Technique on mitre.org

T1137.003: 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.

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.

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MaxPatrol SIEM knowledge base

vulnerabilities: PT-CR-2314: CVE_2024_21378_Outlook_RCE: Exploitation of vulnerability CVE-2024-21378 in Outlook. The vulnerability allows code execution using Outlook forms. mitre_attck_persistence: PT-CR-2649: Outlook_Form_Exploitation: Outlook started a suspicious process after creating a custom form in the Outlook client. This may indicate an attacker's attempt to gain persistence in the system or execute arbitrary code. mitre_attck_persistence: PT-CR-2668: Outlook_Form_Creation: A custom form was created in the Outlook client. This could be an attacker's attempt to escalate privileges or execute arbitrary code.

Detection

ID	DS0009	Data source and component	Process: Process Creation	Description	Monitor newly executed processes that may abuse Microsoft Outlook forms to obtain persistence on a compromised system.

ID	DS0015	Data source and component	Application Log: Application Log Content	Description	Monitor for third-party application logging, messaging, and/or other artifacts that may abuse Microsoft Outlook forms to obtain persistence on a compromised system. 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.

ID	DS0017	Data source and component	Command: Command Execution	Description	Monitor executed commands and arguments that may abuse Microsoft Outlook forms to obtain persistence on a compromised system. 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.

ID	Data source and component	Description
DS0009	Process: Process Creation	Monitor newly executed processes that may abuse Microsoft Outlook forms to obtain persistence on a compromised system.
DS0015	Application Log: Application Log Content	Monitor for third-party application logging, messaging, and/or other artifacts that may abuse Microsoft Outlook forms to obtain persistence on a compromised system. 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.
DS0017	Command: Command Execution	Monitor executed commands and arguments that may abuse Microsoft Outlook forms to obtain persistence on a compromised system. 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.

Mitigation

ID	M1051	Name	Update Software	Description	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. 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.

ID	M1040	Name	Behavior Prevention on Endpoint	Description	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.

ID	Name	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. 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.
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.