional behavior: it encrypts users' personal files directly if it can't obtain administrator privileges to attack the MBR and MFT.
Satana uses the same combination of traditional file encryption and MBR encryption, but in the same program. It first encrypts user files with specific extensions and then waits patiently for the first reboot, at which time it replaces the MBR. The user then sees a screen demanding a ransom payment of 0.5 bitcoin (about US$340).
This routine makes it harder for nontechnical users to restore their systems, because it forces them to use a separate computer to make the payment, as the affected computer can no longer boot into Windows.
"Unfortunately, at this time there is no way to decrypt Satana encrypted files for free," said Lawrence Abrams, founder of the BleepingComputer.com tech support forum, in a blog post.
Users might be able to repair the MBR by using the Windows recovery options, but this requires working with the Windows command line and the bootrec.exe (boot recovery) tool, so it is likely beyond the abilities of typical users.
The current version of Satana hasn’t yet been widely distributed, and the researchers don’t expect that it will because the code is not yet mature and has flaws. However, they believe this version will likely serve as a base for future improvements.