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: Application Log: Application Log Content, Command: Command Execution, Process: Process Creation
Version: 1.0
Created: 07 November 2019
Last Modified: 26 March 2020

Procedure Examples

ID Name Description
S0358 Ruler

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

Mitigations

ID Mitigation Description
M1051 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