237. That's How Adversaries Abuse Expand.exe

Hello everyone!

In many cases adversaries are deobfuscating or decoding downloaded or dropped payloads. Of course, they often use LOLBAS for it. So let's look at an example of Deobfuscate/Decode Files or Information (T1140).

As always, we need some procedure examples. And we'll extract it from this report on Ghostwriter. Here're a few examples of how the threat actors abuse expand.exe:

"C:\Windows\System32\expand.exe" "C:\Users\<USER>\AppData\Local\Temp\sdw9gobh0n" "C:\Users\<USER>\AppData\Local\Logs\sdw9gobh0n.log"

"%windir%\System32\expand.exe" %ProgramData%\OfficeRuntimeBroker.xlam %ProgramData%\~OfficeRuntimeBroker.dat

Usually it's used with CAB files, so we can search for executions without this file type among command line arguments:

event_type: "processcreatewin"

AND

proc_file_path: "expand.exe"

AND NOT

cmdline: *cab

Tune it based on your own telemetry! For example, you may also need to exclude .tmp files.

See you tomorrow!

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