CISA has issued an emergency directive ordering all Federal Civilian Govt Department (FCEB) businesses to mitigate a crucial Microsoft Alternate hybrid vulnerability tracked as CVE-2025-53786 by Monday morning at 9:00 AM ET.
Federal Civilian Govt Department (FCEB) businesses are non-military businesses inside the US govt department, together with the Division of Homeland Safety, Division of the Treasury, Division of Vitality, and Division of Well being and Human Providers.
The flaw tracked as CVE-2025-53786 permits attackers who achieve administrative entry to on-premises Alternate servers to maneuver laterally into Microsoft cloud environments, probably main to finish area compromise.
The vulnerability impacts Microsoft Alternate Server 2016, 2019, and the Subscription Version.
In hybrid configurations, Alternate On-line and on-premises servers share the identical service principal, which is a shared belief relationship used to authenticate with one another.
An attacker with admin privileges on an on-premise Alternate server can probably forge or manipulate trusted tokens or API calls that the cloud aspect will settle for as professional. This method permits the attackers to unfold laterally from the native community into the corporate’s cloud atmosphere, probably compromising the corporate’s complete energetic listing and infrastructure.
To make issues worse, Microsoft says cloud-based logging instruments like Microsoft Purview might not log malicious exercise if it originates from on-prem Alternate, making it onerous to detect exploitation.
This flaw comes after Microsoft launched steerage and an Alternate server hotfix in April 2025 to assist a brand new structure that makes use of a devoted hybrid software, reasonably than the shared one, as a part of its Safe Future Initiative.
Yesterday, safety researcher Dirk-Jan Mollema of Outsider Safety demonstrated how this shared service principal may very well be exploited in a post-exploitation assault throughout a Black Hat presentation.
The researcher instructed BleepingComputer that he reported the flaw three weeks earlier than the discuss, to offer Microsoft advance warning. In coordination with the presentation, Microsoft issued the CVE-2025-53786 CVE and steerage on how one can mitigate it.
“I didn’t initially contemplate this a vulnerability as a result of the protocol that’s used for these assaults was designed with the options lined through the discuss, and is simply normally missing essential safety controls,” Mollema instructed BleepingComputer.
“The report describing the probabilities for attackers was despatched as a heads as much as the MSRC 3 weeks earlier than Black Hat and the disclosure was coordinated with them. Apart from this steerage Microsoft additionally mitigated an assault path that would result in full tenant compromise (International Admin) from on-prem Alternate.”
The excellent news is that Microsoft Alternate clients who beforehand applied the hotfix and the April steerage are already shielded from this new post-exploitation assault.
Nonetheless, those that haven’t applied the mitigations are nonetheless impacted and will set up the hotfix and observe Microsoft’s directions (doc 1 and doc 2) on deploying the devoted Alternate hybrid app.
“Solely making use of the hotfix just isn’t adequate on this case, there are handbook follow-up actions required emigrate to a devoted service principal,” defined Mollema.
“The urgency from a safety perspective depends upon how a lot admins contemplate isolation between on-prem Alternate sources and cloud-hosted sources essential. Within the outdated setup, Alternate hybrid has full entry to all sources in Alternate on-line and in SharePoint.”
Mollema additionally reiterated that his method is a post-exploitation assault, that means an attacker already has to have compromised the on-premises atmosphere or the Alternate servers, and on this case, have administrator privileges.
In accordance with CISA’s Emergency Directive 25-02federal businesses should now mitigate the assault by first taking a listing of their Alternate environments utilizing Microsoft’s Well being Checker script. Any servers which can be not supported by the April 2025 hotfix, resembling end-of-life Alternate variations, should be disconnected.
All remaining servers should be up to date to the most recent cumulative updates (CU14 or CU15 for Alternate 2019, and CU23 for Alternate 2016) and patched with the April hotfix. Afterward, directors should run Microsoft’s ConfigureExchangeHybridApplication.ps1 PowerShell script to modify from the shared to the devoted service principal in Entra ID.
CISA warns that failing to implement these mitigations might end in hybrid environments being fully compromised.
Businesses should full the technical remediation steps by Monday morning and submit a report back to CISA by 5:00 PM the identical day.
Whereas non-government organizations are usually not required to take motion beneath this directive, CISA urges all organizations to mitigate the assault.
“The dangers related to this Microsoft Alternate vulnerability lengthen to each group and sector utilizing this atmosphere,” stated CISA Appearing Director Madhu Gottumukkala.
“Whereas federal businesses are mandated, we strongly urge all organizations to undertake the actions on this Emergency Directive.”
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