Fix Localhost Issues on Windows 11 After KB5066835: IIS, Kestrel, and IIS Express Not Working

After the recent Windows 11 update KB5066835, many developers found their local web servers — including IIS, IIS Express, and Kestrel — suddenly failing to load. Browsers began throwing ERR_CONNECTION_RESET or ERR_HTTP2_PROTOCOL_ERROR messages when trying to reach localhost or 127.0.0.1.

If your development workflow has come to a halt, don’t panic. The issue stems from a regression in how Windows handles HTTP/2 over loopback connections. This guide explains several proven fixes — from Microsoft’s official rollback solution to safe registry tweaks — to restore your local development environment.

1. Fix Localhost Issues Using Windows Update or KIR (Preferred Method)

Microsoft has already acknowledged this bug and released a Known Issue Rollback (KIR) and update-based fix. This solution is the safest because it keeps all security updates intact.

Step-by-Step Fix

Step 1:
Open Settings → Windows Update and click Check for updates. Allow any available updates to install — these may include Microsoft’s rollback or follow-up patch.

Step 2:
Once installation is complete, restart your computer. This reboot activates the KIR rollback and often resolves all HTTP/2 loopback issues.

Step 3 (Enterprise Environments):
For managed devices running Windows 11 24H2, 25H2, or Windows Server 2025, apply the KIR Group Policy package.
Download and install the appropriate MSI for your environment from Microsoft’s official KIR repository.

2. Workaround: Disable HTTP/2 Globally (via HTTP.sys Registry Edit)

If Windows Update hasn’t fixed your issue yet, you can temporarily disable HTTP/2 system-wide. This forces local traffic to use HTTP/1.1, bypassing the broken HTTP/2 handling.

⚠️ Always back up your registry before making changes.

Steps to Disable HTTP/2 Globally

1- Press Win + R, type regedit, and press Enter. Approve the UAC prompt to open Registry Editor.

2- Navigate to: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HTTP\Parameters

3- Create a DWORD (32-bit) Value named EnableHttp2Tls and set its value to 0.
This disables HTTP/2 over TLS (HTTPS).

4- Create another DWORD (32-bit) Value named EnableHttp2Cleartext and set its value to 0.
This disables HTTP/2 for non-TLS (HTTP) traffic.

Restart Windows.
Your localhost sites should now load normally.

To re-enable HTTP/2 later, delete these registry entries or set their values back to 1, then reboot again.

3. Workaround: Disable HTTP/2 for IIS Only

If you prefer to keep HTTP/2 active for other apps but fix IIS and IIS Express specifically, this targeted workaround is safer.

Steps to Disable HTTP/2 in IIS

1- Open Registry Editor and go to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\IIS\Parameters (Create the Parameters key if it doesn’t exist.)

2- Add a DWORD (32-bit) Value named EnableHttp2 and set it to 0.

3- Add another DWORD (32-bit) Value named EnableHttp2OverTls and set it to 0.

4- Restart Windows, then test your IIS and Visual Studio projects.

To restore HTTP/2, delete or set both values back to 1.

4. Last Resort: Uninstall Problematic Windows Updates

Uninstalling the update will restore localhost functionality immediately — but it also removes critical security patches. Use this option only if the other fixes don’t work, and remember to hide the update to prevent automatic reinstallation.

Steps to Uninstall KB5066835 and Related Updates

1- Open Command Prompt as Administrator.
Run the following command: wusa /uninstall /kb:5066835

2- Restart Windows and test your localhost sites.

3- If the issue persists, uninstall other related updates: wusa /uninstall /kb:5065789

wusa /uninstall /kb:5066131

Use the Windows Update Troubleshooter (wushowhide.diagcab) to hide KB5066835, preventing it from reinstalling until Microsoft releases a final patch.

5. Scope, Symptoms, and Notes

This issue primarily affects Windows 11 24H2, 25H2, and Windows Server 2025, though some 22H2 systems may also show symptoms. Developers report the following:

• ERR_CONNECTION_RESET or ERR_HTTP2_PROTOCOL_ERROR when visiting localhost or 127.0.0.1.
• Visual Studio debugging, IIS Express, and Kestrel sessions fail.
• Some OAuth and authentication flows break during local testing.

A few systems recover automatically after installing the latest Windows Defender security intelligence update followed by a reboot. If not, apply one of the fixes above.

Conclusion

The KB5066835 update introduced a temporary but serious disruption for developers relying on local IIS, IIS Express, and Kestrel servers. Thankfully, Microsoft’s rollback and update mechanisms — along with safe registry tweaks — can quickly restore functionality without compromising system security.

Keep Windows Update enabled and periodically recheck for new patches. Once Microsoft delivers a permanent fix, you can safely undo any registry changes and re-enable HTTP/2 for optimal performance.