Researchers have discovered a new phishing campaign in which threat actors distribute the Remcos RAT malware within UUEncoding (UUE) file attachments in emails purporting to be about importing or exporting shipments. The UUEncoding (UUE) file attachments are compressed with Power Archiver, a proprietary and cross-platform archive utility that supports both Windows and MacOS.

Use of UUEncoding (UUE) Files to Distribute Remcos RAT Malware

Researchers from AhnLab discovered that the threat actors behind the campaign, use UUEncoding files with a .UUE extension, which are designed to encode binary data in plain text format. These file formats are suitable for attachment in e-mail or Usenet messages. The malicious .UUE files encode a VBS script attached in phishing emails. The threat actors seem to have leveraged the file format and encoding technique as an attempt to bypass detection. [caption id="attachment_76665" align="alignnone" width="1024"] Source: asec.ahnlab.com[/caption] When decoded, the VBS script is obfuscated, making it difficult for researchers to analyze. The script saves a PowerShell script into the %Temp% directory and executes it. The running script then downloads the Haartoppens.Eft file, which executes an additional PowerShell script. This script is also obfuscated and is designed to load a shellcode to the wab.exe process. [caption id="attachment_76666" align="alignnone" width="638"] Source: asec.ahnlab.com[/caption] The shellcode maintains its persistence by adding a registry key to the infected system, and then accesses a remote C&C server to load additional instructions. The instructions ultimately download the Remcos RAT malware for execution on infected systems.

Remcos RAT malware

The Remcos RAT collects system information from infected systems and stores keylogging data in the %AppData% directory. The malware then sends this data to the remote command-and-control (C&C) server, which is hosted through a DuckDNS domain. [caption id="attachment_76667" align="alignnone" width="894"] Source: asec.ahnlab.com[/caption] Remcos is a commercial remote access tool (RAT) that is advertised as a legitimate tool, but has been observed in numerous threat actor campaigns. Successful loading of Remcos opens a backdoor on targeted systems, allowing for complete control. The researchers have shared the following indicators to help detect and stop this campaign: IOCs (Indicators of Compromise)

• b066e5f4a0f2809924becfffa62ddd3b (Invoice_order_new.uue)
• 7e6ca4b3c4d1158f5e92f55fa9742601 (Invoice_order_new.vbs)
• fd14369743f0ccd3feaacca94d29a2b1 (Talehmmedes.txt)
• eaec85388bfaa2cffbfeae5a497124f0 (mtzDpHLetMLypaaA173.bin)

File Detection

• Downloader/VBS.Agent (2024.05.17.01)
• Data/BIN.Encoded (2024.05.24.00)

C&C (Command & Control) Servers

• frabyst44habvous1.duckdns[.]org:2980:0
• frabyst44habvous1.duckdns[.]org:2981:1
• frabyst44habvous2.duckdns[.]org:2980:0

The researchers also shared the following general recommendations to avoid similar phishing campaigns:

• Refrain from accessing emails from unknown sources.
• Refrain from running or enabling macro commands when accessing downloaded attachment files. Users can set programs to highest levels of security, as lower levels may automatically execute macro commands without displaying any notification.
•  Update anti-malware engines to their latest versions.

The UUE file format has previously been used in several malicious campaigns due to its ability to easily evade detection from security tools, with a researcher previously discovering a UUEncode vulnerability in the main Python program. Media Disclaimer: This report is based on internal and external research obtained through various means. The information provided is for reference purposes only, and users bear full responsibility for their reliance on it. The Cyber Express assumes no liability for the accuracy or consequences of using this information.