Microsoft has disclosed a serious Active Directory replication flaw that can break domain controller synchronization when organizations running Windows Server 2025 as their schema master install recent Exchange Server updates, potentially disrupting enterprise authentication and directory services.
The issue specifically affects environments using Windows Server 2025 in the schema master FSMO (Flexible Single Master Operations) role, a critical position that controls all schema modifications across an Active Directory forest.
When administrators install Exchange Server 2019 CU15 or Exchange Server Subscription Edition RTM on these systems, the schema master may create duplicate attribute values, triggering cascade replication failures across the entire domain infrastructure.
"The replication operation failed because of a schema mismatch between the servers involved."
"Warning 1203 (NTDS Replication): The local domain controller could not replicate the following object from the source domain controller at the following network address because of an Active Directory schema mismatch," reads the Error 8418 message that affected administrators will encounter.
The Windows team has acknowledged this as a known issue in KB5065426, with AD replication tools like repadmin showing persistent sync failures.
The vulnerability stems from how Windows Server 2025 handles schema extensions during Exchange cumulative updates. While the latest operating system can safely function as a domain controller in other roles, its schema master implementation contains a defect that corrupts schema attributes when processing Exchange's Active Directory modifications.
Immediate Workarounds and Timeline
Microsoft Exchange program manager Nino Bilic recommends a temporary mitigation strategy: "Install a single DC (let's say WS 2022) and make it a schema master. Install Exchange SE. Once all is done, you can decommission this DC (after transferring the role to a WS 2025 DC)." This approach allows organizations to proceed with Exchange deployments without risking AD integrity.
For organizations already experiencing replication failures, Microsoft's Windows Support Team can provide manual schema remediation, though this requires opening a dedicated support ticket for hands-on intervention.
A permanent fix is scheduled for release by the end of calendar year 2025, after which the workaround will no longer be necessary. Organizations should avoid using Windows Server 2025 as schema masters until the patch arrives, particularly as Exchange Server 2019's end-of-life approaches on October 14, 2025.