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] Outlook and Outlook Web App (OWA) allow users to create inbox rules for various email functions, including forwarding to a different recipient. 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.[2]

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.

ID: T1114.003
Sub-technique of:  T1114
Tactic: Collection
Platforms: Office 365, Windows
Permissions Required: User
Data Sources: Email gateway, Mail server, Office 365 trace logs, Process monitoring, Process use of network
Contributors: Swetha Prabakaran, Microsoft Threat Intelligence Center (MSTIC)
Version: 1.0
Created: 19 February 2020
Last Modified: 24 March 2020

Mitigations

Mitigation Description
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 to discover and remove potentially malicious auto-forwarding rules.[2]

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.

Detection

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.

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.[2] 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.

References