Ouroboros Ransomware decryption tool

Ouroboros ransomware has been around for more than a year in
various forms, operated by different cybercrime groups. Ouroboros, known to
spread via Remote Desktop Protocol bruteforce attacks and deceptive downloads, has
claimed a significant number of victims worldwide. We’re now happy to announce
the availability of a new decryptor that can restore the .Lazarus, and .Lazarus+
file extensions to their original, unencrypted form.

Please note that this tool only works for the Lazarus and
Lazarus+ extensions. The .Kronos variant is still under analysis.

How to tell if you are
infected with this specific variant of Ouroboros

On an infected computer, most files are encrypted and renamed to the original file name plus the [ID=XXXXXXXXXX][Mail= * ].Lazarus  or [ID=XXXXXXXXXX][Mail= * ].Lazarus+ strings. The ID consists of 10 random characters. Additionally, Ouroboros would create several ransom notes called Read-Me-Now.txt

Fig. 1: Lazarus ransom note
Fig. 2: Lazarus+ ransom note

If you are infected with this variant of Lazarus, you can download the decryptor right away. If you are interested in how Ouroboros works, you may want to read the next chapter.

Download the Ouroboros decryption tool

Brief technical

Affected files are encrypted with the AES 256 CBC algorithm based on the assembly aesenc/aesenclast instructions. This prevents the malware from running on older CPU models (pre-2010) that lack support for these instructions.

Fig. 3: Encryption routine with AES-NI support

The initialization vector used for encryption is hardcoded rather than generated, and it changes with each version:

Fig. 4: Lazarus initialization vector
Fig. 5: Lazarus+ initialization vector

The key is constructed using a combination of two PRNGs (pseudo-random number generators), Mersenne Twister and IsaacRandom, starting from the following alphabet:

After key generation, the ransomware attempts to contact a server to send the key, along with user information. In case of no response, the key will be discarded and replaced with a hard-coded one:

How to decrypt your

Step 1: Download the decryption tool below and save it somewhere on your computer.

Download the Ouroboros decryption tool

This tool does not require an active Internet

Step 2: Double-click BDLockCryptDecryptor.exe and allow it to run
elevated at the UAC prompt.

Step 3: Accept the End User License Agreement

Step 4: Select “Scan Entire System” if you want to search for all
encrypted files. Alternatively, just add the path to the folder containing the
encrypted files. We strongly recommend you also select “Backup files” before
starting decryption, should anything unexpected occur during the process. Start
decryption by clicking the “Scan” button.

At the end of this step, your files should be
decrypted. The tool generates an operation log at the %temp%BDRemovalToolBDRansomDecryptorBitdefenderLog.txt location.

If you need assistance, contact us at the feedback
address listed in the tool. We kindly ask you to attach the above-mentioned log
file to help you isolate potential issues and minimize response time.

deployment across the network

The tool can also be executed via command line. Should
you need to automate deployment of the tool inside a large network, you might
want to use this feature.  

  • -help – provides information on how to run the tool
    silently (this information gets written in the log file, not on the console
  • start – this argument allows the tool to run
    silently (no GUI)  
  • path – this argument specifies the
    path to scan 
  • test – this argument specifies
    the test path to a pair of original/encrypted files 
  • o0:1 – enable Scan entire system option
    (ignoring -path argument) 
  • o1:1 – enable Backup files option 
  • o2:1 – enable Overwrite existing

Command line examples:  

BDOuroborosDecryptor.exe start -path:”C:” -> the tool
starts with no GUI and scans C:  

BDOuroborosDecryptor.exe start o0:1 -> the tool
starts with no GUI and scans the entire system 

BDOuroborosDecryptor.exe start o0:1 o1:1
the tool scans the entire system, backs up the encrypted files and overwrites
present clean files 


This product includes software developed by the OpenSSL Project, for use in the OpenSSL Toolkit.  

*** This is a Security Bloggers Network syndicated blog from Bitdefender Labs authored by Bitdefender Team. Read the original post at: