The recent October 2025 cumulative update for Windows 11, KB5066835, has caused widespread disruption in development environments by breaking localhost functionality, preventing developers from running and testing web applications locally. While the patch addressed important security issues, it inadvertently blocked Windows’ ability to handle internal network communication, a critical feature for local development workflows. Developers rely on localhost to build, test, and debug applications safely before deploying them to production, making this bug especially disruptive.
Reports of the issue quickly surfaced on Microsoft support forums, Stack Overflow, and Stack Exchange, with many developers expressing frustration at the sudden inability to run local servers. Microsoft has confirmed the bug and updated its Windows release health page with guidance on mitigating the problem. Security researcher David Shipley from Beauceron Security highlighted the severity, noting that breaking localhost “is huge” for anyone actively doing software development.
KB5066835 affects Windows 11 versions 24H2 and 25H2 and follows the September 29, 2025, preview update KB5065789. While both updates addressed various security and functionality issues—including print preview problems in Chromium-based browsers, PowerShell Remoting timeouts, and Windows Hello setup errors—developers quickly discovered that the updates also caused unanticipated connection failures and HTTP/2 protocol errors. These issues impact popular development tools like ASP.NET and Visual Studio, disrupting normal coding workflows.
The bug has forced many companies to roll back the update to restore their development environments, raising questions about Microsoft’s quality assurance processes. For developers, it serves as a reminder of the fragility of relying on cumulative updates in active development settings and the need for careful testing before deploying updates in production or development machines.