Malspam campaign uses icon files to delivers NanoCore RAT

Pierluigi Paganini March 12, 2021

Researchers at Trustwave spotted a new malspam campaign that is abusing icon files to trick victims into installing the NanoCore Trojan. 

Researchers at Trustwave have spoted a new malspam campaign that is abusing icon files to trick victims into executing the NanoCore remote access Trojan. 

The emails use a .zipx file attachment, a .zipx file is a ZIP archive compressed using the most recent compression methods of the WinZip archiver to provide optimal results. 

The messages claim to be from a “Purchase Manager” of organizations that are being spoofed by attackers, they use an attachment named “NEW PURCHASE ORDER.pdf*.zipx” which is actually an image binary file.

“The attachments, which have a filename format “NEW PURCHASE ORDER.pdf*.zipx”, are actually image (Icon) binary files, with attached extra data, which happens to be RAR. This file format abuse is similar to what we have seen previously.” reads the analysis published by Trustwave.

malspam icon files

The binary files have attached extra data in a .RAR format. 

The attackers are using an icon file to avoid any scanning email gateways. 

A prerequisite for the success of this campaign is that the victim has installed an unzip tool that can extract the executable file inside the attachment.  Upon clicking on the attachment, an executable file is extracted.

“Interestingly, 7Zip can also extract the content of the latest .zipx sample. 7Zip initially tries to open the files as a ZIP archive and fails, but afterward, 7Zip recognizes the .zipx files as Rar5 archives and can get their contents unpacked. Unlike in the previous blog, there is no need for the extension of the recent attachments to be renamed to something else other than .zipx or .zip just for their executables to be extracted using 7Zip.” states the report. “The executables we gathered have a similar name to that of the .zipx attachment, “NEW PURCHASE ORDER*.exe”. Also, the icon at the start of the .zipx files is actually the icon used on the EXE files within the archive.”

The analysis of the EXE files employed in the campaign revealed that the threat actors attempted to install the NanoCore RAT version on the victims’ systems. Nanocore RAT is a “general purpose” malware with specific client factories available to everyone and easily accessible. The RAT implements information stealer and keylogger capabilities, it also allows to deliver of additional payloads on the victim’s system.  

The Nanocore RAT creates copies of itself in the AppData folder and is able to inject its malicious code at RegSvcs.exe process.

“The recent malspams have the same goal like the ones we investigated almost two years ago and that is to effectively hide the malicious executable from anti-malware and email scanners by abusing the file format of the “.zipx” attachment, which in this case is an Icon file with added surprises.” concludes the report.

Experts also published Indicators of Compromise (IoCs) for the threat.

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Pierluigi Paganini

(SecurityAffairs – hacking, NanoCore Trojan)

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