The described symptoms point to domain controller and network issues after introducing the Windows Server 2025 DC, not to a known specific bug with those three services. From the provided context, only one relevant Windows Server 2025 issue is documented that can directly affect domain connectivity and therefore internet access and policy enforcement.
On Windows Server 2025 domain controllers, there was a known issue where, after a restart, the DC did not apply the domain firewall profile and instead used the standard firewall profile. This could cause:
- The DC to be unreachable or only partially reachable on the domain network.
- Ports and protocols that should be blocked by the domain firewall profile to be open, or required ports to be blocked.
- Applications and services depending on the DC (such as authentication, group policy, and possibly FSSO or similar identity-based firewall integrations) to fail or behave inconsistently.
This aligns with the disruption to user internet access and security policy enforcement after promoting the new Windows Server 2025 DC.
Resolution and mitigation steps from the context:
- Ensure the Windows Server 2025 DC is fully updated
- Install the June 10, 2025 Windows security update (KB5060842) or any later cumulative update on the Windows Server 2025 domain controller. This update resolves the issue where domain controllers manage network traffic incorrectly after restarting.
- Once this update (or a later one) is installed, the DC should correctly use the domain firewall profile after restarts.
- Temporary workaround if the DC is not yet updated
- If the DC is still on a build prior to KB5060842 and cannot be updated immediately, restore expected behavior after each restart by restarting the network adapter on the DC. For example, in an elevated PowerShell session on the DC:
Restart-NetAdapter * - Because the issue reoccurs on every restart, this workaround must be repeated each time the DC restarts. A scheduled task can be created to run
Restart-NetAdapter *at startup to automate this until the update is applied.
- If the DC is still on a build prior to KB5060842 and cannot be updated immediately, restore expected behavior after each restart by restarting the network adapter on the DC. For example, in an elevated PowerShell session on the DC:
- Impact on FSSO and user internet access
- If the firewall/FSSO solution depends on stable connectivity to domain controllers (for user logon tracking, group membership, or security policy application), any intermittent or incorrect firewall profile on the DC can break:
- User authentication visibility for the firewall.
- Group-based or user-based internet access rules.
- Updating the DC to include KB5060842 or later should stabilize DC network behavior and remove this source of disruption.
- If the firewall/FSSO solution depends on stable connectivity to domain controllers (for user logon tracking, group membership, or security policy application), any intermittent or incorrect firewall profile on the DC can break:
- Role of the mentioned services
- The context does not document any known Windows Server 2025 issues specifically tied to Intersite Messaging (IsmServ), Sync Host (OneSyncSvc), or Clipboard User Service (cbdhsvc) causing domain-wide internet access or policy enforcement problems.
- Focus should therefore be on ensuring the DC’s OS build includes the fix for the firewall profile issue and verifying that, after update and reboot, the DC is reachable and correctly applying domain firewall profiles.
Summary of recommended actions:
- On DC5 (Windows Server 2025), install the June 10, 2025 security update KB5060842 or any newer cumulative update.
- Until updated, after each reboot of DC5, run
Restart-NetAdapter *(or use a startup scheduled task) to restore correct domain firewall profile behavior. - After updating, verify:
- Clients can reliably authenticate against DC5.
- FSSO or equivalent firewall integration sees user logons correctly.
- Group policies and security policies are applied as expected.
References: