In build 29667.1000, the SMB functionality encountered a user authentication problem, which resulted in the inability to establish an SMB connection.

Since the issue started after August updates and SMB worked before, first treat it as a secure-by-default behavior change rather than a manual configuration change.

1. Identify which SMB security condition is blocking authentication on the client. Run these on the affected client:

      net use
      Get-SmbConnection
      Get-SmbClientConfiguration
   

   Then check Event Viewer > Applications and Services Logs > Microsoft > Windows > SMBClient > Connectivity and Security. The event message and ID usually identify the exact cause.
2. Check for guest access being blocked. If the share or NAS previously allowed anonymous or guest access, modern Windows blocks insecure guest sign-ins by default. Typical symptom: messages about unauthenticated guest access or SMBClient Event ID 31017. The preferred fix is to connect with valid credentials instead of guest access. Re-enabling insecure guest logons is discouraged except as a temporary, risk-accepted workaround because it increases exposure to rogue-server and man-in-the-middle attacks.
3. Check whether SMB signing is now required. On Windows 11 24H2 and Windows Server 2025, SMB signing is required by default. If the target is a third-party server or NAS that does not support signing, authentication can fail even if credentials are correct. Preferred fix: enable SMB signing on the server or NAS. If the device cannot support signing, evaluate the security impact before changing the client requirement.
4. Check whether NTLM is blocked or unavailable. This commonly affects connections made by IP address, to a workgroup device, or to a NAS where Kerberos cannot be used. In that case, use the server’s Kerberos-capable name/FQDN instead of the IP address, and verify name resolution and SPNs.
5. If Kerberos is expected, verify name and SPN conditions. When the UNC path uses the server FQDN or NetBIOS name, Windows uses Kerberos for authentication. If session setup fails before the SPN is queried, check whether the server is being accessed by an alias or name that does not match its configured SPN. Also ensure the SMB server has an SPN when accessed through a CNAME DNS record.
6. If this is a workgroup or NAS scenario, check authentication compatibility. The context specifically calls out LmCompatibility setting mismatch between the client and server as a possible cause of authentication failure. If the target is a non-domain SMB server or NAS, confirm both sides support the same authentication method.
7. If repeated failed sign-ins have occurred, consider throttling. On Windows Server 2022+, repeated failed SMB sign-ins can be delayed by the SMB authentication rate limiter, which can look like intermittent authentication failure or slowness. Resolve the underlying bad credentials or mapping problem first.

Most likely causes for a system that worked until recent updates are:

• guest access that is now blocked by default
• SMB signing now being required
• NTLM/Kerberos behavior changing because the connection is using IP address, workgroup naming, or a NAS

A practical end-to-end test path is:

1. Remove any existing failed mapping with net use.
2. Reconnect using the server’s name or FQDN, not its IP.
3. Use a known-valid username and password.
4. Review SMBClient event logs immediately after the failed attempt.
5. If the target is a NAS or third-party SMB server, verify it supports SMB signing and is not relying on guest access.

• Access Denied when you access an SMB file share in Windows
• Negotiate, Session Setup, and Tree Connect failures
• Client, service, and program issues can occur if you change security settings and user rights assignments