Microsoft Acknowledges Issues with Windows 11 KB5066835: Localhost, File Explorer Preview, and Installation Errors

Recently, Microsoft acknowledged that the Windows 11 update KB5066835 disrupts HTTP/2 connections on localhost (127.0.0.1).This significant issue prevents locally hosted applications from connecting to networks. However, that is just one of the multiple concerns arising from this update for users operating on versions 24H2 and 25H2:

Failure to install the October 2025 Patch Tuesday update due to errors like 0x800f0922, 0x800f0983, 0x800f081f, 0x80071a2d, and 0x800f0991.
A bug preventing document previews in File Explorer, mistakenly flagging them as security risks.
Compatibility issues with certain Logitech peripherals.

As a critical security update, Windows 11 KB5066835 installs automatically for those using builds 24H2 and 25H2. This particular update introduces several appealing features, such as repositioning on-screen volume indicators and enhancing the responsiveness of the File Explorer context menu. Unfortunately, it has also resulted in substantial disruptions.

Breakdown of Localhost Connections Post-October 2025 Update

As a frequent user of localhost for debugging React applications, I discovered that, post-update, connections to 127.0.0.1 became impossible. All attempts to connect to my web app culminated in error messages such as “ERR_CONNECTION_RESET” and “ERR_HTTP2_PROTOCOL_ERROR, ” irrespective of the browser in use.

Connection reset error in Microsoft Edge

This issue was not just limited to React apps; I encountered similar connectivity problems while debugging Flutter web apps in Microsoft Edge. Microsoft confirmed to Windows Latest that the glitch is not isolated, impacting all server-side applications utilizing HTTP.sys.

In investigative tests conducted by Windows Latest, it was identified that the recent Windows 11 updates triggered a regression within HTTP.sys, the kernel-mode HTTP server. This caused a malfunction during the HTTP/2 handshake when browsers or applications would attempt to connect to services on 127.0.0.1.

Microsoft explained that following the updates, Internet Information Services (IIS) websites may fail to load, generating error messages such as “Connection reset” or similar. This encompasses websites hosted on http://localhost/ and other IIS connections.

The implications of this issue are far-reaching since the update disrupts IIS, affecting all functionalities reliant on HTTP.sys. Essentially, Windows should manage 127.0.0.1 requests by initiating HTTP/2 sessions at the kernel level before routing requests to IIS workers, the ASP. NET Core module, or applications.

With KB5066835, connections reset prematurely due to early termination of sessions at the kernel layer. Consequently, users face two specific error messages:

ERR_CONNECTION_RESET
ERR_HTTP2_PROTOCOL_ERROR

Resolving Localhost Issues After the October 2025 Update (KB5066835)

In response to these issues, Microsoft is issuing an emergency patch aimed at rectifying localhost-related problems for users on Windows 11 versions 24H2 and 25H2. Note that this hotfix might take over 48 hours to appear on impacted systems. Users are encouraged to manually check for updates and restart their PCs, regardless of whether new updates are displayed.

This action facilitates a speedy retrieval of the patch from Microsoft’s servers and aids in the recovery from the errors induced by the problematic code.

New-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Services\HTTP\Parameters' -Name 'EnableHttp2Tls' -PropertyType DWord -Value 0 -Force New-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Services\HTTP\Parameters' -Name 'EnableHttp2Cleartext' -PropertyType DWord -Value 0 -Force

Executing the above PowerShell command will establish two DWORD (32-bit) values, EnableHttp2Tls and EnableHttp2Cleartext, setting them to zero. This action disables HTTP/2 and defaults to using HTTP/1.1, thereby allowing localhost/HTTP/2 connections to function correctly on Windows 11.

File Explorer Preview Pane Issues Linked to Windows 11 KB5066835

Error in File Explorer preview pane — File Explorer bug affects file previews after the October update

File Explorer typically consists of two panes: a Details pane displaying basic information and a Preview pane that provides valuable content insights. The former is often less helpful compared to the Preview pane.

When selecting certain compatible file types, like. PDF, and having the Preview pane active, document previews should be visible on the right side of the window.

Functional File Explorer preview pane — Operational File Explorer preview pane under normal conditions

Users can scroll through pages and even copy text in the Preview pane, greatly enhancing productivity. However, the KB5066835 update has caused critical malfunctions in this feature.

Users confronting this issue receive a warning stating: “The file you are attempting to preview could harm your computer.” This alert surfaces primarily for files obtained from cloud storage services like OneDrive or Google Drive, instead of local files created on their PCs. In testing conducted by Windows Latest, most previews elicited this security alert.

Fixing the “Preview could harm your computer” Error in File Explorer

I experimented by uninstalling Windows 11 KB5066835 and subsequently rebooted my PC, regaining the ability to preview documents sans security alerts. Nevertheless, removing a security update typically isn’t advisable. Thankfully, alternative workarounds exist while Microsoft devises an official fix.

If you encounter the warning, here are a couple of potential solutions:

1. Use PowerShell to Unblock Files (Recommended)

Launch PowerShell with administrator permissions.
Execute the command: Unblock-File -Path "C:\Users\admin\Documents\*.pdf"
Exiting PowerShell will restore functionality to the File Explorer preview.

This command deactivates Windows 11’s security feature for specific file types in the designated directory. Adjust the path to unblock PDF previews in whichever folder suits your needs.

2. Disable File Explorer’s Built-in Security for Previews

Access the Registry Editor (Regedit).
Navigate to this location:

Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\

Create a new key labeled “Attachments.” Select it, then establish a new DWORD (32-bit) value.
Set the DWORD value to “SaveZoneInformation” and assign it a value of 1.