The October cumulative update KB5066835 landed on 17 October 2025 and promptly severed Windows’ ability to talk to itself, according to early reports on Microsoft’s forums that then spilled onto Stack Overflow and Server Fault.
The failure throws HTTP/2 protocol errors and connection refusals across ASP.NET builds and Visual Studio debugging sessions, which is precisely the sort of day-ruiner that empties coffee pots.
The culprit sits in HTTP.sys, the kernel component that handles local HTTP traffic, and the offending build is 26100.6899. Uninstalling KB5066835, and sometimes its sibling KB5065789, brings localhost back to life.
A stopgap involves disabling HTTP/2 in the registry. It works, but it is like using a sledgehammer to swat a fly while hoping you do not dent the desk.
Users report mixed fortunes reinstalling the patch or jumping to newer builds, while clean installs of Windows 11 24H2 appear unaffected, pointing to a messy interaction with existing configs rather than a universal howler.
Meanwhile, Stack Overflow moderators have locked multiple posts and Server Fault threads are clogged with developers trying to coax their local servers into breathing again.
All this landed as Redmond pushed its final Windows 10 update and told the holdouts to get with Windows 11. The Windows 11 media creation tool conked out the day before, and the same patch cycle ticked end-of-support boxes for Office 2019 and several server products.