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Windows duplicate SID authentication failures mitigation guidance

Advisory/Mitigation
First reported
Last updated
Happening score
H score 18
1 unique sources, 1 articles

Summary

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Microsoft issued remediation guidance for Windows systems with duplicate SIDs, after updates released since August 29, 2025 started breaking Kerberos and NTLM authentication. The issue affects Windows 11 24H2, Windows 11 25H2, and Windows Server 2025, causing login failures, Remote Desktop problems, and access denied errors. Microsoft says admins should rebuild affected systems or apply a temporary Group Policy workaround to restore authentication.

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Timeline

  1. 21.10.2025 19:56 1 articles · 8mo ago

    Windows updates enforce SID checks on duplicate-SID systems

    Technical Analysis Update

    Windows updates released on and after August 29, 2025 add security protections that enforce checks on Security Identifiers (SIDs), causing Kerberos and NTLM authentication to fail when devices have duplicate SIDs and disrupting remote desktop connections, login attempts, and access to network resources on Windows 11 24H2, Windows 11 25H2, and Windows Server 2025.

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  2. 21.10.2025 19:56 2 articles · 8mo ago

    Microsoft confirms duplicate-SID authentication failures and mitigation guidance

    Initial Disclosure

    Microsoft’s support document confirms Kerberos and NTLM authentication failures on devices with duplicate SIDs, identifies cloning or duplicating a Windows installation without Sysprep as a cause, and advises IT administrators to rebuild affected systems using supported cloning methods or temporarily install a special Group Policy obtained through Microsoft Support for business.

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