Microsoft Windows 11 October Update Breaks Localhost (127.0.0.1) Connections
Microsoft's cumulative update for Windows 11 in October 2025 has led to issues with localhost functionality, disrupting access to local web applications and services via 127.0.0.1.
Microsoft's cumulative update for Windows 11 in October 2025 has led to issues with localhost functionality, disrupting access to local web applications and services via 127.0.0.1.
The issue is linked to update KB5066835 released on Mon, Oct 14, 2025, affecting builds such as 26100.6899. This has resulted in user complaints across various forums, including Microsoft's support pages.
This disruption affects essential activities like debugging in Visual Studio and testing ASP.NET projects, impacting both individual developers and business environments that depend on local servers.
The update causes HTTP/2 protocol errors, including ERR_HTTP2_PROTOCOL_ERROR or ERR_CONNECTION_RESET, when accessing services hosted on localhost.
Applications using IIS Express or Kestrel servers are reportedly unresponsive, affecting local development. Autodesk has confirmed impacts on its Vault software and recommends rolling back the update when possible.
Community feedback indicates that the issue impacts production desktop applications relying on loopback communication for internal functions.
The issue is linked to update KB5066835 released on Mon, Oct 14, 2025, affecting builds such as 26100.6899.
Modifications in HTTP.sys, the kernel driver managing HTTP traffic, are identified as the cause of the localhost disruption. The changes in KB5066835, intended to address security vulnerabilities, inadvertently affect loopback HTTP/2 communications, especially in configurations updated with the prior KB5065789 from September.
Users have addressed the issue by uninstalling KB5066835 and KB5065789 using command-line tools like wusa.exe, followed by a system restart.
Alternative solutions include disabling HTTP/2 via registry edits under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\IIS\Parameters or updating Microsoft Defender Antivirus definitions through KB2267602, which resolves the issue without requiring a full rollback in some cases.
New installations of Windows 11 do not exhibit this problem, indicating the issue arises from existing system configurations rather than a fundamental flaw.
As of Thu, Oct 17, 2025, Microsoft has not officially acknowledged the bug on the KB5066835 support page, although internal discussions suggest awareness and efforts to rectify the issue.
Some users report that a subsequent Defender intelligence update or minor patch has mitigated the issue for rolled-back systems, though results vary based on hardware and configurations.
With the end of Windows 10 support approaching, this glitch highlights challenges in transitioning to Windows 11 amid recurring update issues.
Developers are advised to temporarily pause updates and monitor release health dashboards for official resolutions to ensure minimal workflow disruptions. This incident is part of a pattern of post-update issues, prompting calls for improved quality control.
Based on reporting by Cyber Security News.