On October 14, 2025, Microsoft released the Windows 11 cumulative update KB5066835. It was intended to deliver security fixes and stability improvements, but instead introduced a serious problem. The update broke localhost functionality for many users, causing major disruption for developers who rely on local testing every day.
Reported on by TechPowerUp, the update broke localhost connections, which use the 127.0.0.1 or localhost address and are essential for testing websites, apps, and APIs on a local machine. After the update, browsers could no longer load anything running on localhost, and local projects stopped responding entirely.
