Experts crack Petya ransomware, enable hard drive decryption for free

The technique is not exactly straightforward, but it works

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.

Join the Good Gear Guide newsletter!

Error: Please check your email address.

Tags securityPetyaransomware

Our Back to Business guide highlights the best products for you to boost your productivity at home, on the road, at the office, or in the classroom.

Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Lucian Constantin

IDG News Service
Show Comments

Essentials

Microsoft L5V-00027 Sculpt Ergonomic Keyboard Desktop

Learn more >

Lexar® JumpDrive® S57 USB 3.0 flash drive

Learn more >

Mobile

Lexar® JumpDrive® S45 USB 3.0 flash drive 

Learn more >

Exec

HD Pan/Tilt Wi-Fi Camera with Night Vision NC450

Learn more >

Lexar® Professional 1800x microSDHC™/microSDXC™ UHS-II cards 

Learn more >

Lexar® JumpDrive® C20c USB Type-C flash drive 

Learn more >

Audio-Technica ATH-ANC70 Noise Cancelling Headphones

Learn more >

Budget

Back To Business Guide

Click for more ›

Most Popular Reviews

Latest News Articles

Resources

PCW Evaluation Team

Azadeh Williams

HP OfficeJet Pro 8730

A smarter way to print for busy small business owners, combining speedy printing with scanning and copying, making it easier to produce high quality documents and images at a touch of a button.

Andrew Grant

HP OfficeJet Pro 8730

I've had a multifunction printer in the office going on 10 years now. It was a neat bit of kit back in the day -- print, copy, scan, fax -- when printing over WiFi felt a bit like magic. It’s seen better days though and an upgrade’s well overdue. This HP OfficeJet Pro 8730 looks like it ticks all the same boxes: print, copy, scan, and fax. (Really? Does anyone fax anything any more? I guess it's good to know the facility’s there, just in case.) Printing over WiFi is more-or- less standard these days.

Ed Dawson

HP OfficeJet Pro 8730

As a freelance writer who is always on the go, I like my technology to be both efficient and effective so I can do my job well. The HP OfficeJet Pro 8730 Inkjet Printer ticks all the boxes in terms of form factor, performance and user interface.

Michael Hargreaves

Windows 10 for Business / Dell XPS 13

I’d happily recommend this touchscreen laptop and Windows 10 as a great way to get serious work done at a desk or on the road.

Aysha Strobbe

Windows 10 / HP Spectre x360

Ultimately, I think the Windows 10 environment is excellent for me as it caters for so many different uses. The inclusion of the Xbox app is also great for when you need some downtime too!

Mark Escubio

Windows 10 / Lenovo Yoga 910

For me, the Xbox Play Anywhere is a great new feature as it allows you to play your current Xbox games with higher resolutions and better graphics without forking out extra cash for another copy. Although available titles are still scarce, but I’m sure it will grow in time.

Featured Content

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?