Email Collection: Email Forwarding Rule

Adversaries may setup email forwarding rules to collect sensitive information. Adversaries may abuse email forwarding rules to monitor the activities of a victim, steal information, and further gain intelligence on the victim or the victim’s organization to use as part of further exploits or operations.[1] Furthermore, email forwarding rules can allow adversaries to maintain persistent access to victim's emails even after compromised credentials are reset by administrators.[2] Most email clients allow users to create inbox rules for various email functions, including forwarding to a different recipient. These rules may be created through a local email application, a web interface, or by command-line interface. Messages can be forwarded to internal or external recipients, and there are no restrictions limiting the extent of this rule. Administrators may also create forwarding rules for user accounts with the same considerations and outcomes.[3][4]

Any user or administrator within the organization (or adversary with valid credentials) can create rules to automatically forward all received messages to another recipient, forward emails to different locations based on the sender, and more. Adversaries may also hide the rule by making use of the Microsoft Messaging API (MAPI) to modify the rule properties, making it hidden and not visible from Outlook, OWA or most Exchange Administration tools.[2]

In some environments, administrators may be able to enable email forwarding rules that operate organization-wide rather than on individual inboxes. For example, Microsoft Exchange supports transport rules that evaluate all mail an organization receives against user-specified conditions, then performs a user-specified action on mail that adheres to those conditions.[5] Adversaries that abuse such features may be able to enable forwarding on all or specific mail an organization receives.

ID: T1114.003
Sub-technique of:  T1114
Tactic: Collection
Platforms: Google Workspace, Linux, Office 365, Windows, macOS
Contributors: Liran Ravich, CardinalOps; Microsoft Security; Swetha Prabakaran, Microsoft Threat Intelligence Center (MSTIC)
Version: 1.3
Created: 19 February 2020
Last Modified: 12 April 2023

Procedure Examples

ID Name Description
G0094 Kimsuky

Kimsuky has set auto-forward rules on victim's e-mail accounts.[6]


LAPSUS$ has set an Office 365 tenant level mail transport rule to send all mail in and out of the targeted organization to the newly created account.[7]

G0122 Silent Librarian

Silent Librarian has set up auto forwarding rules on compromised e-mail accounts.[8]


ID Mitigation Description
M1047 Audit

Enterprise email solutions have monitoring mechanisms that may include the ability to audit auto-forwarding rules on a regular basis.

In an Exchange environment, Administrators can use Get-InboxRule / Remove-InboxRule and Get-TransportRule / Remove-TransportRule to discover and remove potentially malicious auto-fowarding and transport rules.[3][9][10] In addition to this, a MAPI Editor can be utilized to examine the underlying database structure and discover any modifications/tampering of the properties of auto-forwarding rules.[2]

M1042 Disable or Remove Feature or Program

Consider disabling external email forwarding.[11]

M1041 Encrypt Sensitive Information

Use of encryption provides an added layer of security to sensitive information sent over email. Encryption using public key cryptography requires the adversary to obtain the private certificate along with an encryption key to decrypt messages.


ID Data Source Data Component Detects
DS0015 Application Log Application Log Content

Detection is challenging because all messages forwarded because of an auto-forwarding rule have the same presentation as a manually forwarded message. It is also possible for the user to not be aware of the addition of such an auto-forwarding rule and not suspect that their account has been compromised; email-forwarding rules alone will not affect the normal usage patterns or operations of the email account. This is especially true in cases with hidden auto-forwarding rules. This makes it only possible to reliably detect the existence of a hidden auto-forwarding rule by examining message tracking logs or by using a MAPI editor to notice the modified rule property values.[2]Auto-forwarded messages generally contain specific detectable artifacts that may be present in the header; such artifacts would be platform-specific. Examples include X-MS-Exchange-Organization-AutoForwarded set to true, X-MailFwdBy and X-Forwarded-To. The forwardingSMTPAddress parameter used in a forwarding process that is managed by administrators and not by user actions. All messages for the mailbox are forwarded to the specified SMTP address. However, unlike typical client-side rules, the message does not appear as forwarded in the mailbox; it appears as if it were sent directly to the specified destination mailbox.[3] High volumes of emails that bear the X-MS-Exchange-Organization-AutoForwarded header (indicating auto-forwarding) without a corresponding number of emails that match the appearance of a forwarded message may indicate that further investigation is needed at the administrator level rather than user-level.

In environments using Exchange, monitor logs for the creation or modification of mail transport rules.

DS0017 Command Command Execution

On Windows systems, monitor for creation of suspicious inbox rules through the use of the New-InboxRule, Set-InboxRule, New-TransportRule, and Set-TransportRule PowerShell cmdlets.[11][9]