Malicious Microsoft Excel add-ins used to deliver RAT malware

Rat hiding under furniture

Researchers report a brand new model of the JSSLoader distant entry trojan being distributed malicious Microsoft Excel addins.

The actual RAT (distant entry trojan) has been circulated within the wild since December 2020, linked to the financially-motivated Russian hacking group FIN7, also called “Carbanak.”

JSSLoader is a small, light-weight RAT that may carry out knowledge exfiltration, set up persistence, fetch and cargo extra payloads, auto-update itself, and extra.

Excel add-ins

The most recent marketing campaign involving a stealthier new model of JSSLoader was noticed by risk analysts at Morphisec Labs, who say the supply mechanism is presently phishing emails with XLL or XLM attachments.

Abuse of Excel XLL add-ins isn’t new, as they’re generally used for legit functions, akin to importing knowledge right into a worksheet or extending the performance of Excel.

Within the ongoing marketing campaign, nonetheless, the risk actors use an unsigned file, so Excel will present the sufferer a transparent warning in regards to the dangers of executing it.

Warning about unsigned XLL file
Safety warning about unsigned XLL file

When enabled, the XLL information use malicious code inside an xlAutoOpen perform to load itself into reminiscence after which obtain the payload from a distant server and execute it as a brand new course of by way of an API name.

Malware loading and execution flow
Malware loading and execution move (Morphisec)

Extra refined obfuscation

The risk actor repeatedly refreshes the Person-Agent on the XLL information to evade EDRs that consolidate detection info from the whole community.

Changing the User-Agent on each XLL sample
Altering the Person-Agent on every XLL pattern (Morphisec)

In comparison with older variations, the brand new JSSLoader has the identical execution move, but it surely now comes with a brand new layer of string obfuscation that features renaming all capabilities and variables.

String obfuscation added on the new version
String obfuscation added on the brand new JSSLoader (Morphisec)

To evade detection from string-based YARA guidelines utilized by defenders, the brand new RAT has cut up the strings into sub-strings and concatenates them at runtime.

Strings comparison between new and old versions
Strings comparability between new and previous variations (Morphisec)

Lastly, the string decoding mechanism is straightforward in order to depart a minimal footprint and scale back the possibilities of being detected by static risk scanners.

Morphisec experiences that these new additions mixed with the XLL file supply are sufficient to stop detection from next-generation antivirus (NGAV) and endpoint detection and response (EDR) options difficult and even implausible.

This allows FIN7 to maneuver within the compromised community undeterred for a number of days or perhaps weeks earlier than the defenders load matching signatures on instruments that complement AI-based detection options.

FIN7 is a resourceful risk group that has beforehand delivered malware-laced USBs alongside teddy bear presents, tried to rent community penetration specialists by posing as a legit safety agency, and despatched ransomware-carrying USBs by way of submit mail.

The brand new and stealthier model of JSSLoader is just one a part of their arsenal, serving to them conceal in networks for longer with out being detected and stopped.

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