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SquirrelWaffle: New Malware Loader Delivering Cobalt Strike and QakBot

Oct 07 2021

Co-authored by Gustavo Palazolo and Ghanashyam Satpathy

Summary

In September of 2021, a new malware family named SquirrelWaffle joined the threat landscape. It spread through malicious Microsoft Office documents attached in spam emails

The infection flow starts with a ZIP file that contains the malicious Office document. When the file is opened by the victim, the malicious VBA macros download SquirrelWaffle DLL, which eventually leads to deploying another threat, such as CobaltStrike or QakBot.

In this blog post, we will analyze two variants of the malicious Office documents that deliver SquirrelWaffle. We will also analyze the final SquirrelWaffle payload and how the last stage URLs are being protected inside the binary.

SquirrelWaffle Office Documents

We have identified two variants used to deliver SquirrelWaffle, a Microsoft Word document and a Microsoft Excel spreadsheet. 

Screenshot of SquirrelWaffle infected documents
SquirrelWaffle malicious documents

Malicious Word Document

The first variant is a malicious Microsoft Word file that mimics a DocuSign document, asking the victim to click “Enable Editing” and “Enable Content” to view the content. 

Example of a SquirrelWaffle infected Word document
SquirrelWaffle malicious Word document

The file contains several VBA macros, including junk code. The main routine lies in a function named “eFile”, which is executed by the “AutoOpen” functionality.

Window showing Malicious VBA function
Malicious VBA function

Aside from all the junk added by the developer, we can see two important pieces of data when we open the VBA editor: a PowerShell script and a batch script that executes the PowerShell script. 

These routines are kept inside the text property of Visual Basic Control instead of in a regular VBA module. The purpose is to evade AV detection.

Screenshots of Malicious code inside the Word file
Malicious code inside the Word file

Looking at the “eFile” function, we can see that both PowerShell and the batch script are created in the user’s AppData directory, respectively named “www.ps1” and “www.txt”.

Screenshot showing VBA function creating payloads in disk
VBA function creating payloads in disk

This behavior can be observed with Procmon.

Screenshot of VBA function dropping payloads in disk.
VBA function dropping payloads in disk.

Later, the VBA code executes the batch script, using the Windows “cscript.exe” binary.

Screenshot showing Malicious batch script executed by the infected document.
Malicious batch script executed by the malicious document.

Looking at those files closely, we can see that the PowerShell script is responsible for downloading SquirrelWaffle DLL using five distinct URLs, likely to add more resilience to the process. 

The downloaded DLLs are saved into “C:\ProgramData\” and named “www[N].dll” where [N] is a number from 1 to 5.

Screenshot of PowerShell script that downloads SquirrelWaffle DLL.
PowerShell script that downloads SquirrelWaffle DLL.

And the batch script, which is executed by the malicious document, is responsible for executing the PowerShell script and the SquirrelWaffe payload DLL.

Screenshot of Batch script that is executed by the infected document.
Batch script that is executed by the malicious document.

Once downloaded, the DLL is executed through “rundll32.exe”, which calls an exported function named “ldr”.

Both “cscript.exe” and “rundll32.exe” are legitimate files from Windows, used by this sample to connect to the C&C servers and to download and execute the next stage payloads. This technique is known as Living-off-the-Land (LoL), which consists of using legitimate binaries to perform malicious activities. We have already covered other malware families that employ this technique, such as BazarLoader.

Screenshot of Batch script executing SquirrelWaffle DLL.
Batch script executing SquirrelWaffle DLL.

Malicious Excel Document

The second variant identified by Netskope is a malicious Microsoft Excel file, containing a fake message that also tries to deceive the victim into clicking the “Enable Editing” and “Enable Content” buttons.

Example of Infected Microsoft Excel document, delivering SquirrelWaffle.
Malicious Microsoft Excel document, delivering SquirrelWaffle.

The file uses Excel 4.0 (XML) macros that are obfuscated and spread across many hidden sheets in the document.

Screenshot of Hidden sheets inside the infected Excel file.
Hidden sheets inside the malicious Excel file.

The developer also changed the font color to hide the code, which can be revealed when we change the font property as shown below.

Showing Hidden code inside the hidden sheet.
Hidden code inside the hidden sheet.

When the Macros are executed, the obfuscated code is written into seven different cells, containing many calls to Windows APIs.

Example showing Malicious code inside the infected Excel document.
Malicious code inside the malicious Excel document.

Simply put, this code contacts three different URLs to download SquirrelWaffle DLL, which is saved into “C:\Datop\test[N].test”, where [N] is null or a number (1 and 2). The DLL is then executed through Windows “ShellExecuteA” API.

SquirrelWaffle DLL

Regardless of the variants we described, the goal is to download and execute SquirrelWaffle DLL. In this section, we will analyze a payload