Microsoft has confirmed that the KB5066835 security update for Windows 11, scheduled for October 2025, has introduced several issues affecting system performance and connectivity.
Connection and System Errors
The KB5066835 update disrupts HTTP/2 connections over localhost, leading to errors such as ERR_CONNECTION_RESET and ERR_HTTP2_PROTOCOL_ERROR. This occurs because the kernel-mode HTTP server (HTTP.sys) improperly handles HTTP/2 handshakes, causing early session termination. Consequently, locally hosted applications and IIS sites cannot receive data.
Users have also reported installation errors on Patch Tuesday, including codes like 0x800f0922 and 0x800f0983. Additionally, File Explorer's Preview pane throws security warnings for certain files, blocking previews, and Logitech device features may become nonfunctional.
Temporary Solutions and Hotfix
As an interim measure, Microsoft recommends users disable HTTP/2 via PowerShell to ensure fallback to HTTP/1.1, restoring localhost connection functionality. Users facing File Explorer issues can unblock files using PowerShell or modify registry settings, though the latter carries security risks.
For those experiencing input failures in WinRE, IT administrators can temporarily disable WinRE, replace system recovery files, and re-enable the environment.
Microsoft is deploying an emergency hotfix, urging users to install updates and reboot their systems as fixes become available.
