[German]I'm posting a note here on the blog about patch day issues related to Windows 11 24H2/25H2. There are indications that the KB5066835 update from October 14, 2025 causes localhost to become unresponsive. Some users uninstalled the update to get the issue fixed. But there are two other workarounds.
Windows 11 24H2/25H2 Update KB5066835
The cumulative update KB5066835 released on October 14, 2025, for Windows 11 24H2 (and 25H2) includes various quality improvements and security patches, which are listed in the linked support article.
Problems with localhost after update installation
Christian Wenz sent me a private message on Facebook (thanks for that) pointing me to a post Localhost applications failing after installing "2025-10 Cumulative Update for Windows 11 Version 24H2 for x64-based Systems (KB5066835) (26100.6899)" [closed] on Stackoverflow. The thread is currently closed to discussion because it is not a programming topic. However, the problem is likely to affect many Windows 11 users of versions 24H2 and 25H2.
One affected user writes that they have just installed the cumulative update KB5066835 for Windows 11 on their laptop. Now they are receiving error messages when trying to run applications on localhost.
The thread creator states that he has an ASP.NET 4.6.2 application that he runs as a website on his local IIS installation via the hosts file. Specifically, the "app name" is resolved to 127.0.0.1. When the user attempts to execute the request to localhost via http://, they receive the error message "Site cannot be reached" due to an ERR_CONNECTION_RESET error.
According to the user, they also use Visual Studio to create .NET (.NET 8) web applications. Attempting to run (debug) these web applications with IIS Express now also fails with the error message ERR_HTTP2_PROTOCOL_ERROR.
The affected party installed the Windows 11 update "2025-10 Cumulative Update for Version 24H2 for x64-based Systems (KB5066835) (26100.6899)" and the update for .NET Framework 3.5 and 4.8.1 for Windows 11, version 24H2 for x64 (KB5066131)".
When the user asked if anyone else was experiencing this problem, it was confirmed by other affected users. One wrote that he had to uninstall the KB5066835 security update for Windows 11 24H2 / 25H2 in order for access to localhost to work again.
Further hits on the Internet
If you search the Internet, there are a number of hits (e.g., on reddit.com) that confirm that the update breaks the Localhost function in Windows 11 24H2/25H2. On October 15, 2025, Jan van Rhine published an entry on Microsoft Q&A titled Localhost not working anymore after 2025-10 cumulative update Windows 11, which describes the same problem. The error does not occur on a fresh Windows 11 24H2 installation.
May affect some applications
The problem looks exotic, but I have received reports that access control functions have not been working since the October 2025 patch day. This may be exactly the above problem if they use localhost. The post on reddit.com mentions Kantech EntraPass (access control) as affected. The password management safe (formerly SafeInCloud) with the corresponding browser plugin is also affected. Within my German blog post I list more affected applications.
Workarounds for the problem
Beside uninstalling the update, I have seen two workarounds to fix the issue with localhost and IIS.
Does a Defender update fix the problem?
In the Microsoft Q&A thread KB5066835 update causing IIS Service to not work which already spans several pages, I found this answer from today, October 16, 2025.
This morning there was a Defender update, I have installed it an now it works!!!
Sounds crazy, so the question is whether anyone else was affected and whether a Defender update really solves the problem? At least one person in the response thread confirms that IIS is working again for them. Some German blog readers reported also that a Defender update helped, but others wrote, that it didn't solve the issue.
A registry entry also helps
German blog reader Flo Reader Flo posted a comment and refers to this Learn Microsoft post. It lists PowerShell commands that can be used to disable HTTP/2 by setting the appropriate registry entries, which should fix the problem. However, please also note Bolko's comment regarding the implications (disables HTTP/2 on TLS system wide).
Similar articles:
Microsoft Security Update Summary (October 14, 2025)
Patchday: Windows 10/11 Updates (October 14, 2025)
Patchday: Windows Server-Updates (October 14, 2025)
Patchday: Microsoft Office Updates (October 14, 2025)
