Windows 11 KB5066835 Update Breaks Critical System Features
Microsoft’s October 2025 security update for Windows 11 has created widespread disruptions for users running versions 24H2 and 25H2. The mandatory update KB5066835, released on October 14, 2025, breaks critical system functionality including localhost connections, File Explorer previews, and recovery environment tools.
The most severe issue affects localhost connections on the loopback address 127.0.0.1. Microsoft confirmed that the update breaks HTTP/2 connections for locally hosted applications, preventing developers and users from accessing services running on their own machines. This problem stems from a regression in HTTP.sys, the kernel-mode HTTP server component responsible for handling local HTTP traffic. When browsers or applications attempt to connect via HTTP/2 to services hosted on 127.0.0.1, HTTP.sys incorrectly handles the HTTP/2 handshake and resets the connection. Users encounter error messages including ERR_CONNECTION_RESET and ERR_HTTP2_PROTOCOL_ERROR across all browsers and development environments.
The localhost disruption affects a wide range of applications and services. Developers using React, Flutter, ASP.NET Core, and Visual Studio for debugging find their workflow completely blocked. Internet Information Services websites fail to load, affecting anything relying on HTTP.sys infrastructure. Docker containers experience networking failures, and database management systems like MySQL and PostgreSQL cannot establish localhost connections. Password management tools and access control systems that use localhost functionality also stop working.
Microsoft acknowledged the localhost issue and began rolling out an emergency patch. However, the company warned that the hotfix could take longer than 48 hours to reach affected computers. Users experiencing problems should check for updates and reboot their systems even if no new updates appear listed. For those unable to wait, a temporary workaround involves disabling HTTP/2 through registry modifications, forcing the system to fall back to HTTP/1.1 protocol.
The update also breaks File Explorer’s Preview pane functionality. Users attempting to preview documents, particularly PDF files downloaded from cloud storage services like OneDrive or Google Drive, encounter a false security warning stating the file could harm the computer. This affects documents from office networks as well. The Preview pane, normally useful for quickly viewing document contents without opening separate applications, becomes essentially unusable. Microsoft has not yet provided a timeline for fixing this issue, though workarounds exist through PowerShell commands or registry modifications.
Another critical problem affects the Windows Recovery Environment. USB keyboards and mice stop functioning in WinRE after installing KB5066835. This renders the recovery environment effectively useless because users cannot navigate between options or select troubleshooting tools. WinRE serves as an essential component for recovering systems that fail to boot or experience critical errors. Microsoft confirmed awareness of the issue and plans to release an out-of-band update specifically addressing WinRE problems. Until the fix arrives, affected users face significant challenges when attempting system recovery operations.
Users with Logitech peripherals report that custom shortcuts and special features stop working after the update. The MX Anywhere 3 and similar products lose functionality when mouse buttons mapped to keyboard shortcuts fail to respond. Microsoft made changes to Windows input handling that interfere with how Logitech Options and Options Plus software manage custom configurations. This appears unrelated to the HTTP.sys kernel behavior affecting localhost connections.
Installation problems plague many users attempting to apply the update. Error codes including 0x800f0922, 0x800f0983, 0x800f081f, 0x80071a2d, and 0x800f0991 prevent successful installation. These errors typically relate to insufficient space on system reserved partitions or conflicts with .NET Framework components. Microsoft recommends downloading the update directly from the Update Catalog or using the Media Creation Tool to perform an in-place upgrade rather than relying on standard Windows Update mechanisms.
The problems do not affect all Windows 11 installations universally. Some users report no issues after installing KB5066835, while others experience multiple simultaneous failures. Clean installations of Windows 11 24H2 appear unaffected, suggesting the errors stem from conflicts between the update and existing system configurations rather than fundamental bugs in the update itself.
Microsoft advises users without problems to keep the update installed due to its security importance. Those experiencing severe issues can uninstall KB5066835 through Windows Update settings under Update History, though this reduces system security until proper fixes arrive.
