Resolve LocalHost Problems Post KB5066835 Update Installation on Windows 11

October 2025 Security Update for Windows 11 (25H2 and 24H2) causes LocalHost connection issues.
Microsoft has issued a Known Issue Rollback (KIR) to address this concern.
Manual registry edits can also serve as a workaround for affected users.

Following the installation of the October 2025 Security Update (KB5066835) on Windows 11 versions 25H2 and 24H2, users may encounter a significant issue that prevents websites from being accessed through the LocalHost (127.0.0.1) HTTP/2 connection.

LocalHost functions as a reserved hostname linked to your own computer, specifically identified by the IP address 127.0.0.1 in IPv4. Essentially, when you direct a network request to LocalHost, the operating system reroutes the data back to your machine, eliminating the need for external network access.

According to Microsoft’s official communication, after deploying builds 26100.6899 and 26200.6899, users may experience connectivity issues with server-side applications reliant on HTTP.sys. This may result in failure to load IIS websites, often resulting in errors such as “Connection reset – error (ERR_CONNECTION_RESET)” for websites hosted on http://localhost/ or other IIS services.

The issue does not appear uniformly across all systems, as Microsoft acknowledges that a variety of conditions may affect its occurrence.

For those facing this problem, Microsoft is now distributing a Known Issue Rollback (KIR) package that lets users revert the specific issues introduced by the update without having to uninstall it in its entirety. Should this solution not suffice, users can apply several commands to resolve the problem manually.

Resolving LocalHost Issues Post-KB5066835 Update on Windows 11

This issue can be addressed through two primary methods: automatic fixing using the KIR package or manual registry adjustments.

Automatic Resolution via Known Issue Rollback

To manually install the Known Issue Rollback package in Windows 11, follow these succinct steps:

Access the Settings menu.
Navigate to Windows Update.
Select the Check for updates button.
Click the Restart now button.

Upon completing these steps, if a fix is available for your system, it will automatically download and install.

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Manual Resolution of LocalHost Issues

To remedy the LocalHost connection issue affecting IIS, please follow these instructions:

Open the Start menu.
Search for PowerShell (or Terminal), right-click on the top result, and select Run as administrator.
Execute the following command to create the EnableHttp2Tls DWORD with a value of zero:

New-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Services\HTTP\Parameters' -Name 'EnableHttp2Tls' -PropertyType dword -Value 0 -Force
Run this command to create the EnableHttp2Cleartext DWORD, also set to zero:

New-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Services\HTTP\Parameters' -Name 'EnableHttp2Cleartext' -PropertyType dword -Value 0 -Force
Restart your computer.