October updates for Windows 11 disrupted localhost functionality, causing applications that connect to 127.0.0.1 over HTTP/2 to potentially not function properly.
Although developers typically use localhost for website testing and application debugging, it is also used by applications that need to connect to a locally running service to perform certain actions or requests.
After installing the KB5066835 update for Windows 11 and the September preview update KB5065789, users found that applications can no longer establish HTTP connections to the localhost IP address (127.0.0.1). When attempting to connect, they receive errors such as “ERR_CONNECTION_RESET” or “ERR_HTTP2_PROTOCOL_ERROR.”
The issue has been reported on the Microsoft forums, Stack Exchange, and Reddit. All affected users report that they can no longer establish HTTP connections to 127.0.0.1.
The issue has affected many popular applications, including debugging in Visual Studio, SSMS Entra ID authentication, as well as the Duo desktop application, which checks the device’s security posture and requires connecting to web servers running on localhost.
“After installing updates for Windows 11 24H2 and 25H2, an issue may occur in which Duo Prompt cannot access Duo Desktop,” says the Duo support bulletin. “This may prevent successful authentication (or result in limited functionality) in situations where the following are used: Trusted Endpoints, policies such as Duo Desktop & Device Health, Duo Desktop as an authentication method, Duo Passport, Verified Duo Push with Bluetooth Autofill, or Proximity Verification.”
On the BornCity blog, they suggest making the following changes to the registry, which supposedly may help resolve the issue by disabling the HTTP/2 protocol:
[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HTTP\Parameters]
"EnableHttp2Tls"=dword:00000000
"EnableHttp2Cleartext"=dword:00000000
Another method that some users claim helps is installing the latest antivirus definition update for Microsoft Defender. However, others report that this does not fix the issue in their case.
As a result, at the moment the only reliable way to get rid of the error is to uninstall the October update KB5066835 and the September preview update KB5065789. After removing them and restarting Windows, the Windows loopback interface should once again allow HTTP/2 connections, which resolves issues with using applications.