Office Application Startup: Outlook Forms

Adversaries may abuse Microsoft Outlook forms to obtain persistence on a compromised system. Outlook forms are used as templates for presentation and functionality in Outlook messages. Custom Outlook forms can be created that will execute code when a specifically crafted email is sent by an adversary utilizing the same custom Outlook form.[1]

Once malicious forms have been added to the user’s mailbox, they will be loaded when Outlook is started. Malicious forms will execute when an adversary sends a specifically crafted email to the user.[1]

ID: T1137.003
Sub-technique of:  T1137
Tactic: Persistence
Platforms: Office 365, Windows
Permissions Required: Administrator, User
Data Sources: Mail server, Process command-line parameters, Process monitoring
Version: 1.0
Created: 07 November 2019
Last Modified: 26 March 2020

Procedure Examples

Name Description
Ruler

Ruler can be used to automate the abuse of Outlook Forms to establish persistence.[2]

Mitigations

Mitigation Description
Update Software

For the Outlook methods, blocking macros may be ineffective as the Visual Basic engine used for these features is separate from the macro scripting engine.[1] Microsoft has released patches to try to address each issue. Ensure KB3191938 which blocks Outlook Visual Basic and displays a malicious code warning, KB4011091 which disables custom forms by default, and KB4011162 which removes the legacy Home Page feature, are applied to systems.[3]

Detection

Microsoft has released a PowerShell script to safely gather mail forwarding rules and custom forms in your mail environment as well as steps to interpret the output.[4] SensePost, whose tool Ruler can be used to carry out malicious rules, forms, and Home Page attacks, has released a tool to detect Ruler usage.[5]

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.

References