Account Manipulation: Exchange Email Delegate Permissions

Adversaries may grant additional permission levels, such as ReadPermission or FullAccess, to maintain persistent access to an adversary-controlled email account. The Add-MailboxPermission PowerShell cmdlet, available in on-premises Exchange and in the cloud-based service Office 365, adds permissions to a mailbox.[1][2][3]

This may be used in persistent threat incidents as well as BEC (Business Email Compromise) incidents where an adversary can assign more access rights to the accounts they wish to compromise. This may further enable use of additional techniques for gaining access to systems. For example, compromised business accounts are often used to send messages to other accounts in the network of the target business while creating inbox rules (ex: Internal Spearphishing), so the messages evade spam/phishing detection mechanisms.[4]

ID: T1098.002
Sub-technique of:  T1098
Tactic: Persistence
Platforms: Office 365, Windows
Permissions Required: Administrator
Data Sources: Group: Group Modification, User Account: User Account Modification
Contributors: Jannie Li, Microsoft Threat Intelligence Center (MSTIC)
Version: 1.0
Created: 19 January 2020
Last Modified: 04 May 2020

Procedure Examples

ID Name Description
G0016 APT29

APT29 added their own devices as allowed IDs for active sync using Set-CASMailbox, allowing it to obtain copies of victim mailboxes. It also added additional permissions (such as Mail.Read and Mail.ReadWrite) to compromised Application or Service Principals.[5][6]

G0059 Magic Hound

Magic Hound granted compromised email accounts read access to the email boxes of additional targeted accounts. The group then was able to authenticate to the intended victim's OWA (Outlook Web Access) portal and read hundreds of email communications for information on Middle East organizations.[2]


ID Mitigation Description
M1032 Multi-factor Authentication

Use multi-factor authentication for user and privileged accounts.

M1026 Privileged Account Management

Do not allow domain administrator accounts to be used for day-to-day operations that may expose them to potential adversaries on unprivileged systems.


Monitor for unusual Exchange and Office 365 email account permissions changes that may indicate excessively broad permissions being granted to compromised accounts.

A larger than normal volume of emails sent from an account and similar phishing emails sent from  real accounts within a network may be a sign that an account was compromised and attempts to leverage access with modified email permissions is occurring.