[German]With the security updates for Windows 10 and Windows 11 released on September 9, 2025, a problem with accessing SMBv1 network shares has crept in. There are reader comments about this here in the blog. Now there are indications that Microsoft has confirmed the problem, at least internally.
Windows 10/11 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).
SMB issues after the September 2025 update
It appears that the above-mentioned fixes to the compatibility of the SMB client are causing problems with network shares. Shortly after the release of the September 2025 updates, several blog readers reported that network shares were no longer accessible. Bolko writes in this German comment that Windows 11 "with a patch level higher than 26100.4946 (e.g., 26100.5074 or 26100.6584)" has network problems and network shares are no longer accessible.
In a comment, he also links to possible explanations for the issue. In a later comment, Bolko suggested that the problems stem from the correction of vulnerability CVE-2025-55234 (Windows SMB Elevation of Privilege Vulnerability). In subsequent comments, he describes that customers must either configure SMB servers to require SMB server signing or enable EPA on the SMB server to harden their systems against the attack class from CVE-2025-55234. The issue is described in the support article Support for Audit Events to deploy SMB Server Hardening—SMB Server Signing & SMB Server EPA (KB5066913) from September 9, 2025. However, there is currently a new development (see text below).
Microsoft confirms the SMB issue
Bolko left this German comment, 2025, after Microsoft confirmed the SMB1 issue with network shares since the September 2025 updates. He quotes a service alert:
service alert […] known issue affects those connecting to SMBv1 shares over the NetBIOS over TCP/IP (NetBT) networking protocol.
"After installing the September 2025 Windows security update (the Originating KBs listed above) 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)"
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.
Bolko cites the article Microsoft says Windows September updates break SMBv1 shares by colleagues at Bleeping Computer, which I also saw last night. I cannot access the "Service Alert" linked by Bleeping Computer because I do not have an approved account.
The affected clients are Windows 10 21H2 – 22H2 and Windows 11 22H2 – 24H2. The affected servers are Windows Server 2022 and Windows Server 2025. Microsoft is currently working on fixing this issue. In the meantime, affected customers should try the workaround outlined above.
The issue has been fixed for Windows 10 and Windows 11 23H2 with the preview updates September 2025 (see Windows 10 22H2/Windows 11 23H2: Preview Updates (September 23,/ 25, 2025)).
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