Securonix is tracking a phishing campaign that’s targeting the Ukrainian military with malware-laden attachments posing as drone instruction manuals. The threat actor is using Microsoft help files (.chm) to deliver the malware.
“The malicious .chm file was intentionally weaponized to execute a PowerShell one-liner on the victim's machine,” the researchers write. “Microsoft help files have been used maliciously in the past, though today they are less common as Microsoft stopped supporting the .chm file format in 2007. They can, however, be opened and executed in modern Windows versions....Code execution through a .chm file is a well known technique and there are several online tools available for building one. It works by passing in special HTML parameters which can call a child process such as cmd.exe or powershell.exe, along with command line arguments.”
The researchers continue, “The payload is an obfuscated binary that gets XOR’d and decoded to produce a beacon payload for MerlinAgent malware. Once the payload establishes communication back to its C2 server, the attackers would have full control over the victim host. While the attack chain is quite simple, the attackers leveraged some pretty complex TTPs and obfuscation methods in order to evade detection.”
Securonix notes that the social engineering aspect of the campaign allows the documents to bypass technical defenses.
“It’s apparent that this attack was highly targeted towards the Ukrainian military given the language of the document, and its targeted nature,” the researchers write. “Files and documents used in the attack chain are very capable of bypassing defenses, scoring 0 detections for the malicious .chm file. Typically receiving a Microsoft help file over the internet would be considered unusual. However, the attackers framed the lure documents to appear as something an unsuspecting victim might expect to appear in a help themed document or file.”
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