Windows 11 breaks localhost: what's happening, who's affected, and how to fix it

For many developers and IT teams, If “localhost” fails in Windows 11, it means being left without a test bench.Following the latest cumulative updates, several systems have begun dropping connections to the local loopback 127.0.0.1 when using HTTP/2, bringing development servers, debugging servers, and internal tools down.

Reports are repeated on Microsoft forums, Stack Overflow, Server Fault and Reddit: October update KB5066835 and September update KB5065789 (build 26100.6899 on some computers) trigger Errors when trying to connect to local endpoints. It is not a DNS or a problem. hosts file; the fault appears further down the system stack.

What's happening with localhost in Windows 11?

Affected systems stop completing HTTP connections to 127.0.0.1 when negotiating HTTP/2, returning Messages like “ERR_CONNECTION_RESET” or “ERR_HTTP2_PROTOCOL_ERROR”. In practice, Services that should listen on 127.0.0.1 do not respond properly, browsers fail to reach the local endpoint and debugging sessions are interrupted.

The pattern is consistent: with KB5066835/KB5065789 installed, loopback requests failAfter uninstalling them and restarting, the local circuitry works again. Because of this, many have identified this as a specific regression introduced by these builds.

Affected applications and services

In addition to web development scenarios, Incidents have appeared in common tools of work and security that pull local services to operate.

• Debugging and running in Visual Studio (ASP.NET/ASP.NET Core projects) and IIS.
• Authentication and utilities such as SSMS Entra ID.
• Security applications such as Duo Desktop, which check the status of the equipment and require local endpoints.

In organizations with microservices and internal utilities, Loopback failure blocks testing, demos, and validations, which triggers diagnostic times even if the application code is correct.

The technical origin: HTTP.sys and HTTP/2 negotiation

Everything points to HTTP.sys, the kernel-mode component that arbitrates Windows HTTP traffic and on which IIS and ASP.NET Core depend. With recent builds, HTTP/2 negotiation in the local loopback causes connection resets, hence the error messages observed in browsers and clients.

It is important to underline this: It is not a name resolution failure, nor is it fixed by editing hosts. The rerouting occurs at the layer that manages connections within the system itself.

Common symptoms and error messages

On affected computers, when launching the development site or a local service, Requests are cut off with generic codes which point to a connection reset during the handshake.

Reported examples: ERR_CONNECTION_RESET y ERR_HTTP2_PROTOCOL_ERROR. On local servers running IIS or Kestrel, Traces show failed session establishment attempts when the connection attempts to negotiate HTTP/2.

Temporary solutions and mitigations that are working

Until the permanent correction arrives, There are three paths that the community and Microsoft are using, with varying degrees of invasiveness and effectiveness.

• Uninstalling problematic updates and rebooting: This restores loopback behavior to its previous state on most computers. Example commands:
  wusa /desinstalar /kb:5066835
wusa /desinstalar /kb:5065789
• Disable HTTP/2 in the registry to force HTTP/1.1 on local traffic. It's a surgical fix, but not ideal for production:
  
"EnableHttp2Tls"=dword:00000000
"EnableHttp2Cleartext"=dword:00000000
• Apply the Known Issue Rollback (KIR) from Microsoft, which selectively reverts the problematic change without uninstalling the entire update.

Some users comment that Update Microsoft Defender signatures has improved their situation, but the results are not consistent and it is not recommended as a primary solution.

What Microsoft has said and current status

Microsoft has acknowledged the regression and has activated an automatic mitigation with KIR For home and unmanaged computers. In many cases, simply checking for updates and restarting the system will take effect.

1. Open Settings > Windows Update.
2. Click on Search for updates.
3. Restart the device, even if nothing new appears.

In corporate environments, Administrators must deploy the KIR group policy corresponding to Windows 11 24H2/25H2 and Windows Server 2025 to apply the rollback in a controlled manner. The company has indicated that The permanent fix will come in a future update..

Practical recommendations for developers and IT

While the update channel stabilizes, prudent measures should be taken so as not to slow down projects or affect critical services.

• En development machines: if the block is immediate, uninstall KB5066835/KB5065789 or temporarily disable HTTP/2.
• In production and laboratories: validate the KIR and, if necessary, distribute the corresponding group policy.
• Pause the installation of these patches in critical environments until confirming that the loopback works.
• Document the changes (registry, KIR, uninstalls) to revert them as soon as the final update arrives.

For those who work daily with local endpoints, localhost is not a luxury: it is the basis of the test and debug cycleIndications point to a regression in HTTP.sys with HTTP/2, which has already been acknowledged by Microsoft and mitigated using KIR, while a stable patch is being prepared. With the workarounds above, it's possible to continue moving forward without reworking the environment and minimize the impact on timelines.