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Microsoft has resolved a high-impact bug that severed HTTP/2 connections to localhost (127.0.0.1) following recent Windows security updates, disrupting essential developer workflows and enterprise applications. The flaw, introduced in September's KB5065789 preview update and October's KB5066835 Patch Tuesday release, caused widespread ERR_CONNECTION_RESET and ERR_HTTP2_PROTOCOL_ERROR failures across Windows 11 and Windows Server 2025 systems.

The Development Workflow Killer

At its core, the bug corrupted HTTP.sys—Windows' kernel-mode HTTP stack that underpins IIS, ASP.NET Core, and local development servers. Developers suddenly found critical tools failing:
- Visual Studio debugging sessions crashing
- SQL Server Management Studio (SSMS) Entra ID authentication broken
- Duo Desktop and other local services unable to establish connections

"Following installation of updates on or after September 29, server-side applications that rely on HTTP.sys may experience issues with incoming connections," Microsoft acknowledged in its release health dashboard. The company traced the instability to timing conflicts between device restarts, update installations, and network connectivity states—a fragility that amplified disruption.

Emergency Mitigation Strategies

Microsoft deployed a two-tiered remediation approach:
1. Consumer Devices: Automated Known Issue Rollback (KIR) pushed to most home users
2. Enterprise Systems: Manual group policy deployment required for managed Windows 11 24H2/25H2 and Server 2025 environments

Affected users can trigger repairs via:

# Force update check and restart
Start-Process ms-settings:windowsupdate
Restart-Computer -Force

Even if no new updates appear, the restart often clears the corrupted state—a band-aid solution until Microsoft ships a permanent fix in future updates.

The Systemic Risk in Update Pipelines

This incident exposes deeper Windows servicing challenges:
- Critical Path Testing Gaps: HTTP.sys failures should be caught in pre-release validation given its centrality to developer toolchains
- Enterprise Impact: Manual KIR deployments create administrative overhead during critical outages
- Debugging Blindspots: The timing-dependent nature of the bug made reproduction sporadic, delaying diagnosis

As Microsoft prepares a permanent patch, development teams should audit localhost-dependent workflows and monitor IIS health checks. The episode underscores why enterprises increasingly test updates in isolated environments before deployment—especially when core networking components are involved.

Source: BleepingComputer