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Microsoft Windows September 2025 SMBv1 NetBT connection breakage

Service Disruption
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1 unique sources, 1 articles

Summary

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Microsoft confirmed that the September 2025 Windows security updates are causing SMBv1 connection failures over NetBIOS over TCP/IP (NetBT), disrupting access to shared files and folders on affected Windows client and server systems. The issue can occur when either the SMB client or server has the update installed, and Microsoft has issued a temporary workaround while it works on a fix.

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Timeline

  1. 15.09.2025 15:48 2 articles · 8mo ago

    Microsoft confirms September 2025 Windows updates break SMBv1 shares over NetBT and offers a TCP port 445 workaround

    Initial Disclosure

    Microsoft confirmed that the September 2025 Windows security updates are causing connection issues to Server Message Block (SMB) v1 shares over NetBIOS over TCP/IP (NetBT). The known issue affects Windows 11 24H2/23H2/22H2, Windows 10 22H2/21H2, Windows Server 2025, and Windows Server 2022 when either the SMB client or the SMB server has the September 2025 security update installed. Microsoft said it is working to resolve the issue and advised impacted customers to allow traffic on TCP port 445 so SMB can switch from NetBT to TCP until a fix is available.

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