Windows 11 Update KB5066835 Causing Network and File Preview Errors
Microsoft’s October 2025 Windows 11 security update (KB5066835) is causing problems for some users, breaking local web connections and blocking file previews. The company has confirmed the issues and is preparing an emergency fix.
Microsoft’s October 2025 Windows 11 security update (KB5066835) is causing problems for some users.
Microsoft’s latest mandatory update for Windows 11, KB5066835, released in October 2025, has triggered several issues for users running versions 24H2 and 25H2. The update disrupts localhost (127.0.0.1) connections using HTTP/2, affecting developers who rely on local web servers and IIS websites. Microsoft says the problem stems from a kernel-mode HTTP server error.
Another bug causes File Explorer’s preview pane to show false security warnings, preventing users from viewing downloaded documents. A few users have also reported glitches with Logitech devices and input functions in the Windows Recovery Environment.
Microsoft has confirmed the issues and is rolling out an emergency patch to fix the connection problem. Users can apply temporary workarounds, such as disabling HTTP/2, but uninstalling the update is not advised unless necessary.
Quick Facts:
• Localhost (127.0.0.1) HTTP/2 connections fail after update KB5066835.
• File Explorer preview pane falsely flags safe files as harmful.
• Microsoft is releasing an emergency fix for the network issue.
Source: Windows Latest and The Register | Edited by The Fajr Post