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Localhost not working anymore after 2025-10 cumulative update Windows 11

Jan van Rhijn 270 Reputation points
2025-10-15T12:01:18.59+00:00

Hi,

Since the last cumulative update of 2025-10 I can not access applications via localhost anymore. It looks like I am not the only one with this issue: https://serverfault.com/questions/1193405/http-2-protocol-error-after-windows-update and https://stackoverflow.com/questions/79790827/localhost-applications-failing-after-installing-2025-10-cumulative-update-for-w

When the update (KB5066835) is removed, everyhting works again.

This is affecting local development as well as business applications

Cheers,

Jan

Windows for business | Windows 365 Business

37 answers

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  1. Yigit Yildirim 120 Reputation points
    2025-10-15T13:22:51.67+00:00

    Hi,

    What action is expected from Microsoft? Are you planning to release a new Windows update to fix the issue, and if so, when is it expected?

    Thanks

    17 people found this answer helpful.
  2. Yigit Yildirim 120 Reputation points
    2025-10-15T13:28:27.26+00:00

    This is the fix that works for our applications:
    Uninstalling the following KBs
    wusa /uninstall /kb:5066835

    wusa /uninstall /kb:5065789

    restart computer

    7 people found this answer helpful.
  3. Gabhan OLoughlin 25 Reputation points
    2025-10-16T09:02:46.38+00:00

    I've seen some suggestions that Security Intelligence Update for Microsoft Defender Antivirus - KB2267602 (Version 1.439.210.0) - Current Channel (Broad) pushed recently should resolve this issue. Will test when I have time as I have rolled back the previous updates, but if anyone can confirm this resolves the issue?

    4 people found this answer helpful.
  4. Knut Hunstad 15 Reputation points
    2025-10-16T07:05:40.6833333+00:00

    The issue is now resolved! Simply run Windows update to update everything and things work again! You can turn back on automatic updates.

    3 people found this answer helpful.
  5. Vivian Phan 6,665 Reputation points Independent Advisor
    2025-10-15T12:34:55.72+00:00

    Hello,

    You are correct that the October 2025 Cumulative Update (KB5066835) has introduced a regression affecting certain HTTP/2 and loopback traffic scenarios. This may cause requests to fail or connections to drop unexpectedly when hosted locally using IIS, Kestrel, or other web servers relying on HTTP/2 by default.

    In the meantime, there are two safe workarounds you can apply:

    1. Temporarily disable HTTP/2 on the affected system until a fixed update is released.
      • You can do this by setting the following registry keys and restarting the machine: 
             [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\IIS\Parameters]

    "EnableHttp2"=dword:00000000 "EnableHttp2OverTls"=dword:00000000

       
1. Alternatively, as you’ve noted, **uninstalling KB5066835** will also restore functionality. 

If this information helps stabilize your environment, please click **“Accept Answer”** so others in the community can find it easily 😊

Best regards,

VP
    2 people found this answer helpful.

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