| ID | Name |
|---|---|
| T1137.001 | Office Template Macros |
| T1137.002 | Office Test |
| T1137.003 | Outlook Forms |
| T1137.004 | Outlook Home Page |
| T1137.005 | Outlook Rules |
| T1137.006 | Add-ins |
Adversaries may abuse Microsoft Office templates to obtain persistence on a compromised system. Microsoft Office contains templates that are part of common Office applications and are used to customize styles. The base templates within the application are used each time an application starts. [1]
Office Visual Basic for Applications (VBA) macros [2] can be inserted into the base template and used to execute code when the respective Office application starts in order to obtain persistence. Examples for both Word and Excel have been discovered and published. By default, Word has a Normal.dotm template created that can be modified to include a malicious macro. Excel does not have a template file created by default, but one can be added that will automatically be loaded.[3][4] Shared templates may also be stored and pulled from remote locations.[5]
Word Normal.dotm location:C:\Users\<username>\AppData\Roaming\Microsoft\Templates\Normal.dotm
Excel Personal.xlsb location:C:\Users\<username>\AppData\Roaming\Microsoft\Excel\XLSTART\PERSONAL.XLSB
Adversaries may also change the location of the base template to point to their own by hijacking the application's search order, e.g. Word 2016 will first look for Normal.dotm under C:\Program Files (x86)\Microsoft Office\root\Office16\, or by modifying the GlobalDotName registry key. By modifying the GlobalDotName registry key an adversary can specify an arbitrary location, file name, and file extension to use for the template that will be loaded on application startup. To abuse GlobalDotName, adversaries may first need to register the template as a trusted document or place it in a trusted location.[5]
An adversary may need to enable macros to execute unrestricted depending on the system or enterprise security policy on use of macros.
| ID | Name | Description |
|---|---|---|
| S0475 | BackConfig |
BackConfig has the ability to use hidden columns in Excel spreadsheets to store executable files or commands for VBA macros.[6] |
| S0154 | Cobalt Strike |
Cobalt Strike has the ability to use an Excel Workbook to execute additional code by enabling Office to trust macros and execute code without user permission.[7] |
| G0069 | MuddyWater |
MuddyWater has used a Word Template, Normal.dotm, for persistence.[8] |
| ID | Mitigation | Description |
|---|---|---|
| M1040 | Behavior Prevention on Endpoint |
On Windows 10, enable Attack Surface Reduction (ASR) rules to prevent Office applications from creating child processes and from writing potentially malicious executable content to disk. [9] |
| M1042 | Disable or Remove Feature or Program |
Follow Office macro security best practices suitable for your environment. Disable Office VBA macros from executing. Disable Office add-ins. If they are required, follow best practices for securing them by requiring them to be signed and disabling user notification for allowing add-ins. For some add-ins types (WLL, VBA) additional mitigation is likely required as disabling add-ins in the Office Trust Center does not disable WLL nor does it prevent VBA code from executing. [10] |
Many Office-related persistence mechanisms require changes to the Registry and for binaries, files, or scripts to be written to disk or existing files modified to include malicious scripts. Collect events related to Registry key creation and modification for keys that could be used for Office-based persistence.[11][12] Modification to base templates, like Normal.dotm, should also be investigated since the base templates should likely not contain VBA macros. Changes to the Office macro security settings should also be investigated.[5]