KB5066835 causing HTTP/2 protocol errors

[neufuse Veteran]

Anyone else install yesterday's update and now have issues with HTTP/2 protocol errors?

After installing it our IIS servers at not letting anyone talk to them and give that error...

Uninstalling the update and it works again

[neufuse Veteran]

Just saw this https://stackoverflow.com/questions/79790827/localhost-applications-failing-after-installing-2025-10-cumulative-update-for-w

 

guess I'm not the only one, wish MS would not push stuff out with stupid issues like this

[NXTwoThou]

On 15/10/2025 at 08:49, neufuse said:

Just saw this https://stackoverflow.com/questions/79790827/localhost-applications-failing-after-installing-2025-10-cumulative-update-for-w

 

guess I'm not the only one, wish MS would not push stuff out with stupid issues like this

FYI, adding the two registry keys from that post got my machine back up and running without having to uninstall the update:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\HTTP\Parameters

EnableHttp2Tls  (DWORD32) 0

EnableHttp2Cleartext (DWORD32) 0

Reboot

[ViperAFK]

the latest windows defender definition updates seems to fix this:

https://learn.microsoft.com/en-us/answers/questions/5585440/kb5066835-update-causing-iis-service-to-not-work?page=7#answers

I also got feedback from one of my impacted users that applying the latest updates resolved the issue

[NXTwoThou]

On 16/10/2025 at 09:43, ViperAFK said:

the latest windows defender definition updates seems to fix this:

https://learn.microsoft.com/en-us/answers/questions/5585440/kb5066835-update-causing-iis-service-to-not-work?page=7#answers

I also got feedback from one of my impacted users that applying the latest updates resolved the issue

Interesting, because I'm a "check for updates every morning" type of person and still ran into the issue.  But just checked and KB2267602(version 1.439.220.0) just installed.  I'm going to remove the keys and reboot.

[neufuse Veteran]

defender update fixes it

[NXTwoThou]

On 16/10/2025 at 10:07, neufuse said:

defender update fixes it

Confirmed with W11 27965.1/VS 2022 after installing KB2267602 and removing EnableHttp2Tls/EnableHttp2Cleartext