Tuesday, June 25, 2024

Cloud-Based Malware Attack Abusing Google Drive & Dropbox

A phishing email with a malicious zip attachment initiates the attack. The zip contains a single executable disguised as an Excel file using Left-To-Right Override characters (LTRO). 

LTRO makes the filename appears to have a harmless .xlsx extension (e.g., RFQ-101432620247flexe.xlsx) while it’s actually an executable (.exe).

Unsuspecting users, deceived by the icon and filename, launch the malware by opening the executable disguised as an Excel spreadsheet.

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A small (560KB) executable compiled with Visual Studio 2015 heavily employs XOR encoding (hex offset E2) to obfuscate strings and embedded files, hindering static analysis, and the executable drops VBScript payloads and a legitimate Excel file. 

Binary executable analysis: Payload overview

The VBScript execution is believed to be the core malicious functionality, while the Excel file likely serves as a decoy to prevent user suspicion in case the VBScript fails. 

Malware dropper executed the payload and wrote several files to the C:\ProgramData directory, a common location for malware due to its hidden nature and write permissions.

These files (20240416.xlsx, 3156.vbs, etc.) were embedded within the payload itself, encoded with XOR to hide their content from basic analysis. 

Payload dynamic analysis: ProgramData directory

The transitions from binary execution to a VBScript stage, which writes a heavily obfuscated script (3156.vbs) to the ProgramData folder and executes it using wscript.exe, suppressing errors, setting the working directory, and then creating objects to interact with the file system and shell. 

The core functionality involves executing two subsequent VBScripts (i4703.vbs and i6050.vbs) and a lure file (20240416.xlsx) using ShellExecute.

To ensure the completion of these tasks, the script sleeps for 3000 seconds before deleting all .vbs and .jse files from ProgramData.  

3156.vbs deobfuscated code

The malicious VBScript, i4703.vbs, creates a scheduled task disguised as a Google Chrome update to achieve persistence on the infected system, which runs a separate VBScript, located at “C:\Programdata\97468.tmp”, every minute with the highest privileges by mimicking legitimate system processes. 

According to Securonix, the obfuscated script further complicates analysis but its purpose of downloading additional malicious payloads will be investigated later.  

XML file for newly created scheduled task from i4703.vbs

The malware analyzed employs a multi-stage attack using VBScript and PowerShell.

In stage 4, two VBScripts (i6050.vbs and a scheduled task) are executed, which create additional scheduled tasks that launch temporary VBScript files (97468.tmp and 68904.tmp) every minute. 

These temporary VBScripts then use WScript.Shell to bypass execution policies and run PowerShell scripts (Tmp912.tmp, tmpdbx.ps1, Tmp703.tmp, zz.ps1) using various obfuscated techniques.  

The attacker leverages two sets of scheduled tasks to execute malicious PowerShell scripts every minute, as the first set executes VBScripts that download and run Tmp912.tmp and Tmp703.tmp.

Tmp912.tmp interacts with Dropbox by refreshing an access token, uploading a log file, and downloading tmpdbx.ps1. 

example of tmpdbx.ps1 contents

Tmp703.tmp interacts with Google Drive by refreshing an access token and downloading zz.ps1.

The second set of scheduled tasks executes tmpdbx.ps1 and zz.ps1, which download additional files from Dropbox and Google Drive, respectively, based on predefined patterns. 

The attackers deployed a PowerShell script to download a compressed binary and execute it in memory using reflection, which bypasses file scanning by antivirus and EDR software. 

The script extracts the binary (skipping the first 10 bytes), loads it as a .NET assembly, and invokes the “start” method within the assembly to establish a network connection with the attacker’s C2 server at a predefined IP and port. 

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