Office Application Startup: Add-ins

Adversaries may abuse Microsoft Office add-ins to obtain persistence on a compromised system. Office add-ins can be used to add functionality to Office programs. [1] There are different types of add-ins that can be used by the various Office products; including Word/Excel add-in Libraries (WLL/XLL), VBA add-ins, Office Component Object Model (COM) add-ins, automation add-ins, VBA Editor (VBE), Visual Studio Tools for Office (VSTO) add-ins, and Outlook add-ins. [2][3]

Add-ins can be used to obtain persistence because they can be set to execute code when an Office application starts.

ID: T1137.006
Sub-technique of:  T1137
Tactic: Persistence
Platforms: Office 365, Windows
Permissions Required: Administrator, User
Version: 1.1
Created: 07 November 2019
Last Modified: 16 August 2021

Procedure Examples

ID Name Description
S0268 Bisonal

Bisonal has been loaded through a .wll extension added to the %APPDATA%\microsoft\word\startup\ repository.[4]

G0019 Naikon

Naikon has used the RoyalRoad exploit builder to drop a second stage loader, intel.wll, into the Word Startup folder on the compromised host.[5]


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. [6]


ID Data Source Data Component
DS0017 Command Command Execution
DS0022 File File Creation
File Modification
DS0009 Process Process Creation
DS0024 Windows Registry Windows Registry Key Creation
Windows Registry Key Modification

Monitor and validate the Office trusted locations on the file system and audit the Registry entries relevant for enabling add-ins.[7][2]

Collect process execution information including process IDs (PID) and parent process IDs (PPID) and look for abnormal chains of activity resulting from Office processes. Non-standard process execution trees may also indicate suspicious or malicious behavior