Meet Buran: The New Delphi Ransomware Delivered via RIG Exploit Kit
Buran is a new version of the Vega ransomware strain (a.k.a. Jamper, Ghost, Buhtrap) that attacked accountants from February through April 2019. The new Buran ransomware first was discovered by nao_sec in June 2019, delivered by the RIG Exploit Kit, as reported by BleepingComputer.
A specific trait of this ransomware is that the cryptolockers’ code was written in Object Pascal in Delphi IDE. That’s a programming language that was popular – mostly in Latin American and former Soviet Union countries – around two decades ago when it was being taught in colleges and technical universities.
The ransomware code contains a homemade RSA algorithm implementation that does not use Fermat primes (such as 65537, the largest Fermat prime) for public exponent e.
Let’s take a close look at this ransomware to find out the installation process, communication details, and encryption model.
The ransomware file is PE32 executable for MS Windows. It is 796160 bytes in size. The payload code is written and compiled in Borland Delphi, while the protector’s code is Microsoft Visual C++. According to the compilation timestamp, the binary was compiled on June 5, 2019.
The ransomware is downloaded from an IP address (188.8.131.52) using the following obfuscated Windows shell script, executed by an SFW exploit from the web site http://makemoneyeasy.live/.
You can find the infection process with details in app.any.run sandbox (https://app.any.run/tasks/4e32f20f-1228-4b2d-ae8d-4d472e586d87/).
Once executed, the ransomware checks its process command line arguments for the presence of “*”.
If there are no arguments, it executes the following batch script to copy the downloaded ransomware and register the reference to it in the autorun key.
The ransomware will then be started when Windows boots up.
After that, the service key is created in the Windows Registry where it stores the flags and encrypted session keys.
On start, Buran sends a check-in request to the IPlogger service that is uses to track a victim’s IP address.
It checks the ‘Knock’ parameter in the Windows registry and, if it is not found, the ransomware initiates the connection with the IPlogger service. After sending the request, Buran adds the ‘Knock’ parameter set to the ‘666’ string, which is supposed to be some hardcoded value that has no specific meaning. The same value is later inserted at the beginning of the file’s content for verification during encryption.
The encryption model has three layers:
- The ransomware generates a 256-bit key for every file and uses it to encrypt file data with AES-256-CBC.
- The ransomware also generates the session key pair (RSA-512) to encrypt a file key and store it in the file’s footer. The session key pair is also stored in the Windows Registry.
- The session private key is encrypted with the hardcoded master public RSA 2048-bit key and stored in the footer as well.
It is not possible to decrypt the affected files without knowing the master private key. The decryption tool sold by an attacker is generated for each specific computer and includes only the decrypted session private key for this system.
Once started, Buran generates the session RSA key pair.
An example of a Session Public Key that contains modulus (N) and public exponent (E):
An example of a session private key that contains modulus (N) and private exponent (D).
Then, the cryptolocker encrypts the session key pair with the master public RSA 2048-bit key hardcoded in the ransomware, and stores the encrypted session key pair Base64 encoded in the ‘[HKCU\Software\Buran\Service]’ Windows registry key.
A file key is generated using a variation of a linear congruential generator (LCG), with the time stamp counter value as a seed:
The key generation algorithm looks as follows:
The generated key has a length of 256 bits:
Buran targets files with the following extensions, which may contain sensitive information for a user:
html, html,htm, htm, xml, xml, doc, doc, xls, xls, ppt, ppt, rtf, rtf, dot, dot, xlw, xlw, pps, pps, xlt, xlt, hta, hta, pot, pot, pdf, pdf, xps, xps, xbap, xbap, xlsx, xlsx, pptx, pptx, docx, docx
The system files are not encrypted which prevents Windows from crashing. Also the cryptolocker does not encrypt its unlocker’s file.
boot.ini, bootfont.bin, bootsect.bak, desktop.ini, defender.exe, iconcache.db, master.exe, master.dat, ntdetect.com, ntldr, ntuser.dat, ntuser.dat.log, ntuser.ini, temp.txt, thumbs.db, unlock.exe, unlocker.exe
The file with the following extensions will be skipped as well:
bat, cmd, com, cpl, dll, msc, msp, pif, scr, sys, log, exe, buran
Buran skips the files in the following folders as they do not contain a user’s sensitive data, or may cause the operation system to crash:
:\System Volume Information\
\Windows Media Player\
\Windows Multimedia Platform\
\Windows Photo Viewer\
\Windows Portable Devices\
\Windows Photo Viewer\
\Embedded Lockdown Manager\
\Windows Defender Advanced Threat Protection\
If the file size is bigger than 66560 bytes, Buran splits the file into 65536-byte blocks.
In case of big files, it does not encrypt data in files after the 65572 (10024h) byte due to the performance and reliability considerations – one big file may delay encryption and could lead to a crash if the application hasn’t been thoroughly tested.
It also copies 1024 bytes of original data starting at 10000 and appends it, untouched after encrypted data block. This is done because the encrypted data contains 24 extra bytes added by Buran – namely the first 21 bytes of the header and 3 verification bytes that represent the string “666” that are added to the beginning of the data block to be encrypted.
Buran uses AES-256-CBC to encrypt file’s data.
Interestingly, Buran adds “666” to the file content before AES encryption. That is supposedly used to verify if the decryption has been successful. As mentioned before, the ‘Knock’ parameter in the Windows registry has the same value.
The session private RSA key is in the memory before it is encrypted and stored in the footer. Also, the session RSA key pair is stored in the Windows registry.
Once the file content has been encrypted, it is renamed by adding an additional extension that represents the victim’s computer Reference ID.
The overall file encryption process looks like the following in ProcessMonitor:
Buran checks the value of “Stop” parameter in [HKCU\Software\Buran\Stop]. If it equals the personal Reference ID, it terminates the encryption. This parameter is used by the Buran unlocker to stop encryption in case it is in progress at the moment the unlocker is running.
In the folder with encrypted files, Buran leaves the file “!!! YOUR FILES ARE ENCRYPTED !!!.TXT” with the following ransom note.
Final clean up
Once the mission is completed, the session RSA key pair is wiped from the Windows registry.
Buran executes the following shell script to delete itself at the end:
"C:\Windows\system32\cmd.exe" /c for /l %x in (1,1,999) do ( ping -n 3 127.1 & del "c:\Users\IEUser\AppData\Roaming\Microsoft\Windows\ctfmon.exe" & if not exist "c:\Users\IEUser\AppData\Roaming\Microsoft\Windows\ctfmon.exe" exit )
Buran/Vega ransomware has continually evolved since the beginning of 2019 and is now using the RIG Exploit Kit as protection against analysis. The ransomware is unusual from a programming perspective. The code is written in Object Pascal and compiled in Borland Delphi using the _fastcall/_register calling convention. The encryption code includes a “homemade” implementation of the RSA crypto algorithm without using the Fermat primes. The cryptographic functions are statically linked within the body. In the end, a PE protector is applied that complicates the malware analysis and makes it harder to find the encryption model.
Even though the cryptography code is not conventional, a victim still has to pay a ransom to get back the encrypted files or pay a ransom. One victim reported at the forum that $3000 was paid to get the unlocker.
As usual, Acronis Backup and Acronis True Image can protect your computer against Buran ransomware -- and service providers can protect their customers with Acronis Backup Cloud. That’s because these cyber protection solutions not only offer backup, but they also include our integrated Acronis Active Protection, our AI-enabled and behavior-based technology that is uniquely able to deal with zero-day ransomware threats.