Microsoft Exchange Server Hybrid Deployment Elevation of Privilege Vulnerability

What privileges could be gained by an attacker who successfully exploited the vulnerability within the organization’s cloud environment?
In an Exchange hybrid deployment, an attacker who first gains administrative access to an on-premises Exchange server could potentially escalate privileges within the organization’s connected cloud environment without leaving easily detectable and auditable trace. This risk arises because Exchange Server and Exchange Online share the same service principal in hybrid configurations.

According to the CVSS metric, successful exploitation could lead to a scope change (S:C). What does this mean for this vulnerability?
In this scenario, successfully exploiting the vulnerability could enable an attacker to escalate privileges within the organization’s connected cloud environment without leaving easily detectable and auditable traces.

According to the CVSS metric, the attack complexity is high (AC:H). What does that mean for this vulnerability?
Successful exploitation of this vulnerability requires an attacker to first gain or possess administrator access on an Exchange Server.

Microsoft Exchange Server Subscription Edition RTM didn't exist in April when the hot fix was released. Why is it listed in the Security Updates Table?
Support for the dedicated Exchange hybrid app feature is already part of the initial release of the Exchange Server SE version. If you have already migrated to the newest version you already have this level of protection from the vulnerability. All you need to do is to follow the steps as outlined in the documentation to enable the feature and clear the certificates from the shared service principals keyCredentials.

Is there new information that I should be aware of?
Yes, please see this blog post for more information: MDVM Guidance for CVE-2025-53786: Exchange Hybrid Privilege Escalation
If you're using Exchange hybrid, install the Hot Fix (or newer release) on your on-premises Exchange servers and follow the configuration instructions outlined in Deploy dedicated Exchange hybrid app. For additional details, refer to Exchange Server Security Changes for Hybrid Deployments. After completing the steps, be sure to reset the service principal's keyCredentials.

If you’ve previously configured Exchange hybrid or OAuth authentication between Exchange Server and your Exchange Online organization but no longer use it, make sure to reset the service principal's keyCredentials.

Security updates
15.02.2562.017 — http://support.microsoft.com/kb/5047155
15.02.1748.024 — http://support.microsoft.com/kb/KB5050672
15.01.2507.055 — http://support.microsoft.com/kb/KB5050674
15.02.1544.025 — http://support.microsoft.com/kb/KB5050673