IIS and IIS Express Issues on Windows 11 25H2 — MAWi v2 Now Working Normally after KB2267602

RESOLVED: IIS / IIS Express Breakage After Windows 11 25H2 Update — Impact on MAWi

16.10.2025 - Microsoft Update KB2267602 Resolves IIS / IIS Express Issue


Microsoft has released a Windows Defender Security Intelligence Update – KB2267602 (Version 1.439.216.0) – which, according to multiple verified user reports, resolves the IIS and IIS Express malfunction that affected some Windows 11 25H2 systems after installing the earlier cumulative updates KB5066835 and KB5065789.

✅ What’s Fixed

After installing KB2267602 v1.439.210.0 (or newer) and rebooting the system, IIS and IIS Express resume normal operation.
This means:

MAWi v2 instances hosted via IIS Express now launch and serve content correctly.


Summary



After applying Windows 11 25H2 (especially with the October 2025 cumulative update, KB5066835), many users have reported that IIS or IIS Express stops functioning correctly.
This issue can disrupt web-based integrations and testing environments, including our MAWi v2 system.

Symptoms
Customers have observed one or more of the following after upgrading to 25H2 + KB5066835:
  • The IIS Express site (e.g. MAWi v2) cannot start or throws errors during startup
  • Browser shows ERR_HTTP2_PROTOCOL_ERROR or ERR_CONNECTION_RESET when connecting to the MAWi internal IP address or hostname.
  • Breakage only showing up after the update (i.e. MAWi v2 was working fine prior to 25H2 upgrade)


Root Causes (Current Understanding)


While Microsoft has not publicly confirmed a full root cause yet, the leading theories based on community reports are:
  1. Interaction with KB5066835 — This October 2025 update is implicated in many IIS / IIS Express failures on 24H2 / 25H2 systems.
  2. Changes in HTTP/2 / TLS 1.3 / handshake behavior — New defaults or stricter processing in newer Windows versions may conflict with how IIS Express handles certain security/renegotiation flows, especially in the presence of client certificate settings.
  3. Client certificate / post-handshake requests — These were more permissive in older Windows versions; changes in 25H2 may render them unsupported in some scenarios.


Temporary Workarounds - Roll back the update



Navigate to:
 Settings → Windows Update → Update history → Uninstall updates,
 and remove KB5066835 (some users also removed KB5065789).
Uninstalling this update has restored normal IIS and IIS Express functionality for many affected systems.

You can prevent a specific KB update from being installed in Windows 11 using the "Show or hide updates" troubleshooter tool using this link