KB5066835 update causing IIS Service to not work

For my part I didn’t see the update either.... I rebooted the PC but didn’t see it, and it wasn’t installed. Maybe the update is being rolled out later in some countries? My solution: I uninstalled update KB5066835 and now everything works again. I’ve paused Windows Update for 5 weeks and will check later if there’s an update that fixes this issue.

I have installed the "Security Intelligence Update for Microsoft Defender Antivirus - KB2267602 (Version 1.439.210.0)" today and for me it is now working again!

Installed the update and now working!

No need to roll back or disable updates

Repair the WCF Services installation by re-enabling all associated features. This process forces the system to properly reconfigure the affected components without requiring a full rollback of the Windows update.

1. Open the Control Panel. You can do this by searching for "Control Panel" in the Start Menu.
2. Navigate to Programs > Programs and Features.
3. Click on Turn Windows features on or off in the left-hand menu.
4. In the list of features, scroll down and expand .NET Framework 4.8 Advanced Services.
5. Expand WCF Services.
6. Select all of the WCF Services by clicking the checkbox next to the parent "WCF Services" folder. If any are already checked, uncheck and then re-check them to ensure they are re-enabled.
7. Click OK to apply the changes.
8. You may be prompted to restart your computer. It is critical to do this restart for the changes to take effect and for the services to be properly re-registered.

Just tested that. I acknowledge that the Defender update resolved the issue

Worked for me as well (wasn't able to remove updates before so this works without updating). For the ones who couldn't update online can download and install it from (I did like that):
https://www.microsoft.com/en-us/wdsi/defenderupdates

Just keep in mind, the file you downloaded seems like doing nothing. Just run it with admin privilages and wait couple of minutes and restart your pc. It should work.

To force Windows Defender updates, as per https://www.microsoft.com/en-us/wdsi/defenderupdates

From an Administrator CMD run the following to update definitions. You must reboot after for the fix to take effect:

cd %ProgramFiles%\Windows Defender
MpCmdRun.exe -removedefinitions -dynamicsignatures
MpCmdRun.exe -SignatureUpdate

Don't uninstall anything at this point and turn back on windows updates. Originally uninstalling KB5066835 and KB5065789 resolved this - but we had dozens of machines unable to uninstall KB5065789 due to an error. Reinstalling KB5066835 later in the day on the machines with failed KB5065789 resolved the issue for us on ALL machines.

This is all dumb - and cost so many people wasted time...

I can confirm that this indeed corrects the problem. My use of HTTP.sys now functions normally with all windows/defender updates applied.

Yeah! working now, in my case, using Windows 11, went to:

• Windows Security, Virus & threat protection, Virus & threat protection settings, updated to latest (last update 10/16/2025).
• Back to settings, Windows Update, re-enable Get latest updates, updated, restarted.

Now IISExpress working and able to run localhost, thanks!

Installing this Microsoft Defender Antivirus update - KB2267602 solved my problem

In my case, I uninstalled IIS Express from Windows applications. I restarted my computer. And I downloaded and installed the latest version from Microsoft's page. It worked perfectly.

We're seeing that many machines automatically got the defender update, and we just need to reboot them, after which all is working again.

Some of our devs were having this issue yesterday along with other staff using applications that relied on the .net httplistener. Not all staff with the KB5066835 update ran into those issues though. Just some staff oddly enough. So we paused pushing the updates to anyone else.

Then this morning all the staff who had issues reported in that their apps started working again after a reboot. We made no changes on our end so were very confused.

Then I saw this answer about updating Defender and that could explain what we were seeing. If a combination of the KB5066835 and a specific Windows Defender update was the cause of the issue or if Microsoft placed a patch in a Defender update this morning. Then that could be the resolution for some users.

I suggest making sure you are on the latest Defender update and rebooting as one of your first trouble shooting steps before attempting to remove updates.