**Update October 17, 2025:** Microsoft has resolved the issue through a Known Issue Rollback (KIR) update, with additional details provided below.

Microsoft’s October Windows 11 updates introduced a significant bug that disabled localhost functionality, preventing applications from connecting to the local IP address 127.0.0.1 over HTTP/2 protocols.

## What is Localhost?

Localhost refers to your local computer, accessible through the IP address 127.0.0.1. Developers rely on localhost for testing websites and debugging applications, while various programs use it to connect to local services for essential operations.

## The Problem

Following the installation of Windows 11 updates KB5066835 (October Patch Tuesday) and KB5065789 (September preview), users experienced connection failures when applications attempted to reach localhost. Common error messages included “ERR_CONNECTION_RESET” and “ERR_HTTP2_PROTOCOL_ERROR.”

The bug affected numerous critical applications:
– **Visual Studio** debugging tools
– **SQL Server Management Studio (SSMS)** authentication
– **Duo Desktop app** for device security verification

Reports flooded Microsoft forums, Stack Exchange, and Reddit from frustrated users unable to establish HTTP connections to 127.0.0.1.

## Temporary Workarounds

Before Microsoft’s official fix, users found several temporary solutions:

**Registry Modification:** Some users reported success by disabling HTTP/2 protocol through registry entries, though results varied.

**Update Removal:** The most reliable temporary fix involved uninstalling the problematic updates using these commands:
“`
wusa /uninstall /kb:5066835
wusa /uninstall /kb:5065789
“`

## Microsoft’s Official Fix

Microsoft confirmed the issue stems from problems with the HTTP.sys driver affecting server-side applications. The company released a Known Issue Rollback (KIR) update to resolve the bug.

**To apply the fix:**
1. Open Windows Settings
2. Navigate to Windows Update
3. Click “Check for updates”
4. Install any available updates
5. Restart your device (even if no updates install)

## Root Cause

Microsoft explained that the flaw affects applications relying on the HTTP.sys driver, causing IIS websites and localhost connections to fail. The issue’s occurrence depends on various factors including internet connectivity and update installation timing, which explains why some users weren’t affected despite having the problematic updates installed.

## Resolution

After restarting Windows with the KIR update, localhost connection issues should be completely resolved, restoring normal functionality to affected applications and development environments.

This incident highlights the importance of thorough testing before releasing system updates, particularly those affecting core networking functionality that developers and enterprise applications depend on daily.