Guest access in SMB2 disabled by default in Windows

This article describes information about Windows disabling guest access in SMB2 by default, and provides settings to enable insecure guest logons in Group Policy. However, this is generally not recommended.

Applies to:   Windows 10 - all editions, Windows Server 2019
Original KB number:   4046019

Symptoms

Starting in Windows 10, version 1709 and Windows Server 2019, the SMB2 client no longer allows the following actions:

  • Guest account access to a remote server.
  • Fall back to the Guest account after invalid credentials are provided.

SMBv2 has the following behavior in these versions of Windows:

  • Windows 10 Enterprise and Windows 10 Education RS3 and later 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 Pro version 1809 and later 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 2019 Datacenter and Standard editions 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 is unchanged from its 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.

Also, if a remote server tries to force you to use guest access, or if an administrator enables guest access, the following entries are logged in the SMB Client event log:

Log entry 1

Log Name: Microsoft-Windows-SmbClient/Security  
Source: Microsoft-Windows-SMBClient  
Date: Date/Time  
Event ID: 31017  
Task Category: None  
Level: Error  
Keywords: (128)  
User: NETWORK SERVICE  
Computer: ServerName.contoso.com  
Description: Rejected an insecure guest logon.  
User name: Ned  
Server name: ServerName

Guidance

This event indicates that the server tried to log on the user as an unauthenticated guest but was denied by the client. Guest logons do not support standard security features such as signing and encryption. So, guest logons are vulnerable to man-in-the-middle attacks that can expose sensitive data on the network. Windows disables insecure (nonsecure) guest logons by default. We recommend that you don't enable insecure guest logons.

Log entry 2

Log Name: Microsoft-Windows-SmbClient/Security  
Source: Microsoft-Windows-SMBClient  
Date: Date/Time  
Event ID: 31018  
Task Category: None  
Level: Warning  
Keywords: (128)  
User: NETWORK SERVICE  
Computer: ServerName.contoso.com  
Description: The AllowInsecureGuestAuth registry value is not configured with default settings.

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

Guidance

This event indicates that an administrator has enabled insecure guest logons. An insecure guest logon occurs when a server logs on the user as an unauthenticated guest. It typically occurs in response to an authentication failure. Guest logons do not support standard security features, such as signing and encryption. So, allowing guest logons makes the client vulnerable to man-in-the-middle attacks that can expose sensitive data on the network. Windows disables insecure guest logons by default. We recommend that you don't enable insecure guest logons.

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. We recommend that you don't 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:

  1. Open the Local Group Policy Editor (gpedit.msc).
  2. In the console tree, select Computer Configuration > Administrative Templates > Network > Lanman Workstation.
  3. For the setting, right-click Enable insecure guest logons and select Edit.
  4. Select Enabled and select OK.

Note

By enabling insecure guest logons, this setting reduces the security of Windows clients.

More information

This setting has no effect on SMB1 behavior. SMB1 continues to use guest access and guest fallback.

Note

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 version 1709, Windows Server version 1709 and later versions.