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Windows 11 Updates Disrupt Localhost HTTP/2 Connections

Benhcmark Administrator

14/11/2025

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Windows 11 Update Breaking Your Localhost? How to Fix HTTP/2 Errors

If you’re a developer who has recently updated Windows 11, you may have run into a frustrating and unexpected issue: your local development servers are suddenly failing. Specifically, applications attempting to connect to localhost using the HTTP/2 protocol are being met with connection resets and TLS handshake failures. This isn’t a problem with your code; it’s a direct result of recent changes within the operating system.

This issue can bring development to a standstill, making it impossible to test web applications locally. Here’s a breakdown of what’s happening and, more importantly, how you can fix it.

Understanding the Problem: Connection Failures After Update

Developers using various frameworks, especially .NET and Node.js, have reported that after installing specific Windows 11 updates, their local web servers are no longer accessible over HTTP/2. When a browser or client tries to connect, the connection is abruptly terminated.

The primary symptoms include:

• TLS handshake failures, even with valid local development certificates.
• Browser errors such as ERR_HTTP2_PROTOCOL_ERROR in Chrome or similar messages in other browsers.
• Applications that rely on local APIs failing to connect.

The problem has been linked to two cumulative updates for Windows 11: KB5031354 and KB5030310. These updates appear to have introduced a bug or a change in how Windows handles high-performance TCP loopback connections, which is the underlying mechanism for all localhost traffic.

Who is Affected by This Issue?

This issue primarily impacts web developers and software engineers who:

• Are running Windows 11 (versions 22H2 and 23H2).
• Use local development servers that support or require HTTP/2 over TLS (HTTPS).
• Rely on localhost or the loopback address 127.0.0.1 for testing and development.

If your development workflow involves running a local backend and a frontend that communicates with it, you are highly likely to encounter this problem after installing the latest Windows updates.

The Solution: Manually Enabling TCP Fast Loopback

Fortunately, there is a straightforward workaround that resolves the connection issue by explicitly enabling a feature designed to optimize loopback performance. This involves adding a new key to the Windows Registry.

While editing the registry can seem daunting, following these steps carefully will restore your local development environment.

Important: This process requires administrative privileges on your machine.

Step-by-Step Instructions to Fix the Localhost Bug:

1. Open PowerShell as an Administrator. You can do this by right-clicking the Start button and selecting “Windows Terminal (Admin)” or “PowerShell (Admin)”.

2. Execute the Registry Command. Copy and paste the following command directly into the administrative PowerShell window and press Enter:

   New-ItemProperty -Path "HKLM:\System\CurrentControlSet\Services\HTTP\Parameters" -Name "EnableFastLoopback" -Value "1" -PropertyType "DWord" -Force
   

3. Understand the Command. This command adds a new DWORD value named EnableFastLoopback to the registry under the HTTP service parameters and sets its value to 1. This effectively turns on the TCP loopback optimization that the recent updates seem to have disrupted.

4. Reboot Your Computer. This is a critical step. The changes to the registry will not take effect until you have completely restarted your system. A simple sign-out is not enough; you must perform a full reboot.

After your computer restarts, your localhost connections using HTTP/2 should function correctly again. You can resume your development and testing without encountering the TLS handshake errors.

A Note on Future Updates

This registry edit is a workaround for a problem introduced in recent updates. It’s possible that Microsoft will release a permanent fix in a future Windows update that makes this manual change unnecessary. For now, this is the most reliable method to get your local environment back up and running.

Always exercise caution when making changes to the Windows Registry. Using the provided PowerShell command is a safe and precise way to apply this specific fix without navigating the Registry Editor manually, reducing the risk of accidental changes.

Source: https://www.bleepingcomputer.com/news/microsoft/windows-11-updates-break-localhost-127001-http-2-connections/