Windows 11 Update Bug Breaks Localhost Functionality

Recent Windows 11 updates from Microsoft have caused a major issue with the “localhost” functionality, affecting applications that rely on connecting back to 127.0.0.1 over HTTP/2.

Localhost, which refers to the local computer or device being used, can be accessed through the special IP address 127.0.0.1. This functionality is commonly used by developers to test websites or debug applications.

Following the installation of Windows 11 KB5066835 Patch Tuesday and the September KB5065789 preview update, users have reported that their applications are no longer able to establish HTTP connections to the localhost IP address (127.0.0.1).

Errors such as “ERR_CONNECTION_RESET” or “ERR_HTTP2_PROTOCOL_ERROR” are being encountered when attempting to make these connections.

Numerous Windows users on platforms like Microsoft forums, Stack Exchange, and Reddit have voiced their concerns about this bug, stating that they are unable to make HTTP connections to 127.0.0.1.

Popular applications such as Visual Studio debugging, SSMS Entra ID authentication, and the Duo Desktop app have been impacted by this bug, as they require connections back to web servers running on the localhost.

The Duo support bulletin acknowledges the issue, stating that after certain Windows updates, the Duo Prompt may be unable to reach Duo Desktop, affecting authentication processes and functionality.

One suggested solution involves modifying Registry entries to disable the HTTP/2 protocol, although this fix has not been independently verified.

    [HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HTTP\Parameters]

    "EnableHttp2Tls"=dword:00000000

    "EnableHttp2Cleartext"=dword:00000000 

Another method reported to potentially resolve the issue is installing the latest Microsoft Defender intelligence update. However, some users have found that this does not completely fix the problem.

To ensure a complete resolution, users have resorted to uninstalling the October KB5066835 update and the September KB5065789 preview update using specific commands.

    wusa /uninstall /kb:5066835

    wusa /uninstall /kb:5065789

Following the uninstallation of these updates and a system restart, the loopback interface should once again support HTTP/2 connections, resolving the issues faced by affected applications.

BleepingComputer has reached out to Microsoft for a response regarding this bug and will provide updates as necessary.