Windows 11 KB5066835 is a mandatory security update released in October 2025 for versions 24H2 and 25H2. While it introduces new features such as faster File Explorer context menus and improved volume controls, it has also caused multiple serious issues, now officially acknowledged by Microsoft.
Localhost (127.0.0.1) Connection Error
After installing KB5066835, many developers reported that localhost (127.0.0.1) stopped working. This affected applications such as React, Flutter, IIS, and other local development environments, preventing them from connecting locally.
Microsoft confirmed that the issue is caused by a bug in the kernel HTTP.sys component, which breaks HTTP/2 connections on localhost. Affected systems often show errors such as ERR_HTTP2_PROTOCOL_ERROR.
Fixing the Localhost Issue
Microsoft states that an emergency patch is rolling out, though it may take up to 48 hours to reach all devices. Until then, you can apply a manual workaround.
Steps:
- Open PowerShell as Administrator
- Run the two commands provided by Microsoft
- Restart your PC
This workaround disables HTTP/2, switches the system to HTTP/1.1, and restores localhost connectivity.
File Explorer Preview Pane Not Working
Another major bug affects File Explorer’s Preview Pane. When attempting to preview files such as PDFs, users may see the warning:
“The file you are attempting to preview could harm your computer.”
Fix Option 1: Unblock Files Using PowerShell
- Open PowerShell as Administrator
- Run the unblock command (replace Documents with your folder name, such as Downloads)
- Close PowerShell
The Preview Pane should work again.
Fix Option 2: Disable the Security Warning via Registry
- Open Registry Editor
- Navigate to the appropriate path
- Create a new key named Attachments
- Inside it, create a DWORD (32-bit) value named SaveZoneInformation
- Set its value to 1
- Restart File Explorer
Logitech Devices Not Working Properly
After installing KB5066835, some Logitech users, particularly those using MX Master or MX Anywhere series, reported that custom shortcuts and zoom functions stopped working.
- Logitech Options / Options+ may fail to recognize mapped keys
- The issue appears related to Windows input handling
Current workaround:
The only reliable fix at the moment is to roll back the KB5066835 update.
Keyboard and Mouse Not Working in WinRE
One of the most serious issues reported is that keyboard and mouse input stop working inside the Windows Recovery Environment (WinRE). This makes system recovery nearly impossible if Windows fails to boot.
Advanced Manual Fix (For IT Admins / Advanced Users)
- Disable WinRE using:
reagentc /disable - Copy winre.wim from a Windows 11 25H2 ISO
- Replace the existing file in:
C:\Windows\System32\Recovery - Re-enable WinRE:
reagentc /enable
Should You Uninstall KB5066835?
Not every system is affected. If your PC is running normally, you can safely keep the update installed.
However, if you are experiencing:
- Localhost connection failures
- Broken File Explorer previews
- Logitech device issues
- WinRE input problems
Then uninstalling KB5066835 is a reasonable temporary solution until Microsoft releases a full fix.
