151. Adversaries Abuse MST Transforms to Install Malware

Hello everyone!

Another day - another interesting technique. Today we'll look how adversaries, possible APT32, abuse MST transforms to drop malware to the compromised system.

The threat actors distribute ISO files with an LNK file with double extension (pdf.lnk), and two hidden files - an MSI installer and an MST file.

The LNK runs the following command:

"C:\Windows\System32\msiexec.exe" /qn /i WindowsPCHealthCheckSetup.msi TRANSFORMS=5ACXP.mst

As the result, a legitimate PcHealthCheck executable along with the malicious tbs.dll dropped by the MST transform under %LocalAppData%\PCHealthCheck\.

Can we hunt for suspicious MST transforms? Of course! For example:

event_type: "processcreatewin"

AND

proc_file_name: "msiexec.exe"

AND

cmdline: "TRANSFORMS"

See you tomorrow!

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