Localhost applications failing after installing 2025-10 Cumulative Update for Windows 11 Version 24H2 for x64-based Systems (KB5066835) (26100.6899)

Question

I have an Winui application in production which has a dependency app [Standalone rest service].

Recently one of my customer has updated the windows version to below version,

2025-10 Cumulative Update for Windows 11 Version 24H2 for x64-based Systems (KB5066835) (26100.6899)

After this update the request sent to the dependency app [localhost service] is getting terminated before it reaches the service itself.

System.Net.Http.HttpRequestException: An error occurred while sending the request.

---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host..

---> System.Net.Sockets.SocketException (10054): An existing connection was forcibly closed by the remote host.

--- End of inner exception stack trace ---

at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ThrowException(SocketError error, CancellationToken cancellationToken)

at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.System.Threading.Tasks.Sources.IValueTaskSource<System.Int32>.GetResult(Int16 token)

at System.Net.Http.HttpConnection.InitialFillAsync(Boolean async)

at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)

--- End of inner exception stack trace ---

at System.Net.Http.HttpConnection.SendAsyncCore(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)

at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)

at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)

at System.Net.Http.HttpClient.<SendAsync>gCore|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)

at Zakya.POS.Platform.SyncEngine.Client.APIClient.GetAPIResultAsync(CancellationToken token, Double timer, Boolean PrintEncruptedLog, Boolean parseDesc, Boolean WriteResponseInLog)

Answer 1

https://learn.microsoft.com/en-us/answers/questions/5585563/localhost-not-working-anymore-after-2025-10-cumula

Answer 2

Hi, I appreciate you reaching out and I understand how unexpected behavior like this can impact your workflow. I’ve looked into the issue, and it appears that the recent Windows 11 update (KB5066835) introduced changes to how the system handles HTTP/2 and socket connections, which is affecting applications that rely on localhost communication, like your WinUI app and its REST service.

To get things working again, I recommend starting with a few basic steps: first, try disabling HTTP/2 if your service is hosted via IIS. You can do this in IIS Manager under your site’s advanced settings. If that doesn’t apply, you can also try rolling back the update via Settings then go to Windows Update then click Update History then Uninstall Updates. Additionally, running sfc /scannow and DISM /Online /Cleanup-Image /RestoreHealth in Command Prompt (Admin) can help repair any system-level issues that may be contributing.

Just to narrow things down further, can you confirm if the REST service is hosted via IIS, IIS Express, or a custom .NET host? Also, is the service using HTTP/2 or HTTP/1.1? These details will help me guide you more precisely.

Let me know what you find, and I’ll help you take the next steps.

Answer 3

As of now for this issue the only solution which works is uninstalling the problematic updates.

Steps :
Go to Windows Update -> Update History -> Uninstall Updates

1. Uninstall the version KB5066835, Restart the Machine.

2. Then uninstall the version KB5065789, Restart the Machine.

Answer 4

Thanks for the update. That’s really helpful to know. It looks like uninstalling both KB5066835 and KB5065789 is currently the most effective workaround to restore communication between your WinUI app and the standalone REST service.

Here’s a quick summary of the steps you can share with your team or users: Go to Settings then click Windows Update then Update History then Uninstall Updates Uninstall KB5066835, then restart the machine. After reboot, uninstall KB5065789, then restart again.

This should bring the system back to a stable state where the localhost communication works as expected. While this is a solid temporary fix, I’ll keep monitoring for any official patches or updates from Microsoft that address this behavior directly.

Let me know if the issue persists even after the rollback.