Word Library Add-Ins

It' possible to persist in the userland by abusing word library add-ins by putting your malicious DLL into a Word's trusted location. Once the DLL is there, the Word will load it next time it is run.

Execution

Get Word's trusted locations where library add-ins can be dropped:

attacker@target

 Get-ChildItem "hkcu:\Software\Microsoft\Office\16.0\Word\Security\Trusted Locations"

Those trusted locations are actually defined in Word's Security Center if you have access to the GUI:

Let's create a simple DLL that will launch a notepad.exe once the DLL addin is loaded:

Compile the DLL and copy it over to Startup folder and rename it to evilm64.wll:

mv .\evilm64.dll .\evilm64.wll

Next time the victim opens up Word, evilm64.wll will be loaded and executed:

Interesting to note that Process Explorer does not see the evilm64.wll loaded in any of the currently running processes:

...although we can definitely see that the add-in is now recognized by Word:

This technique did not work for me on Office 365 version, but worked on Office Professional. Not sure if there's a bug in the 365 version or it's just a limitation of that version.

References

Persistence: "the continued or prolonged existence of something": Part 1 - Microsoft Office - MDSecMDSec

Add-In Opportunities for Office PersistenceF-Secure Labs