KB5066835 update causing IIS Service to not work

Sashi Kumar 405 Reputation points
2025-10-15T10:06:37.1166667+00:00

Recently my local hosted IIS services is not running after the new update. My visual studio projects also can't be run as well. I have to delete the new update in order to run my projects and local hosted services again. May I know what is the problem?

Developer technologies | ASP.NET Core | Other

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Answer accepted by question author
xavier ubikos 175 Reputation points
2025-10-16T05:23:40.57+00:00

Hi there!

This morning there was a Defender update, I have installed it an now it works!!!

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  1. Marcin Pabian 15 Reputation points
    2025-10-15T20:31:06.27+00:00

    It seems the issue is not limited to IIS.
    We have a WCF service self-hosted with ServiceHost (.NET Framework), and after installing the October 2025 cumulative update, the service stopped working.

    Clients now get this exception:

    System.ServiceModel.CommunicationException: An error occurred while receiving the HTTP response to http(s)://<host>:<port>/ServicePath This could be due to the service endpoint binding not using the HTTP protocol. This could also be due to an HTTP request context being aborted by the server (possibly due to the service shutting down). See server logs for more details.

    Changing the binding from BasicHttpBinding or BasicHttpsBinding to NetTcpBinding makes the service work again without uninstalling the update.

    It looks like the update changed how HTTP.sys handles self-hosted HTTP/HTTPS endpoints (possibly related to CVE-2025-55248).
    Could Microsoft confirm if this change is intentional or if it’s an unexpected issue breaking WCF SelfHost applications?
    Do you plan to release a fix or provide an official workaround for affected systems?

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  2. John Hatton 25 Reputation points
    2025-10-15T21:04:06.3566667+00:00

    Not just an IIS problem. It's breaking our app on some subset of customer machines (and, luckily, mine). Minimal repro:

    using System;
using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
class MinimalRepro
{
    static async Task Main()
    {
        var listener = new HttpListener();
        listener.Prefixes.Add("http://localhost:8089/");
        listener.Start();
        listener.BeginGetContext(ar =>
        {
            try
            {
                var context = listener.EndGetContext(ar); // FAILS HERE
                context.Response.StatusCode = 200;
                context.Response.Close();
            }
        }, null);
        await Task.Delay(200);
        // Trigger the error
        using var client = new HttpClient();
        await client.GetAsync("http://localhost:8089/");
        listener.Stop();
    }
}

    ERROR: 50 - The request is not supported.

    Unhandled exception. 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.

    at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.CreateException(SocketError error, Boolean forAsyncThrow)

    at System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.ReceiveAsync(Socket socket, CancellationToken cancellationToken)

    at System.Net.Sockets.Socket.ReceiveAsync(Memory`1 buffer, SocketFlags socketFlags, Boolean fromNetworkStream, CancellationToken cancellationToken)

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  3. John Hatton 25 Reputation points
    2025-10-15T21:30:47.6+00:00

    As someone pointed out, the problem is not just IIS, but anything opening a port using .net HTTPListener. Minimal reproduction : https://github.com/BloomBooks/httplistener-test

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  4. Constantin Philippou 10 Reputation points
    2025-10-15T22:10:50.2233333+00:00

    Uninstalled KB5066835 AND KB5065789 and back to normal.

    I feel the guy with the 500 computers...

    And MS Devs, how is it possible to deploy a Windows Update you BUILT with Visual Studio and not see VS is not working after you deploy on your test servers? Wow...

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    2 people found this answer helpful.