Microsoft’s Latest Update Breaks SMBv1, Here’s How to Fix It

16 September 2025: Microsoft has confirmed that the September 2025 security updates have caused connectivity issues with SMB v1 shares, affecting a range of Windows versions including all Windows 11 builds, Windows 10 22H2/21H2, and Windows Server 2022/2025, when connections are made via the NetBIOS over TCP/IP (NetBT) protocol. In its support bulletin, Microsoft stated that after installing the update, you might fail to connect to shared files and folders using SMB v1 on NetBT.

As a workaround, affected users are advised to allow traffic on TCP port 445, forcing Windows SMB connections to use direct TCP until a patch is released, with a permanent fix in development expected by the end of the month or early next. The article notes that SMBv1 is obsolete and insecure, deprecated in 2014 and not installed by default since Windows 10 version 1709, and that Microsoft has long encouraged migration to SMBv2 or SMBv3. It stresses that a small number of enterprises still run SMBv1, underscoring the ongoing security rationale for moving away from the protocol.