The Newest Malicious Actor: “Squirrelwaffle” Malicious Doc.

November 10, 2021

Authored By Kiran Raj

Due to their widespread use, Office Documents are commonly used by Malicious actors as a way to distribute their malware. McAfee Labs have observed a new threat “Squirrelwaffle” which is one such emerging malware that was observed using office documents in mid-September that infects systems with CobaltStrike.

In this Blog, we will have a quick look at the SquirrelWaffle malicious doc and understand the Initial infection vector.

Geolocation based stats of Squirrelwaffle malicious doc observed by McAfee from September 2021

 

Figure1- Geo based stats of SquirrelWaffle Malicious Doc
Figure1- Geo-based stats of SquirrelWaffle Malicious Doc

 

Infection Chain

  1. The initial attack vector is a phishing email with a malicious link hosting malicious docs
  2. On clicking the URL, a ZIP archived malicious doc is downloaded
  3. The malicious doc is weaponized with AutoOpen VBA function. Upon opening the malicious doc, it drops a VBS file containing obfuscated powershell
  4. The dropped VBS script is invoked via exe to download malicious DLLs
  5. Thedownloaded DLLs are executed via exe with an argument of export function “ldr”
Figure-2: Infection Chain
Figure-2: Infection Chain

Malicious Doc Analysis

Here is how the face of the document looks when we open the document (figure 3). Normally, the macros are disabled to run by default by Microsoft Office. The malware authors are aware of this and hence present a lure image to trick the victims guiding them into enabling the macros.

Figure-3: Image of Word Document Face
Figure-3: Image of Word Document Face

UserForms and VBA

The VBA Userform Label components present in the Word document (Figure-4) is used to store all the content required for the VBS file. In Figure-3, we can see the userform’s Labelbox “t2” has VBS code in its caption.

Sub routine “eFile()” retrieves the LabelBox captions and writes it to a C:\Programdata\Pin.vbs and executes it using cscript.exe

Cmd line: cmd /c cscript.exe C:\Programdata\Pin.vbs

Figure-4: Image of Userforms and VBA
Figure-4: Image of Userforms and VBA

VBS Script Analysis

The dropped VBS Script is obfuscated (Figure-5) and contains 5 URLs that host payloads. The script runs in a loop to download payloads using powershell and writes to C:\Programdata location in the format /www-[1-5].dll/. Once the payloads are downloaded, it is executed using rundll32.exe with export function name as parameter “ldr”

Figure-5: Obfuscated VBS script
Figure-5: Obfuscated VBS script

De-obfuscated VBS script

VBS script after de-obfuscating (Figure-6)

Figure-6: De-obfuscated VBS script
Figure-6: De-obfuscated VBS script

MITRE ATT&CK

Different techniques & tactics are used by the malware and we mapped these with the MITRE ATT&CK platform.

  • Command and Scripting Interpreter (T-1059)

Malicious doc VBA drops and invokes VBS script.

CMD: cscript.exe C:\ProgramData\pin.vbs

 

  • Signed Binary Proxy Execution (T1218)

Rundll32.exe is used to execute the dropped payload

CMD: rundll32.exe C:\ProgramData\www1.dll,ldr

IOC

Type Value Scanner Detection Name
Main Word Document 195eba46828b9dfde47ffecdf61d9672db1a8bf13cd9ff03b71074db458b6cdf ENS,

WSS

 

 W97M/Downloader.dsl

 
Downloaded DLL

 

 85d0b72fe822fd6c22827b4da1917d2c1f2d9faa838e003e78e533384ea80939 ENS,

WSS

 RDN/Squirrelwaffle
URLs to download DLL ·       priyacareers.com

·       bussiness-z.ml

·       cablingpoint.com

·       bonus.corporatebusinessmachines.co.in

·       perfectdemos.com

 WebAdvisor Blocked

 

 

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