KB5066835 asp.net web forms iis express ERR_HTTP2_PROTOCOL_ERROR

Question

KB5066835 asp.net web forms iis express ERR_HTTP2_PROTOCOL_ERROR

Gareth Williams 20

After installing KB5066835 this morning , every project web forms in visual studio 2022 asp.net errors when try to boot , iis express raises ERR_HTTP2_PROTOCOL_ERROR. Tried various suggestions after googling and asking chatgpt but still get HTTP/2 error.

Danny Nguyen (WICLOUD CORPORATION) 3,500 Reputation points Microsoft External Staff

2025-10-16T02:30:37.77+00:00

Hi, thanks for posting. May I ask which suggestions have you tried regarding this problem?
Danny Nguyen (WICLOUD CORPORATION) 3,500 Reputation points Microsoft External Staff

2025-10-16T03:06:18.23+00:00
Hi @Gareth Williams

This issue appears to be linked to recent Windows updates that affect IIS Express and ASP.NET Web Forms. Based on current guidance, the recommended workaround is:

Uninstall the following updates temporarily:

KB5066835

KB5065789

After removing these updates, restart your system and verify if IIS Express starts working as expected.

Please note: This is a temporary measure which is working currently.
Gareth Williams 20 Reputation points

2025-10-16T05:16:32.86+00:00

Hello, i did try earlier this earlier, but just KB5066835 - wusa /uninstall /kb:5066835. railed on reboot because and repair re added it.
Gareth Williams 20 Reputation points

2025-10-16T09:14:10.65+00:00

Thank you Danny. I checked updates and there was non to install. I use Bitdefender for antivirus not Microsoft Defender Antivirus. Would you suggest i remove Bitdefender and use Microsoft Defender Antivirus instead?
Danny Nguyen (WICLOUD CORPORATION) 3,500 Reputation points Microsoft External Staff

2025-10-16T09:44:18.1933333+00:00

Unless you currently have Microsoft Defender disabled, I don't think you have to remove Bitdefender.You can see the update in question here https://www.microsoft.com/en-us/wdsi/defenderupdates

Please let me know if this works out for you.

Answer accepted by question author

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Danny Nguyen (WICLOUD CORPORATION) 3,500 Reputation points Microsoft External Staff

2025-10-16T02:30:37.77+00:00

Hi, thanks for posting. May I ask which suggestions have you tried regarding this problem?
Danny Nguyen (WICLOUD CORPORATION) 3,500 Reputation points Microsoft External Staff

2025-10-16T03:06:18.23+00:00

Hi @Gareth Williams

This issue appears to be linked to recent Windows updates that affect IIS Express and ASP.NET Web Forms. Based on current guidance, the recommended workaround is:

Uninstall the following updates temporarily:

KB5066835

KB5065789

After removing these updates, restart your system and verify if IIS Express starts working as expected.

Please note: This is a temporary measure which is working currently.
Gareth Williams 20 Reputation points

2025-10-16T05:16:32.86+00:00

Hello, i did try earlier this earlier, but just KB5066835 - wusa /uninstall /kb:5066835. railed on reboot because and repair re added it.
Gareth Williams 20 Reputation points

2025-10-16T09:14:10.65+00:00

Thank you Danny. I checked updates and there was non to install. I use Bitdefender for antivirus not Microsoft Defender Antivirus. Would you suggest i remove Bitdefender and use Microsoft Defender Antivirus instead?
Danny Nguyen (WICLOUD CORPORATION) 3,500 Reputation points Microsoft External Staff

2025-10-16T09:44:18.1933333+00:00

Unless you currently have Microsoft Defender disabled, I don't think you have to remove Bitdefender.You can see the update in question here https://www.microsoft.com/en-us/wdsi/defenderupdates

Please let me know if this works out for you.

Answer 1

Danny Nguyen (WICLOUD CORPORATION) 3,500 Microsoft External Staff

Hi, according to this thread https://free.blessedness.top/en-us/answers/questions/5585440/kb5066835-update-causing-iis-service-to-not-work

Several users have reported that installing the latest Microsoft Defender update resolves the IIS Express/ASP.NET Web Forms issue without removing KB5066835 or KB5065789. If you haven’t already:

Check for updates manually:
- Go to Settings → Windows Update → Check for updates.
- Ensure Microsoft Defender Antivirus and Security Intelligence Updates are up to date.
After updating, restart your machine and test IIS Express again.

If you still want to go through with the uninstallation, try this:

Uninstall KB5066835 and KB5065789 from Installed Updates.
If they come back, use DISM: BATdism /online /get-packages | findstr 5066835 dism /online /remove-package /packagename:<PackageName> Show more lines
Pause Windows Update to block auto-reinstall.
Restart and confirm IIS Express works.

Gareth Williams 20 Reputation points

2025-10-16T10:13:45.5333333+00:00

Thank you, let me clarity, currently Microsoft Defender is disabled as Bitdefender Free is used for virus protection. Will uninstalling Bitdefender and activating and updating Microsoft Defender Antivirus work do you think?
Danny Nguyen (WICLOUD CORPORATION) 3,500 Reputation points Microsoft External Staff

2025-10-16T10:20:29.89+00:00

You can temporarily uninstall Bitdefender to install this update for Microsoft Defender. After that make sure to check if IIS is working, and you can also consider reinstall Bitdefender after that if you want to keep it.
Gareth Williams 20 Reputation points

2025-10-16T10:46:58.5866667+00:00

Danny THANK YOU ! Removing Bitdefender and using Microsoft Defender instead fixed IIS express KB5066835 ERR_HTTP2_PROTOCOL_ERROR
Danny Nguyen (WICLOUD CORPORATION) 3,500 Reputation points Microsoft External Staff

2025-10-16T10:52:46.7033333+00:00

That's good to hear. I have changed the recommendation of installing update into an answer. Please consider accepting my answer so others can benefit from it.

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KB5066835 asp.net web forms iis express ERR_HTTP2_PROTOCOL_ERROR

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