278. Hunting for Suspicious XLL Files

Hello everyone!

Adverasries keep experimenting with file types they use for phishing. And this time we'll look at UAC-0245 and XLL files.

According to the report, the adversary leverarage malicious XLL files to deliver CABINETRAT. Such files had some very interesting behaviors. For example, the file created another XLL file in "%APPDATA%\Microsoft\Excel\XLSTART\" enabling the adversary to use Office Template Macros (T1137.001) for persistence.

First of all, we can hunt for any suspicious XLL files:

event_type: "processcreatewin"

AND

cmdline: *.xll

Another detection opportunity is to look for suspicious XLL file creation events related to XLSTART folder:

event_type: "filecreatewin"

AND

file_path: "xlstart"

AND

file_extension: "xll"

See you tomorrow!

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