Microsoft has addressed a significant issue affecting HTTP/2 localhost (127.0.0.1) connections and IIS websites following the installation of recent Windows security updates. This bug has been particularly troublesome for users of both Windows 11 and Windows Server 2025, leading to connection reset errors when attempting to load web pages or utilize applications that connect to the localhost IP address.
Details of the Issue
As reported by BleepingComputer, the problem emerged after the installation of the Windows 11 KB5066835 Patch Tuesday update, as well as the September KB5065789 preview update. Users have encountered errors such as ERRCONNECTIONRESET and ERRHTTP2PROTOCOL_ERROR, rendering many applications, including the Duo Desktop app and features within Visual Studio debugging and SSMS Entra ID authentication, inoperable.
Feedback from users on platforms such as Stack Exchange, Reddit, and Microsoft’s own forums highlights a common frustration: the inability to establish HTTP connections to 127.0.0.1. Microsoft has acknowledged this issue, attributing it to a bug within the HTTP.sys Windows-based web server utilized by ASP.NET Core. The company noted that various factors, including the timing of device restarts and update installations, as well as the device’s internet connectivity, can trigger this bug.
In a Thursday update on the Windows release health dashboard, Microsoft stated, “Following installation of updates released on or after September 29 (KB5066835), server-side applications that rely on HTTP.sys may experience issues with incoming connections.” This has resulted in IIS websites failing to load, often displaying messages like ‘Connection reset – error (ERRCONNECTIONRESET)’ or similar notifications, which also affects websites hosted on http://localhost/.
Steps for Resolution
To assist affected users, Microsoft has outlined a straightforward procedure to resolve the issue on impacted devices:
- Open “Windows Update” in the “Windows Settings” app. This can be done by accessing the start menu, typing “check for updates,” and selecting the appropriate option from the results.
- Click on “Check for updates” and allow any updates to install.
- Restart your device, even if no updates were installed in the previous step.
For non-managed business devices and most home users, Microsoft has implemented an automatic resolution through Known Issue Rollback (KIR), a feature designed to reverse problematic updates delivered via Windows Update.
For enterprise-managed devices running Windows 11 24H2, Windows 11 25H2, and Windows Server 2025, IT administrators are required to install and configure the relevant KIR group policy. Additional guidance on deploying and configuring KIR group policies can be found on Microsoft’s support website.
Looking ahead, Microsoft has indicated that a permanent fix will be included in a future Windows update, eliminating the need for organizations to implement a group policy to rectify this issue.
