Microsoft has confirmed that Windows updates released since August 29, 2025, are breaking authentication on systems sharing Security Identifiers.
Windows uses unique alphanumeric strings known as Security Identifiers (SIDs) to track and manage user accounts, groups, and computer accounts. SIDs are also used internally by the operating system for access control, permissions management, and security auditing, instead of relying on account names.
"You might experience Kerberos and New Technology LAN Manager (NTLM) authentication failures across devices that have duplicate Security IDs (SIDs)," Microsoft said in a support document published on Tuesday.
"Windows updates released on and after August 29, 2025 include added security protections that enforce checks on SIDs, causing authentication to fail when devices have duplicate SIDs. This design change blocks authentication handshakes between such devices."
These authentication failures can lead to a wide range of issues on Windows 11 24H2, Windows 11 25H2, and Windows Server 2025 systems, including failing remote desktop connections and "access denied" errors when trying to access various resources on the network.
The list of potential symptoms also includes failed login attempts when using valid credentials, with the following errors:
Login attempt failed.
Login failed/your credentials didn't work.
There is a partial mismatch in the machine ID.
The username or password is incorrect.
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