Microsoft Defender app (Windows 11 26H2): crashes on launch with access violation in OneAuthInterop.dll, no window appears

The Microsoft Defender app on my Windows 11 PC crashes about 2 seconds after every launch. No window ever appears. It started on Oct 5, 2026, after repeated "sign back in" errors in the app the same day.

Steps to reproduce:

1. Install Microsoft Defender from the Microsoft Store.
2. Launch it from the Start menu.
3. The process starts and exits about 2 seconds later. No window, no error message.

Crash details (full memory dump analyzed in WinDbg):

• Faulting module: OneAuthInterop.dll 8.2.1.0 (bundled with the app)
• Exception: c0000005, attempt to execute address 0x00000000 (null function pointer) on a OneAuth worker thread
• Failure bucket: SOFTWARE_NX_FAULT_NULL_INVALID_POINTER_EXECUTE_c0000005_OneAuthInterop.dll!Unknown
• Failure hash: {6bd8b1a4-f0ad-4f05-9615-37cf7716a55c}
• No third-party modules loaded in the process

Environment:

• App package: Microsoft.6365217CE6EB4 102.2608.18002.0 (MicrosoftSecurityApp.exe 1.2.2608.18002)
• OS: Windows 11 26H2, build 26300.9550

Already tried, no change:

• Uninstall and reinstall from the Microsoft Store
• Reset-AppxPackage
• Cleared the TokenBroker cache, OneAuth and IdentityCache folders
• Signed out and back in, rebooted
• A brand-new local Windows user (same crash, so it is not my profile or account)
• Launching with the network disconnected
• sfc /scannow and DISM /RestoreHealth run by Microsoft Support

The app's own trace logs in LocalState\Logs are 0 bytes, so it dies before logging anything. Because it cannot start or sign in, this PC no longer appears in my Defender device list.

Microsoft Support told me this is a known issue affecting multiple users and to wait for an app update, but could not give a reference or an ETA.

Feedback Hub link: https://aka.ms/AA13rsb3