Microsoft Windows 11 October Update Disrupts Localhost (127.0.0.1) Connectivity

Microsoft’s October 2025 Windows 11 update, KB5066835, released on 14 October 2025 for OS Builds 26200.6899 and 26100.6899, disrupted localhost connections by blocking access to services on 127.0.0[.]1. The patch, which aimed to fix security vulnerabilities and included updates for PowerShell Remoting, Windows Hello face recognition, and Chromium-based browser print preview, inadvertently broke local network connectivity for many users and development environments.

Developers relying on localhost for testing web applications, databases, and APIs reported connection timeouts and unresponsive local servers, with Docker containers and certain API testing tools affected as well. According to KB5066835, other fixes addressed in this release included a resolved print preview screen freezing, improved PowerShell Remoting and WinRM timeouts, and the removal of the ltmdm64.sys driver affecting fax modem hardware.

Until a formal fix is issued, rolling back the update via DISM and testing in isolated environments are suggested, as is monitoring the Windows Release Health Dashboard and @WindowsUpdate for guidance. The article notes that while Microsoft acknowledged several known issues, the networking regression itself appears to have emerged specifically with the October security patch.