Symptoms
In Windows 10, version 1709, Windows Server version 1709, and Windows Server 2019, the SMB2 client no longer allows the following actions:
- Guest account access to a remote server
- Fallback to the Guest account after invalid credentials are provided
SMBv2 has the following behavior in Windows 10, version 1709, Windows Server version 1709, and Windows Server 2019:
- Windows 10 Enterprise and Windows 10 Education no longer allow a user to connect to a remote share by using guest credentials by default, even if the remote server requests guest credentials.
- Windows Server 2016 Datacenter and Standard edition no longer allow a user to connect to a remote share by using guest credentials by default, even if the remote server requests guest credentials.
- Windows 10 Home and Professional editions are unchanged from their previous default behavior.
If you try to connect to devices that request credentials of a guest instead of appropriate authenticated principals, you may receive the following error message:
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
This change in default behavior is by design and is recommended by Microsoft for security.
A malicious computer that impersonates a legitimate file server could allow users to connect as guests without their knowledge. Microsoft recommends that you do not change this default setting. If a remote device is configured to use guest credentials, an administrator should disable guest access to that remote device and configure correct authentication and authorization.
Windows and Windows Server have not enabled guest access or allowed remote users to connect as guest or anonymous users since Windows 2000. Only third-party remote devices might require guest access by default. Microsoft-provided operating systems do not.
Resolution
If you want to enable insecure guest access, you can configure the following Group Policy settings:
Computer configuration\administrative templates\network\Lanman Workstation
"Enable insecure guest logons"
Note By enabling insecure guest logons, this setting reduces the security of Windows clients.
Example :Default Registry Value: [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters] "AllowInsecureGuestAuth"=dword:0 Configured Registry Value: [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters] "AllowInsecureGuestAuth"=dword:1
More Information
This setting has no effect on SMB1 behavior. SMB1 continues to use guest access and guest fallback.
SMB1 is uninstalled by default in latest Windows 10 and Windows Server configurations. For more information see SMBv1 is not installed by default in Windows 10 Fall Creators Update and Windows Server, version 1709.
