Guidance for "PrivExchange" Elevation of Privilege Vulnerability

An elevation of privilege vulnerability exists in Microsoft Exchange Server. An attacker who successfully exploited this vulnerability could attempt to impersonate any other user of the Exchange server. To exploit the vulnerability, an attacker would need to execute a man-in-the-middle attack to forward an authentication request to a Microsoft Exchange Server, thereby allowing impersonation of another Exchange user.

To address this vulnerability, a Throttling Policy for EWSMaxSubscriptions could be defined and applied to the organization with a value of zero. This will prevent the Exchange server from sending EWS notifications, and prevent client applications which rely upon EWS notifications from functioning normally. Examples of impacted applications include Outlook for Mac, Skype for Business, notification reliant LOB applications, and some iOS native mail clients. Please see Throttling Policy, for more information.

An example:

New-ThrottlingPolicy -Name AllUsersEWSSubscriptionBlockPolicy -EwsMaxSubscriptions 0 -ThrottlingPolicyScope Organization

A planned update is in development. If you determine that your system is at high risk then you should evaluate the proposed workaround.

After installing the update you can undo the above action with this command:

Remove-ThrottlingPolicy -Identity AllUsersEWSSubscriptionBlockPolicy