A recent Windows 11 update has caused significant disruptions for developers worldwide, leading to the complete failure of localhost applications. If you’ve installed the October 2025 cumulative update and suddenly found yourself unable to access your local development environment, you’re not alone. This comprehensive guide will walk you through understanding the problem and implementing proven solutions to restore your localhost functionality.
The Problem: Localhost Applications Failing After Windows Update
After installing the “2025-10 Cumulative Update for Windows 11” (KB5066835), developers immediately experienced critical failures when attempting to run or access their local applications. The issue manifests in several ways, affecting a wide range of development scenarios.
When trying to connect to services via 127.0.0.1 or localhost, applications fail with various error messages. The most commonly reported errors include ERR_CONNECTION_RESET and ERR_HTTP2_PROTOCOL_ERROR. These failures impact numerous development tools and workflows that developers rely on daily.
Local web servers running on IIS or IIS Express become completely inaccessible. ASP.NET applications configured through the hosts file fail to load. Visual Studio debugging sessions crash immediately when attempting to launch applications. Database connections to local MySQL, PostgreSQL, or SQL Server instances timeout or refuse connections.
Docker containers that expose ports to localhost stop responding to requests. API testing tools like Postman or Insomnia cannot reach local endpoints. Development frameworks across the board—whether you’re working with .NET, Node.js, Python Django, or Ruby on Rails—all experience the same devastating connectivity issues.Real-World Impact on Developers
For many developers, this meant their entire workflow came to a grinding halt. Projects with tight deadlines were suddenly blocked. Testing environments became inaccessible. The inability to run applications locally meant no way to debug code, test features, or verify bug fixes before deployment.
One developer reported trying to access an ASP.NET 4.6.2 application running on their local IIS installation. The application was configured via the hosts file, mapping a custom domain name to 127.0.0.1. After the update, attempting to access the site via HTTP resulted in a “Site cannot be reached” error with an ERR_CONNECTION_RESET message.
Another common scenario involved Visual Studio users building .NET 8 web applications. When attempting to run these applications in debug mode using IIS Express, they immediately encountered an ERR_HTTP2_PROTOCOL_ERROR. This made it impossible to develop and test their applications locally.
Identifying the Root Cause: HTTP.sys Bug
The source of this widespread disruption has been traced back to a critical component of Windows called HTTP.sys. This kernel-mode driver is responsible for handling all HTTP and HTTPS traffic on Windows systems. It operates at a low level, managing connections before they even reach application codeThe KB5066835 update introduced a critical bug in how HTTP.sys processes loopback traffic. Loopback traffic refers to network connections that a computer makes to itself, typically through the 127.0.0.1 IP address or the localhost hostname. This type of traffic is essential for local development work.
The bug specifically affects build 26100.6899 and causes HTTP.sys to incorrectly handle HTTP/2 protocol connections to localhost. When applications attempt to establish connections, the driver fails to process them correctly, resulting in connection resets or protocol errors. This explains why developers saw both ERR_CONNECTION_RESET and ERR_HTTP2_PROTOCOL_ERROR messages.
The Official Solution: Install the Latest Windows Update
Microsoft has acknowledged this critical bug and released a fix. The most straightforward and recommended solution is to update Windows to the latest version, which includes the patch for this localhost connectivity issue.How to Update Windows 11
Follow these steps to check for and install the latest Windows updates:
- Open the Settings app by pressing Windows + I on your keyboard
- Navigate to Windows Update in the left sidebar
- Click on “Check for updates” button
- If updates are available, Windows will automatically download them
- Once downloaded, click “Install now” or “Restart now” to apply the updates
- Your computer will restart to complete the installation
- After reboot, verify that your localhost applications work correctly
This solution ensures you have all the latest security patches and bug fixes. Keeping your system updated is always the best practice for maintaining system stability and security.
Alternative Solution 1: Uninstall the Problematic Update
If you cannot immediately install the latest Windows update, or if the official fix hasn’t been released yet, you can temporarily restore localhost functionality by uninstalling the problematic update.Using Command Prompt to Uninstall Updates
To uninstall KB5066835, open Command Prompt or PowerShell as Administrator and run:
wusa /uninstall /kb:5066835After running this command, your computer will prompt you to restart. Accept the restart to complete the uninstallation process.
Some developers found that they also needed to uninstall the September preview update (KB5065789) to fully resolve the issue:
wusa /uninstall /kb:5065789Again, restart your computer after the uninstallation completes. Test your localhost applications to verify they’re working correctly.
Important Security Consideration
Uninstalling security updates can leave your system vulnerable to security threats. This should only be a temporary measure until you can install the official fix from Microsoft. Consider temporarily limiting your network exposure or working in a controlled environment while these updates are uninstalled.Alternative Solution 2: Disable HTTP/2 via Registry Edit
Another effective workaround involves disabling HTTP/2 protocol support in the Windows registry. This prevents the buggy HTTP/2 implementation in HTTP.sys from affecting your localhost connections.
Step-by-Step Registry Modification
- Press Windows + R to open the Run dialog
- Type “regedit” and press Enter to open Registry Editor
- Navigate to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HTTP\Parameters - Right-click on the Parameters key and select New > DWORD (32-bit) Value
- Name the first value “EnableHttp2Tls” and set its data to 0
- Create another DWORD value named “EnableHttp2Cleartext” and set it to 0
- Close Registry Editor
- Restart your computer for the changes to take effect
Using a Registry File
Alternatively, you can create a .reg file with the following content and run it:Windows Registry Editor Version 5.00[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HTTP\Parameters]
“EnableHttp2Tls”=dword:00000000
“EnableHttp2Cleartext”=dword:00000000
Save this as a file with a .reg extension, then double-click it to apply the changes. You’ll need administrator privileges to modify the registry.
Registry Edit Warning
Editing the Windows registry can cause system instability if done incorrectly. Always create a backup of your registry before making changes. To backup, in Registry Editor, click File > Export and save a copy of your current registry settings.
Alternative Solution 3: Update Windows Defender
Some developers reported success by updating Windows Defender to the latest security intelligence version and then restarting their computer.
Test your localhost applicationsThis solution has worked for some developers, though results may vary depending on your specific system configuration.
Preventing Future Issues
To avoid similar problems in the future, consider these best practices:
Enable Optional Updates Cautiously
Windows distinguishes between critical security updates and optional preview updates. Preview updates often contain new features but may also introduce bugs. Consider waiting a few days after a major update is released to see if other developers report issues.
Maintain System Backups
Regular system backups or restore points allow you to quickly roll back problematic updates. Before installing major Windows updates, create a system restore point manually through System Properties > System Protection.
Test Updates on Non-Production Systems
If possible, test major Windows updates on a non-critical development machine before applying them to your primary workstation. This gives you early warning of potential compatibility issues.Conclusion
The localhost failure issue caused by Windows 11 update KB5066835 significantly impacted developers worldwide, disrupting local development environments and testing workflows. The root cause lies in a bug within HTTP.sys that incorrectly handles HTTP/2 protocol connections to localhost addresses.
The primary and recommended solution is to install the latest Windows updates, which include Microsoft’s official fix for this issue. This ensures you have the most secure and stable system while restoring localhost functionality.
If you cannot immediately update, several alternative solutions exist. You can temporarily uninstall the problematic update, disable HTTP/2 via registry modifications, or update Windows Defender to the latest security intelligence version. Each workaround has its trade-offs, particularly regarding system security.
Remember that uninstalling security updates should only be a temporary measure. Always prioritize keeping your system updated with the latest patches from Microsoft to maintain security and stability.
By following this guide, you should be able to restore your localhost functionality and continue your development work without further interruption..
Open Windows Security from the Start menu
Go to Virus & threat protection
Click on “Check for updates” under Virus & threat protection updates
Install Security Intelligence Update for Microsoft Defender Antivirus (KB2267602)
Restart your computer