The Windows 11 KB5066835 update from October 2025 is causing major issues for developers and system administrators who use Internet Information Services (IIS) to host local websites. This problem primarily affects localhost environments, where users are reporting failures when accessing local web applications through secure (HTTPS) connections.
🔍 What’s Happening
After installing KB5066835, many IIS users found that their browsers could no longer connect properly to local sites. The issue seems to stem from changes in how HTTP/2 and TLS negotiation are handled within Windows networking components.
This means that when you try to run or test a website locally, the browser may:
- Refuse the connection,
- Display SSL/TLS handshake errors, or
- Time out during the HTTP/2 protocol negotiation.
Applications that rely on local web services — such as Autodesk Vault, SharePoint development setups, or custom API testing servers — are also affected. These tools depend on IIS to host or connect to internal web services, and the update’s changes are breaking those dependencies.
🧩 Microsoft’s Response
Microsoft acknowledged the issue on their support forums. Their team suggested two main steps:
- Update Microsoft Defender Definitions:
- Some users have found success simply by updating Windows Security (Defender) to the latest virus and platform definitions.
- This appears to reset or patch certain networking settings that restore IIS functionality.
- Uninstall Problematic Updates (if necessary):
- If updating Defender doesn’t help, Microsoft recommends uninstalling KB5066835 and the related KB5065789.
- After uninstalling, pause Windows Updates to prevent automatic reinstallation until a fixed version is released.
🧠 Community Workaround (Registry Fix)
For users who can’t uninstall the updates — for example, in enterprise environments — developers have discovered a registry-based workaround to disable HTTP/2 protocol negotiation.
Steps:
- Open Registry Editor (
regedit.exe). - Navigate to:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HTTP\Parameters - Add or modify the following DWORD values:
EnableHttp2Tls→ 0EnableHttp2Cleartext→ 0
- Restart the computer.
What it does:
This change forces IIS and Windows networking to use HTTP/1.1 instead of HTTP/2. It prevents the broken negotiation process from interfering with localhost connections.
⚠️ Note: This is only a temporary workaround. Disabling HTTP/2 can affect performance and isn’t recommended for production servers.
🧰 Best Practices Before Applying Fixes
- Test any changes (like registry edits) on a non-production machine first.
- Back up your registry before making modifications.
- If uninstalling updates, ensure your system has restore points or backups ready.
- Re-enable Windows Update once Microsoft releases a corrected patch.
🚨 Why It Matters
This update’s behavior has caught the attention of system administrators, developers, and IT support teams worldwide. Threads on Reddit and Microsoft Tech Community are filled with reports of:
- Broken development environments (Visual Studio, .NET APIs, IIS Express),
- Third-party software failures, and
- Unexpected SSL/TLS errors on localhost setups.
Several software vendors are warning their customers to avoid installing KB5066835 until Microsoft provides an official fix.
✅ Recommended Next Steps
- Update Microsoft Defender first — it’s the safest and quickest attempt.
- If that fails, apply the registry HTTP/2 workaround.
- As a last resort, uninstall KB5066835 and pause updates temporarily.
- Monitor official channels like Microsoft’s Windows Release Health Dashboard for patch updates.
If you’ve been impacted by this bug, you can track ongoing discussions or share your own experience in community threads — it’s helping Microsoft prioritize a fix.
