Microsoft’s October 2025 cumulative replace for Home windows 11 has disrupted localhost performance, stopping builders and customers from accessing native net purposes and companies by way of 127.0.0.1.
The difficulty, tied to replace KB5066835 launched on October 14, impacts builds like 26100.6899 and has sparked widespread complaints on boards, together with Microsoft’s help pages, Stack Overflow, and Server Fault.
This breakage halts important duties comparable to debugging in Visible Studio and testing ASP.NET tasks, impacting each particular person builders and enterprise environments reliant on native servers.
HTTP/2 Protocol Errors
The issue manifests as HTTP/2 protocol errors, comparable to ERR_HTTP2_PROTOCOL_ERROR or ERR_CONNECTION_RESET, when making an attempt to connect with localhost-hosted companies.
Customers report that purposes utilizing IIS Categorical or Kestrel servers fail to reply, turning routine native growth right into a irritating ordeal. For example, Autodesk confirmed disruptions to its Vault software program, urging prospects to roll again the replace the place potential.
Group threads spotlight how the bug extends past hobbyists, affecting manufacturing desktop purposes that depend upon loopback communication for inside processes.
One developer famous that even easy net previews in Chromium-based browsers now encounter unresponsive connections post-update.
Investigations level to modifications in HTTP.sys, the kernel driver dealing with HTTP visitors, because the offender behind the localhost severance.
This part, up to date in KB5066835 to deal with safety vulnerabilities, inadvertently breaks loopback HTTP/2 negotiations, particularly in configurations with the prior September preview replace KB5065789.
Affected customers have discovered aid by typically uninstalling KB5066835 alongside KB5065789 utilizing command line instruments like wusa.exe, adopted by a restart.
Different workarounds embrace disabling HTTP/2 by way of registry edits beneath HKEY_LOCAL_MACHINESOFTWAREMicrosoftIISParameters or updating Microsoft Defender Antivirus definitions by way of KB2267602, which resolves the difficulty with out full rollback in some circumstances.
Contemporary Home windows 11 installations seem immune, suggesting the error stems from interactions with current setups moderately than a core flaw.
As of October 17, 2025, Microsoft has not publicly acknowledged the bug on its KB5066835 help web page, although discussion board responses from engineers point out inside consciousness and ongoing fixes.
Some stories declare a subsequent Defender intelligence replace or minor patch has mitigated it for rolled-back programs, however variability persists throughout {hardware} and configurations.
With Home windows 10’s help ending quickly, this glitch underscores challenges in transitioning customers to Home windows 11 amid recurring replace regressions.
Builders are suggested to pause updates quickly and monitor launch well being dashboards for official resolutions, guaranteeing minimal downtime of their workflows. The incident joins a sample of post-patch points, prompting requires improved high quality management at Redmond.
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