New JSSLoader Trojan Delivered Through XLL Files

Morphisec Labs has observed a new wave of JSSLoader infections this year. We’ve tracked JSSLoader activity since December 2020 and published a thorough report on the Russian criminal hacking group FIN7’s JSSLoader: The Evolution of the FIN7 JSSLoader. JSSLoader is a small, very capable .NET remote access trojan (RAT). Its capabilities include data exfiltration, persistence, auto-updating, additional payload delivery, and more. 

Attackers are now using .XLL files to deliver a new, obfuscated version of JSSLoader. We explain how this new malware variant utilizes the Excel add-ins feature to load the malware and inspect the changes inside.

Infection Chain

Figure 1: From .xll file to JSSLoader

This infection chain is similar to other XLL infections. The victim receives a malicious attachment, either an XLM or XLL file, inside an email. Once the attachment is downloaded and executed, Excel loads and executes the malicious code inside the .xll file, which then downloads the payload from a remote server. The payload is a new, similar variant of JSSLoader.

XLL Excel Add-in

The first stage of the malware responsible for downloading JSSLoader into an infected machine uses an Excel add-in file, denoted by an XLL file extension. Because the file isn’t signed, a popup

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