Windows 11 update breaks localhost, prompting mass uninstall workaround

Steven

Windows 11 update breaks localhost, prompting mass uninstall workaround

Summary

Microsoft’s October 2025 cumulative update (notably KB5066835, build 26100.6899) caused localhost connections to fail on many Windows 11 machines by breaking HTTP.sys’s handling of local HTTP traffic. Developers reported HTTP/2 protocol errors and failed connections affecting ASP.NET builds, Visual Studio debugging and other local web workflows. Uninstalling KB5066835 (and in some cases KB5065789) restores functionality; a registry tweak to disable HTTP/2 also works as a temporary, blunt workaround. The issue appears tied to interactions with existing system configurations (it often does not occur on clean 24H2 installs). Microsoft later acknowledged the problem and pointed users to guidance and mitigations on its release-health page.

Key Points

  • October 2025 cumulative update KB5066835 (build 26100.6899) disrupted localhost by affecting HTTP.sys.
  • Symptoms include HTTP/2 protocol errors and failed local connections impacting ASP.NET, Visual Studio and local servers.
  • Workarounds: uninstall KB5066835 (and sometimes KB5065789) or disable HTTP/2 via the registry.
  • The bug seems to stem from update/configuration conflicts rather than every installation — clean Windows 11 24H2 installs often work fine.
  • Microsoft confirmed the issue and published mitigations on its Windows release-health guidance page.

Context and relevance

This comes during a major Windows transition week — Windows 10 reached end of support and Microsoft shipped multiple high-profile updates. The bug highlights how a single cumulative update can derail local development workflows and tooling, creating widespread disruption for dev teams and sysadmins. It also underlines the fragility of complex OS update interactions and the importance of testing patches against varied system states.

Why should I read this?

Short answer: if you develop on Windows, you need to know this now. It’s the kind of patch that will silently break your local servers, mess with debugging and waste a day’s work unless you either uninstall it or apply a workaround. We’ve done the legwork so you don’t have to trawl Stack Overflow threads — read the fixes and the Microsoft guidance before you click “Update”.

Source

Source:https://go.theregister.com/feed/www.theregister.com/2025/10/16/windows_11_update_localhost/