Overview
- Microsoft confirmed a regression in HTTP.sys that blocked HTTP/2 connections to 127.0.0.1 after the September preview KB5065789 and October update KB5066835.
- The bug affected Windows 11 (24H2 and 25H2) and Windows Server 2025, producing errors such as ERR_CONNECTION_RESET and ERR_HTTP2_PROTOCOL_ERROR.
- IIS-hosted local sites and tools that rely on loopback traffic were disrupted, including Visual Studio debugging, SSMS Entra ID authentication, and Duo Desktop.
- Microsoft activated Known Issue Rollback to reverse the change for most home and non‑managed devices and advised users to check for updates and reboot.
- Enterprise-managed systems require administrators to apply a KIR group policy, and Microsoft says a permanent fix will ship in a future Windows update; documented workarounds include uninstalling the updates or disabling HTTP/2 via registry.