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Experts have uncovered a new cybercrime campaign abusing Excel spreadsheets to distribute nasty trojan malware. 

Cybersecurity researchers from Morphisec Labs have spotted the Russian threat actor, FIN7 (aka Carbanak), distributing a small, lightweight Remote Access Trojan (RAT), a variant of JSSLoader, through mailed XLL and XLM files.

These files carry weaponized add-ins, which allow the attackers to exfiltrate data, establish persistence on the target endpoint, and have the RAT perform auto-updates, among other things.

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This particular RAT has been around since December 2020. In this campaign, though, the attackers are trying to distribute an unsigned file, meaning Excel will show a clear warning that running the file comes with risks. 

The researchers explain that these XLL files, should the victim enable them, use malicious code found in the xlAutoOpen function, load themselves into memory, after which they download the stage-two malware from a remote server.

After that, they use an API call to run the process.

Even though it has the same execution flow, this JSSLoader variant is a bit different from the older ones, as it is capable of renaming all functions and variables, in a bid to stay below the radar of antivirus and other security solutions.

It also splits the strings into sub-strings and chains them at runtime, to further avoid being detected by string-based YARA rules.

These new detection-avoiding methods, together with the way the payload is delivered, are enough for the RAT to remain out of sight of most antivirus and endpoint protection solutions, Morphisec added.

FIN7 can use it for unabated lateral movement throughout the compromised network, for days, or even weeks, before being spotted, the company said. 

The threat actor is a relatively creative criminal group, which recently made headlines in January 2022 when it was found to be mailing malicious thumb drives to victims.

Via: BleepingComputer