Office Add-ins

Office Add-ins provide enhanced functionality and customization for Microsoft Office applications, enabling users to tailor their experience with additional features.

Overview

• Registry Location: The specific registry path for Office Add-ins depends on the Office application and version. A general pattern for these paths is:

  • HKCU\Software\Microsoft\Office\[Office Version]\[Application]\Addins

• Purpose and Use: Add-ins can range from productivity tools, such as grammar checkers and citation managers, to enterprise-specific integrations like CRM and project management tools. They're developed using various technologies, including COM add-ins, VSTO (Visual Studio Tools for Office), and Office Web Add-ins.

Abuse by Threat Actors

• Malicious Add-ins: Threat actors can create or modify registry entries to load malicious add-ins when an Office application starts. These add-ins can perform a wide range of malicious activities, from data exfiltration to executing further payloads.

• Persistence and Stealth: Once installed, a malicious add-in can provide persistent execution of malicious code each time the Office application is launched. Given that users frequently open Office applications, this provides a reliable method for maintaining access. Moreover, users might not suspect add-ins as a source of compromise, granting malware stealth.

Detection and Mitigation

• Monitoring Registry Changes: Regular monitoring of the Add-ins registry keys for unexpected or unauthorized changes can help identify the installation of malicious add-ins. This can be achieved through security solutions that track registry modifications.

• Office Application Behavior Analysis: Analyzing behavior anomalies in Office applications, such as unexpected network connections or file modifications, can help detect malicious add-in activity.

• Security Solutions and Policies: Employ endpoint security solutions capable of detecting and blocking malicious add-ins. Establishing policies that control the installation and management of Office add-ins can limit the attack surface.

Example of a Suspicious Entry

For Microsoft Word 2016, a suspicious add-in entry might look like this:

[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Word\Addins\MyMaliciousAddIn]
"Description"="My Malicious Add-In"
"FriendlyName"="MyMaliciousAddIn"
"LoadBehavior"=dword:00000003

• LoadBehavior: Determines how the add-in is loaded. A value of 3 indicates the add-in is loaded at startup.

• Description and FriendlyName: These might be crafted to appear benign or mimic legitimate add-ins, making detection harder.

Conclusion

While Office Add-ins are a powerful feature for extending the capabilities of Office applications, their potential misuse for malicious purposes cannot be ignored. Organizations should implement strict controls over add-in management, conduct regular audits of registry settings related to add-ins, and employ advanced security solutions to detect and mitigate threats posed by malicious add-ins. Awareness and proactive management of add-ins are crucial for maintaining a secure computing environment.