[German]After installing the security updates for Windows 10 and Windows 11 (and also some Server versions) released on September 9, 2025, a problem with accessing SMBv1 network shares occurs for users. As of September 23, 2025, Microsoft has fixed this bug, at least with the preview update KB5065790 for Windows 11 23H2.
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SMB issues after the September 2025 update
On September 9, 2025, Microsoft released cumulative security updates for the versions of Windows 10 and Windows 11 that are still supported. The updates fix several bugs and also close vulnerabilities. These include fixes for the UAC error and SMB client compatibility. The updates are listed in the blog post Patchday: Windows 10/11 Updates (September 9, 2025).
Shortly after the September 2025 updates were released, several blog readers reported that network shares were no longer accessible. The fixes to SMB client compatibility mentioned in the previous paragraph are apparently causing problems with network shares. In a comment, Bolko pointed out that the problems are due to the fix for the CVE-2025-55234 vulnerability (Windows SMB Elevation of Privilege Vulnerability).
Microsoft confirms the issue
Microsoft then confirmed the issue described above in an internal service alert (see my blog post Windows 10/11: September 2025 updates cause SMBv1 issues). Susan Bradley quoted the Microsoft Service Alert (Message ID WI1152308) on patchmanagement.org in this post.
Unable to connect to shared files/folders on Server Message Block (SMB) v1
"After installing the September 2025 Windows security update [Windows 11 23H2 Update KB5065431] from September 9, 2025 or later updates, you might fail to connect to shared files and folders using the Server Message Block (SMB) v1 protocol on NetBIOS over TCP/IP (NetBT)"
According to my blog post Windows 10/11: September 2025 updates cause SMBv1 issues, this affects Windows 10 21H2 – 22H2 and Windows 11 22H2 – 24H2 clients. Windows Server 2022 and Windows Server 2025 are affected on the server side.
Microsoft confirmed at the time that it was working on fixing this issue. In the meantime, affected customers should try the workaround outlined below.
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Workaround: allow traffic on TCP port 445, which will cause the Windows SMB connection to resume successfully by switching to using TCP instead of NetBT.
Microsoft pointed out that the SMBv1 protocol is outdated and is no longer installed by default in newer versions of Windows clients and Windows Server. The newer versions of the protocol, SMBv2 or SMBv3, are not affected by this issue. However, many devices still use the outdated SMBv1 protocol.
Windows 11 23H2: Microsoft fixes the problem
On September 23, 2025, Microsoft released the preview update KB5065790 for Windows 11 Version 23H2. One of the fixes mentioned there relates to the SMBv1 bug (see Windows 11 23H2: Preview Update KB5065790 (September 23, 2025)).
The fix is mentioned in the Microsoft Service Alert (Message ID WI1152308) cited in this post on patchmanagement.org. It is currently unclear when a fix for the remaining affected Windows versions will be available (the fix is expected to be rolled out by October 2025 at the latest, with a security update on October 14, 2025).
Similar articles:
Microsoft Security Update Summary (September 9, 2025)
Patchday: Windows 10/11 Updates (September 9, 2025)
Patchday: Windows Server-Updates (September 9, 2025)
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Windows 11 24H2: Out-of-Band-Update KB5068221
Windows 11 23H2: Preview Update KB5065790 (September 23, 2025)
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