Windows 11 October 2025 Update Breaks Localhost Functionality Forcing Developers to Revert the Update

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KB5066835 (Oct 2025) caused localhost/HTTP/2 failures that broke local dev servers; many developers removed the update while Microsoft prepares a proper fix.

Quick Report

After the Windows 11 October cumulative update (KB5066835), developers reported localhost and HTTP/2 failures that broke local development servers and debugging tools; many reverted the update while Microsoft works on a fix.

The issue surfaced across Microsoft Support Forums, Stack Overflow and community channels where users noted that HTTP.sys regressions in build 26100.6899 disrupted connections to localhost. Workarounds have involved uninstalling the October package (and the related September KB5065789) while Microsoft investigates and prepares a patch.

Affected tools include ASP.NET, Visual Studio debugging, and other frameworks relying on local HTTP services. Microsoft acknowledged the problem and said it is working on a resolution in a future update.

Written using GitHub Copilot GPT-4o-mini in agentic mode instructed to follow current codebase style and conventions for writing articles.

Source(s)

  • TPU
  • Microsoft Learn — release health status (Windows 11)
  • Stack Overflow thread reporting localhost failures