Security experts have devised a method that allows users to recover data from computers infected with the Petya ransomware program without paying money to cybercriminals.

Petya appeared on researchers' radar last month when criminals distributed it to companies through spam emails that masqueraded as job applications. It stood out from other file-encrypting ransomware programs because it overwrites a hard disk drive's master boot record (MBR), leaving infected computers unable to boot into the operating system.

The program replaces the drive's legitimate MBR code, which normally starts the operating system, with code that encrypts the master file table (MFT) and shows a ransom note. The MFT is a special file on NTFS volumes that contains information about all other files: their name, size and mapping to hard disk sectors.

The actual contents of the user's files are not encrypted, but without the MFT, the OS no longer knows where those files are located on disk. Using data recovery tools to reconstruct files might be possible, but it is not guaranteed to work perfectly and would be time-consuming.

Fortunately, resorting to that method is no longer necessary, and neither is paying Petya's authors. Someone using the online handle leostone devised an algorithm to crack the key needed to restore the MFT and recover from a Petya infection.

Computer experts from the popular tech support forum BleepingComputer.com confirmed that the technique works, but it requires extracting some data from an affected hard drive: 512 bytes starting at sector 55 (0x37h) with an offset of 0 and an 8-byte nonce from sector 54 (0x36) offset 33 (0x21).

If that sounds complicated, no worries: Fabian Wosar from security firm Emsisoft created a simple and free tool that can do it for you. However, because the infected computer can no longer boot into Windows, using the tool requires taking out the affected hard drive and connecting it to a different computer where the tool can run. An external, USB-based hard drive docking station can be used.

The data extracted by the tool must be inputted into a Web application created by leostone that will use it to crack the key. The user must then put the affected hard drive back into the original computer, boot from it, and input the key on the ransom screen displayed by Petya.

"Once the hard drive is decrypted, the ransomware will prompt you to reboot your computer and it should now boot normally," BleepingComputer.com founder Lawrence Abrams, wrote in a blog post.