"Microsoft's October Windows 11 update has managed the impressive feat of breaking localhost, leaving developers unable to access web applications running on their own machines. The problem first surfaced on Microsoft's own support forums and quickly spread to Stack Overflow and Server Fault after the October 2025 cumulative update (KB5066835) landed, which appears to have severed Windows' ability to talk to itself. Developers describe HTTP/2 protocol errors and failed connections affecting everything from ASP.NET builds to Visual Studio debugging sessions."
"The bug, introduced in build 26100.6899, has been traced to HTTP.sys, the Windows kernel component that handles local HTTP traffic. Developers have found that uninstalling KB5066835, and in some cases its sibling KB5065789, restores localhost functionality. Others have discovered a temporary workaround that involves manually disabling HTTP/2 in the registry, which works but feels a bit like using a sledgehammer to swat a fly."
An October 2025 cumulative update for Windows 11 (KB5066835, build 26100.6899) broke localhost by disrupting HTTP.sys and HTTP/2, preventing local web access. Developers report HTTP/2 protocol errors and failed connections across ASP.NET builds and Visual Studio debugging sessions. Uninstalling KB5066835, and sometimes KB5065789, restores localhost functionality. A temporary workaround disables HTTP/2 in the registry. Clean installs of Windows 11 24H2 do not exhibit the problem, indicating an interaction with existing system configurations. Microsoft had not acknowledged the issue when reported. The update coincided with Windows 10 end-of-support and media creation tool failures affecting upgrades.
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