Oct 21, 2025 10:01:00

Reports of 'localhost' broken after Windows 11 update

In environments where the cumulative update for Windows 11 released by Microsoft in October 2025 (KB5066835) has been applied, an issue has occurred where localhost is no longer accessible. This has caused developers and others to be unable to connect to web apps they are developing and testing on their own PCs via localhost, interfering with tasks such as debugging in Visual Studio and

ASP.NET builds .

Localhost not working anymore after 2025-10 cumulative update Windows 11 - Microsoft Q&A
https://learn.microsoft.com/en-us/answers/questions/5585563/localhost-not-working-anymore-after-2025-10-cumula

Microsoft update breaks localhost in Windows 11 • The Register
https://www.theregister.com/2025/10/16/windows_11_update_localhost/

This issue has been confirmed to occur after installing the October 2025 cumulative update ( KB5066835 ). When users try to access a web app through localhost, an HTTP/2 protocol error occurs and the connection fails. This makes it impossible to debug using Visual Studio or test the operation of apps built with ASP.NET.

The Register, an IT news site, reports that the issue has been traced to a bug in HTTP.sys, a Windows kernel component that handles local HTTP traffic. However, since the issue has been reported to not occur on clean installations of Windows 11 24H2, it is believed that the bug is not inherent in the OS itself, but is likely caused by a conflict when updates are applied to existing system configurations.

The most commonly reported solution on Microsoft forums is to uninstall the update that is causing the issue. If uninstalling KB5066835 doesn't resolve the issue, some users have also reported that uninstalling

KB5065789 also resolves the issue.

You can uninstall KB5066835 by running the command prompt as an administrator and entering the following command. However, since Windows Update may automatically reinstall it after uninstallation, you will need to temporarily stop updates or use a tool to hide specific updates.
[code]wusa /uninstall /kb:5066835[/code]

At the time of writing, there has been no official announcement from Microsoft regarding this issue. However, an independent advisor on the Microsoft forum acknowledged that the update introduced a regression in HTTP/2 and loopback communication.

The Register criticized, 'Microsoft caused a glitch in its media creation tool in the same week that a new version of its operating system ruined local development. The multi-million-dollar upgrade drive highlights how fragile the Windows ecosystem remains.'