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Office Application Startup: Outlook Rules

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 Outlook rules to obtain persistence on a compromised system. Outlook rules allow a user to define automated behavior to manage email messages. A benign rule might, for example, automatically move an email to a particular folder in Outlook if it contains specific words from a specific sender. Malicious Outlook rules can be created that can trigger code execution when an adversary sends a specifically crafted email to that user.[1]

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

ID: T1137.005
Sub-technique of:  T1137
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Tactic: Persistence
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Platforms: Office 365, Windows
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Permissions Required: Administrator, User
Contributors: Microsoft Security
Version: 1.1
Created: 07 November 2019
Last Modified: 15 October 2021

Procedure Examples

ID Name Description
S0358 Ruler

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

Mitigations

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. [3]
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.[4] 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.[5]

Detection

ID Data Source Data Component
DS0015 Application Log Application Log Content
DS0017 Command Command Execution
DS0009 Process Process Creation

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.[6] This PowerShell script is ineffective in gathering rules with modified PRPR_RULE_MSG_NAME and PR_RULE_MSG_PROVIDER properties caused by adversaries using a Microsoft Exchange Server Messaging API Editor (MAPI Editor), so only examination with the Exchange Administration tool MFCMapi can reveal these mail forwarding rules.[7] 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.[8]

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

  1. Landers, N. (2015, December 4). Malicious Outlook Rules. Retrieved February 4, 2019.
  2. SensePost. (2016, August 18). Ruler: A tool to abuse Exchange services. Retrieved February 4, 2019.
  3. Microsoft. (2021, July 2). Use attack surface reduction rules to prevent malware infection. Retrieved June 24, 2021.
  4. Stalmans, E. (2017, April 28). Outlook Forms and Shells. Retrieved February 4, 2019.
  1. Stalmans, E. (2017, October 11). Outlook Home Page – Another Ruler Vector. Retrieved February 4, 2019.
  2. Fox, C., Vangel, D. (2018, April 22). Detect and Remediate Outlook Rules and Custom Forms Injections Attacks in Office 365. Retrieved February 4, 2019.
  3. Damian Pfammatter. (2018, September 17). Hidden Inbox Rules in Microsoft Exchange. Retrieved October 12, 2021.
  4. SensePost. (2017, September 21). NotRuler - The opposite of Ruler, provides blue teams with the ability to detect Ruler usage against Exchange. Retrieved February 4, 2019.