Yet Another Archive Format Smuggling Malware

guest · Jun 24, 2021

Phishing attack's unusual file attachment is a double-edged sword
June 24, 2021
https://www.bleepingcomputer.com/ne...sual-file-attachment-is-a-double-edged-sword/

A threat actor uses an unusual attachment to bypass security software that is a double-edged sword that may work against them.

As secure email gateways and security software become more advanced and adapt to ever-changing phishing campaigns, threat actors resort to more unusual file formats to bypass detection.
In the past, phishing scams switched to unusual attachments such as ISO files or TAR files which are not commonly found as email attachments.
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Using WIM to bypass security
In a new report by Trustwave, researchers explain how a threat actor has begun to utilize WIM (Windows Imaging Format) attachments to distribute the Agent Tesla remote access trojan.

"All the WIM files we gathered from our samples contain Agent Tesla malware. This threat is a Remote Access Trojan (RAT) written in .Net which can take full control over a compromised system and can exfiltrate data via HTTP, SMTP, FTP, and Telegram," explains Trustwave security researcher Diana Lopera in the report.
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Windows Imaging Format (WIM) files are a file-based disk image format that Microsoft developed to aid in deploying Windows Vista and later operating systems.

WIM files are used to pack an entire drive, with all of its files and folders, into a single file for easy distribution.
However, while WIM files may be less likely to be detected, phishing campaigns that use them have a bigger problem as Windows has no built-in mechanism to open a WIM file.
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Trustwave: Yet Another Archive Format Smuggling Malware

The use of novel disk image files to encapsulate malware distributed via spam has been a theme that we have highlighted over the past couple of years. As anticipated, we have seen more disk image file formats being used, in addition to .ISO, .IMG, and .DAA which we blogged about.

We recently spotted another file format being used. These spams, spoofing courier companies, contain a malicious WIM (Windows Imaging Format) file disguised as an invoice or consignment note. WIM is a file-based disk image format developed by Microsoft. The file format serves to deploy Windows software components and updates ever since Windows Vista. This format uses a ".wim" extension and its content can be extracted using archiving tools like 7Zip, PowerISO, and PeaZip. Below are some samples from this campaign.
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Summary
Encapsulating malware in an unusual archive file format is one of the common ways to bypass gateways and scanners. However, this strategy also poses a hurdle – the target system must recognize the file type or at least have a tool which can unpack and process the file. In contrast to the more popular .IMG and .ISO disk image files, WIM files are not supported by Windows built-in ability to mount disk image files. Moreover, the other popular archive utilities WinRAR and WinZip do not recognize the WIM disk image. WIM files can be processed with the widely used 7Zip.

In terms of suspiciousness, an inbound email containing a WIM file is not usual and should raise a few flags. Administrators could look to block these outright. Trustwave MailMarshal detects these threats bundled inside unusual disk imaging containers using a combination of its unpacking engine and its multi-layered threat detection technology.
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