KB5066835 went terribly wrong

Yesterday, on 15.10.2025, KB5066835 was released for Windows 11. And guess, what, it causes IIS and IIS Express to reject client connections.

This effectively not only stops people who develop using VS + IIS Express but seems that some websites are affected

kb5066835 breaks IIS Express - Developer Community

KB5066835 update causing IIS Service to not work - Microsoft Q&A

Localhost not working anymore after 2025-10 cumulative update Windows 11 - Microsoft Q&A

Localhost applications failing after installing "2025-10 Cumulative Update for Windows 11 Version 24H2 for x64-based Systems (KB5066835) (26100.6899)" - Stack Overflow

windows - Http 2 Protocol error connecting to localhost web sites - Server Fault

Visual Studio - IIS Express - Site won't run / connect / SSL error - Umbraco community forum

Most people suggest uninstalling the KB, however, if your Windows refuses it, try the workaround from the last link above:

1. In the registry, navigate to: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HTTP\Parameters
2. Under the Parameters folder, right-click in the right-hand pane and select New > DWORD (32-bit) Value.
3. Name the value EnableHttp2Tls and set its data to 0 (zero).
4. Repeat the process to add another DWORD (32-bit) Value named EnableHttp2Cleartext and set its data to 0.
5. Restart the machine.

Edit:Seems that it has been patched in a seemingly unrelated Security Intelligence Update for Microsoft Defender Antivirus - KB2267602 (Version 1.439.210.0).