Microsoft’s October 2025 Windows 11 update has introduced an unexpected connectivity issue affecting developers and IT professionals worldwide.
The security patch KB5066835, released on October 14, 2025, for OS Builds 26200.6899 and 26100.6899, has disrupted localhost connections, preventing applications from accessing services running on the loopback address 127.0.0.1.
The update, which primarily addressed security vulnerabilities and included fixes for PowerShell Remoting, Windows Hello face recognition, and Chromium-based browser print preview issues, inadvertently broke local network connectivity for numerous users.
Developers relying on localhost for testing web applications, databases, and APIs have reported connection failures immediately following the update installation.
Key issues reported by users include:
- Connection timeouts when accessing services on 127.0.0.1.
- Local web servers failing to respond to requests.
- Database management systems unable to establish localhost connections.
- Docker containers experiencing networking failures.
- API testing tools reporting unreachable endpoints.
Impact on Development Environments
The localhost connectivity disruption has created significant challenges for software developers and system administrators. Applications attempting to connect to local services through 127.0.0.1 or localhost are encountering connection timeouts and errors.
This affects popular development tools including local web servers, database management systems like MySQL and PostgreSQL, and containerized applications running through Docker.
Testing environments that depend on localhost connectivity have become temporarily inoperable, forcing developers to seek workarounds or delay critical projects.
The timing proves particularly problematic as many organizations rely on stable development environments for ongoing software releases and maintenance cycles.
Web development frameworks, API testing platforms, and microservices architectures have all experienced disruptions due to this networking regression.
Other issues fixed in KB5066835:
- Chromium-based browser print preview screen freezing resolved.
- Gamepad sign-in input responsiveness restored.
- PowerShell Remoting and WinRM 10-minute timeout issue addressed.
- Windows Hello face recognition setup error corrected.
- Removal of ltmdm64.sys driver affecting fax modem hardware.
Microsoft’s Response and Known Issues
While Microsoft’s official release notes for KB5066835 acknowledge several known issues—including problems playing protected content in BluRay, DVD, and Digital TV applications—the localhost connectivity problem was not initially documented.
The update also removed the ltmdm64.sys driver, affecting fax modem hardware, and addressed timeout issues with PowerShell Remoting and Windows Remote Management commands.
The security update incorporates improvements from the September 29, 2025 preview update KB5065789, which included fixes for gaming input responsiveness and browser functionality.
However, the networking regression appears to have emerged specifically with the October security patch.
Temporary Solutions and Recommendations
Until Microsoft releases an official fix, affected users can consider rolling back the update using DISM command-line tools.
The company’s documentation provides instructions for removing the latest cumulative update while preserving the servicing stack update, though this approach may leave systems vulnerable to the security issues the patch was designed to address.
IT administrators are advised to test the update in isolated environments before widespread deployment.
Organizations heavily dependent on localhost connectivity for development operations may want to postpone installation until Microsoft acknowledges and resolves the networking issue.
Microsoft has not yet issued a statement regarding when a corrective patch will be available. Users experiencing localhost connectivity problems should monitor the Windows Release Health Dashboard and follow @WindowsUpdate on social media for the latest information on this developing situation.
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