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.
Adversaries may also assign mailbox folder permissions through individual folder permissions or roles. Adversaries may assign the Default or Anonymous user permissions or roles to the Top of Information Store (root), Inbox, or other mailbox folders. By assigning one or both user permissions to a folder, the adversary can utilize any other account in the tenant to maintain persistence to the target user’s mail folders.
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.
APT29 added their own devices as allowed IDs for active sync using
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.
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.
|ID||Data Source||Data Component|
|DS0015||Application Log||Application Log Content|
|DS0002||User Account||User Account Modification|
Monitor for unusual Exchange and Office 365 email account permissions changes that may indicate excessively broad permissions being granted to compromised accounts.
Enable the UpdateFolderPermissions action for all logon types. The mailbox audit log will forward folder permission modification events to the Unified Audit Log. Create rules to alert on ModifyFolderPermissions operations where the Anonymous or Default user is assigned permissions other than None.
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.