026. Threat Actors Abuse Printui.exe for DLL Search Order Hijacking

Hello everyone! In some cases even threat actors involved in cryptomining malware distribution leverage quite curious techniques. Today we'll look at an activity cluster colleagues from Red Canary track as Tangerine Turkey (I like their taxonomy as well (Hi Katie and team :)).

So, it's a worm, it's delivered via USB (yes, again), and it abuses printui.exe to run a cryptominer. Let's look how it works.

Once the victim opens a malicious LNK file on an infected USB drive, wscript.exe executes a script:

  • Look for suspicious VBS files executions from USB drives, for example: WScript.exe "D:\rootdir\x455198.vbs"
The script executes a BAT file from the same location. This file creates a new folder (mkdir "\\?\C:\Windows \System32"), abuses xcopy to copy printui.exe (xcopy "C:\Windows\System32\printui.exe" "C:\Windows \System32" /Y), copies malicious DAT file (xcopy "x805299.dat" "%SystemDrive%\Windows \System32" /Y), renames this DAT file (ren "%SystemDrive%\Windows \System32\x805299.dat" "printui.dll") and executes printui.exe for DLL Search Order Hijacking (start "" "%SystemDrive%\Windows \System32\printui.exe"):

  • Look for creation of system folders with a trailing space (e.g. "Windows ")
  • Look for copying of system binaries to suspicious locations
  • Looks for printui.exe executions from unexpected locations
  • Look for renaming DAT files to DLL files
Cryptominers are extremely noisy, but still quite interesting from detection perspective!

See you tomorrow!

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