After Upgrading to Windows 10 1709 (Fall Creators Update) I couldn’t access my Synology NAS anymore. Therefore I started troubleshooting the Windows 10 1709 Cannot Access SMB2 Share Guest Access error:

An error occurred while reconnecting X: to \\nas\data Microsoft Windows Network: You can’t access this shared folder because your organization’s security policies block unauthenticated guest access. These policies help protect your PC from unsafe or malicious devices on the network.

Cause

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Starting with Windows 10 1709, Windows prevents you from accessing network shares with guest access enabled. Guest access means connecting to network shares without authentication, using the built-in “guest” account.

This has no reference to the SMB1 protocol which was disabled in the latest Windows 10 release.

Resolution

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To enable guest access again, configure the following GPO:

Computer configuration > administrative templates > network > Lanman Workstation: "Enable insecure guest logons" = Enabled

Registry Key

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The according registry key is located under:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters]"AllowInsecureGuestAuth"=dword:1

Download

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You can also download the reg file to simply click and set the registry key from here:AllowInsecureGuestAuth.Reg

MDM Policy

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Theres also an MDM Policy available, starting with Windows 10 1803: https://docs.microsoft.com/en-us/windows/client-management/mdm/policy-csp-lanmanworkstation

About guest access

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Guest access is often used to access data stored on Network Attached Storage, e.g. on a Synology NAS. Every user from any device has access to these shares. Generally it’s never recommended to use guest access because it’s a huge security risk. Within the time of ransomware, encrypting whole drives it’s definitely not a good idea. I strongly recommend to use LDAP Support to authenticate against your NAS.

Reference

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• Microsoft Support: guest-access-smb2-disabled-by-default-in-windows-10-server-2016