Microsoft’s October 2025 cumulative update for Windows 11 has disrupted localhost functionality, preventing developers and users from accessing local web applications and services via 127.0.0.1.
The issue, tied to update KB5066835 released on October 14, affects builds like 26100.6899 and has sparked widespread complaints on forums, including Microsoft’s support pages, Stack Overflow, and Server Fault.
This breakage halts critical tasks such as debugging in Visual Studio and testing ASP.NET projects, impacting both individual developers and business environments reliant on local servers.
HTTP/2 Protocol Errors
The problem manifests as HTTP/2 protocol errors, such as ERR_HTTP2_PROTOCOL_ERROR or ERR_CONNECTION_RESET, when attempting to connect to localhost-hosted services.
Users report that applications using IIS Express or Kestrel servers fail to respond, turning routine local development into a frustrating ordeal. For instance, Autodesk confirmed disruptions to its Vault software, urging customers to roll back the update where possible.
Community threads highlight how the bug extends beyond hobbyists, affecting production desktop applications that depend on loopback communication for internal processes.
One developer noted that even simple web previews in Chromium-based browsers now encounter unresponsive connections post-update.
Investigations point to modifications in HTTP.sys, the kernel driver handling HTTP traffic, as the culprit behind the localhost severance.
This component, updated in KB5066835 to address security vulnerabilities, inadvertently breaks loopback HTTP/2 negotiations, especially in configurations with the prior September preview update KB5065789.
Affected users have found relief by often uninstalling KB5066835 alongside KB5065789 using command line tools like wusa.exe, followed by a restart.
Alternative workarounds include disabling HTTP/2 through registry edits under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\IIS\Parameters or updating Microsoft Defender Antivirus definitions via KB2267602, which resolves the issue without full rollback in some cases.
Fresh Windows 11 installations appear immune, suggesting the error stems from interactions with existing setups rather than a core flaw.
As of October 17, 2025, Microsoft has not publicly acknowledged the bug on its KB5066835 support page, though forum responses from engineers indicate internal awareness and ongoing fixes.
Some reports claim a subsequent Defender intelligence update or minor patch has mitigated it for rolled-back systems, but variability persists across hardware and configurations.
With Windows 10’s support ending soon, this glitch underscores challenges in transitioning users to Windows 11 amid recurring update regressions.
Developers are advised to pause updates temporarily and monitor release health dashboards for official resolutions, ensuring minimal downtime in their workflows. The incident joins a pattern of post-patch issues, prompting calls for improved quality control at Redmond.
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