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Cumulative Update KB5066835 Bug affect localhost applications.

João Filipi 0 Reputation points
2025-10-16T14:07:47.5333333+00:00

After cumulative update KB5066835 all applications under localhost stopped. I tried any configurations to solve this, but the only solution was uninstall this update. I have any application that use SignalR to provide local tools to another applications and de hosts 172.0.0.1 and localhost not response more. We need that issue be solved because is critical and affect all develop enviroment. See another post about this problem: https://free.blessedness.top/pt-br/answers/questions/5585958/relat-rio-de-bug-cr-tico-falha-de-funcionalidade-d

Windows development | Internet Information Services
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  1. Raymond Huynh (WICLOUD CORPORATION) 2,215 Reputation points Microsoft External Staff
    2025-10-17T06:16:23.24+00:00

    Hi João Filipi , 

    This issue often occurs after installing that update because it introduces changes to the HTTP/2 and TLS handling in the networking stack, which can break loopback (localhost) connections used by IIS and IIS Express. These changes can cause protocol negotiation failures, resulting in errors like ERR_CONNECTION_RESET or ERR_HTTP2_PROTOCOL_ERROR when accessing local sites. Visual Studio projects that rely on IIS Express are also affected because they depend on these local HTTP services.  

    # You can resolve it by installing the latest Windows Defender update; once completed, the problem should go away. 

    You can also check for other methods here: KB5066835 update causing IIS Service to not work - Microsoft Q&A 

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