Microsoft releases a security advisory that fixes the Privilege Escalation Vulnerability With Exchange Server. By exploiting this vulnerability an attacker could impersonate any other exchange user.
An attacker could exploit this vulnerability by executing a man-in-the-middle attack and forwarding an authentication request to a Microsoft Exchange Server which allows 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, reads Microsoft security advisory.
- Microsoft Exchange Server 2010 Service Pack 3 Update Rollup 26 – Elevation of Privilege
- Microsoft Exchange Server 2013 Cumulative Update 22 – Elevation of Privilege
- Microsoft Exchange Server 2016 Cumulative Update 12 – Elevation of Privilege
- Microsoft Exchange Server 2019 Cumulative Update 1 – Elevation of Privilege
Mitigations & Workarounds
From Microsoft Exchange 2013, the NTLM authentication over HTTP fails to set the NTLM Sign and Seal flags which makes the NTLM relay attacks possible.
If you are using an exchange server then it is recommended to block the EWS subscriptions from being created. Microsoft recommends “Customers are strongly encouraged to test workarounds prior to deploying them into production to understand the potential impact.”