Windows 11 October Update Disrupts Localhost Services — Developers Urged to Roll Back KB5066835
Windows 11 October Update Disrupts Localhost Services — Developers Urged to Roll Back KB5066835
Microsoft’s latest cumulative update for Windows 11, KB5066835, released on October 14, has unexpectedly disrupted localhost (127.0.0.1) functionality — a critical element for developers and IT professionals running local web applications and testing environments.
Critical Impact on Local Development Environments
After installing the update (affecting builds such as 26100.6899), users began reporting widespread failures when accessing local web servers and debugging tools. Tasks that rely on IIS Express, Kestrel, or ASP.NET environments suddenly return connection errors like:
ERR_HTTP2_PROTOCOL_ERROR
ERR_CONNECTION_RESET
These disruptions have halted essential workflows across individual and enterprise systems, including software from major vendors such as Autodesk, which confirmed issues with its Vault application.
Root Cause: HTTP.sys Modification
Technical analysis points to changes in the HTTP.sys driver, the Windows kernel component responsible for managing HTTP traffic.
While the update aimed to patch security vulnerabilities, it inadvertently broke loopback (localhost) HTTP/2 connections — particularly for systems that also installed the September preview update KB5065789.
Community Workarounds and Temporary Fixes
Developers have identified several temporary solutions to restore functionality:
Uninstall KB5066835 and KB5065789 using wusa.exe commands, then reboot.
Disable HTTP/2 in the registry under
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\IIS\ParametersUpdate Microsoft Defender Antivirus definitions via KB2267602, which has resolved the issue for some users without requiring full rollback.
Interestingly, fresh Windows 11 installations appear unaffected, suggesting that the bug arises from update conflicts in existing configurations.
Microsoft Response and Ongoing Monitoring
As of this writing, Microsoft has not published an official advisory on the KB5066835 page, though engineers in community forums have acknowledged internal awareness and ongoing investigations.
Some users report partial recovery following a minor Defender intelligence update, but inconsistencies remain across various hardware and configurations.
Elliptic Systems’ Advisory
With Windows 10’s end of support approaching, this incident highlights ongoing update reliability challenges for Windows 11 deployments.
Elliptic Systems recommends that IT teams and developers:
Pause Windows Updates temporarily until Microsoft issues a confirmed fix.
Test patch rollbacks in isolated environments before applying organization-wide.
Monitor the Microsoft Release Health Dashboard and Elliptic Systems’ cybersecurity updates for resolution guidance.
Maintaining secure, stable, and predictable development environments is essential — especially as organizations accelerate modernization under the Windows 11 ecosystem. Elliptic Systems Corporation
