In October 2025, a pair of bugs emerged within Microsoft's September optional update (KB5065789) and October cumulative update (KB5066835) for Windows 11. These bugs hindered developers and applications from establishing connections to the local host (denoted as localhost and 127.0.0.1) through the HTTP/2 protocol.

Once the updates were installed, users who were affected started facing connection problems whenever they attempted to use localhost or 127.0.0.1. The system would promptly return error messages. Moreover, applications that depended on HTTP/2 for communicating with local services ceased to function properly.

This issue has had a profound and detrimental effect on development and testing environments. Additionally, it has disrupted the smooth operation of certain enterprise applications. At present, the sole solution that has been widely confirmed to be effective is to fully uninstall the relevant updates.