CyberNew CyberNew
Threat Intelligence & APT BleepingComputer 4 weeks ago

Microsoft shares workaround for Windows domain login issues

CN
CyberNew Threat Research Desk
• Original Reporting: BleepingComputer
CyberNew Threat Intel Assessment
Assessed Severity: INFORMATIONAL
Impacted Surface
Client Endpoints & Operating Systems
Intelligence Sector
Threat Intelligence & APT

Analyst Takeaway

"Microsoft shared a temporary fix on Wednesday for a known issue that prevents Windows 11 users from logging in with valid domain credentials after installing the September 2026 security updates. According to widespread reports from users and IT administrators on Microsoft's Q&A forums , Reddit , and other online platforms , this bug breaks domain trust relationships on some enterprise systems and..."

Recommended Defense & Mitigation

  • Threat Monitoring: Ingest published IOCs into SIEM/EDR detection rules for continuous telemetry monitoring.
  • Security Awareness: Alert staff and maintain baseline endpoint hardening across endpoints.
Curated & Verified by CyberNew Threat Intelligence Desk Standard Defense Framework (NIST / MITRE ATT&CK)

Windows 11

Microsoft shared a temporary fix on Wednesday for a known issue that prevents Windows 11 users from logging in with valid domain credentials after installing the September 2026 security updates.

According to widespread reports from users and IT administrators on Microsoft's Q&A forums, Reddit, and other online platforms, this bug breaks domain trust relationships on some enterprise systems and causes affected users to see domain trust errors and credential errors even though their usernames or passwords are valid.

Admins who investigated this issue have linked the failures to the Machine Identity Isolation Windows security mechanism, which is being set to enforcement mode after this month's KB5124008 (Windows 11 24H2/25H2) or KB5124012 (Windows 11 26H1) updates are installed.

Microsoft's documentation also warns that enabling Machine Identity Isolation in enforcement mode and then disabling it will break domain authentication and require the device to be unjoined and rejoined to the Windows domain.

On Wednesday, Microsoft confirmed that these authentication issues are triggered by the September 2026 security updates that enable Machine Identity Isolation, causing domain trust failures and sign-in issues with valid domain credentials.

"While the update does not directly enable Machine Identity Isolation enforcement, it does cause Windows to begin honoring any existing or policy-provisioned settings that enabled Machine Identity Isolation enforcement," it said in a release health dashboard update.

"However, this feature is only supported for environments connected to domain controllers running at a Windows Server 2025 Domain Functional Level (DFL) and above. The feature should be disabled elsewhere."

1. On a Windows 11 (version 24H2 or 25H2) device, locate the following registry paths: 
    HKLM\SYSTEM\CurrentControlSet\Control\Lsa\MachineIdentityIsolation 
    HKLM\SOFTWARE\Policies\Microsoft\Windows\DeviceGuard\MachineIdentityIsolation 
2. For either of these registry keys, if the value for MachineIdentityIsolation = 2, then set MachineIdentityIsolation = 0.
3. After you disable Machine Identity Isolation, restart the device.
4. Then reset the secure channel using the following command: 
    'Test-ComputerSecureChannel -Repair -Credential (Get-Credential)'
Save your seat

Source Verification & Original Intelligence

This threat report was reviewed by CyberNew analysts and cross-referenced with primary security disclosures from BleepingComputer.

Primary Advisory →