Our Threat Response Unit (TRU) publishes security advisories, blogs, reports, industry publications and webinars based on its original research and the insights driven through proactive threat hunts.
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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).
Figure 1: The malicious ZIP archive shared by an attacker
The ZIP archive contains a decoy image and a shortcut file disguised as a PDF (Figure 2).
Figure 2: Contents of the password-protected ZIP archive
The shortcut file leverages PowerShell to retrieve additional payloads from the website. Here is the example of the spawned PowerShell one-liner command:
Figure 3: The web server hosting Remcos RAT and GuLoader payloads
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).
Figure 4: PDF decoy document
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
Figure 5: The obfuscated VBS file Figure 6: GuLoader shellcode payload written to the registry from the VBS file
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'.
The “State” registry key contains the obfuscated PowerShell script that reflectively loads the GuLoader shellcode in memory (Figure 8).
Figure 8: Obfuscated PowerShell script
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.
Figure 9: Decoded PowerShell secondary script
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).
Figure 10: Decoding the shellcode
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:
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.
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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eSentire Threat Response Unit (TRU)
The eSentire Threat Response Unit (TRU) is an industry-leading threat research team committed to helping your organization become more resilient. TRU is an elite team of threat hunters and researchers that supports our 24/7 Security Operations Centers (SOCs), builds threat detection models across the eSentire XDR Cloud Platform, and works as an extension of your security team to continuously improve our Managed Detection and Response service. By providing complete visibility across your attack surface and performing global threat sweeps and proactive hypothesis-driven threat hunts augmented by original threat research, we are laser-focused on defending your organization against known and unknown threats.
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