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Technical background to the SMB protocol

Server Message Block is susceptible to NTLM relay attacks if packets are not signed. This man-in-the-middle attack hijacks authentications between clients and servers. Authentication can be used to start a session on the server and steal data. The SMB signing explicitly assigns each initiated session to the client. This means that it is still possible to hijack the authentication, but it is discarded as the session is not signed.

In SMBv1, SMB signing is deactivated by default. This protocol is outdated, error-prone, insecure and should generally be switched off. SMBv2 activates SMB signing as the default value, but only uses it if the server or client require it as a prerequisite. This is usually not the case.

	Server
Client	Requires	Activated	Deactivated (SMBv1)
Required	Signed	Signed	Not supported
Enabled	Signed	Signed SMBv1, not signed SMBv2	not signed
Disabled (SMBv1)	not supported	not signed	not signed

This means that SMB signing must be activated on the server as a prerequisite.

Procedure

flowchart Z[Flow] A[Disable SMBv1] B[SMB signing] C[Client) D[Server) E(optional activation) F(forced activation) Z--Step 1-->A Z-->B B-->C B-->D C--Step 2-->E D--Step 3-->F

Step 1 - Deactivate SMBv1

Clients

Clients can be managed via the login script. The status can be queried via Powershell (admin rights required) as follows:

Get-WindowsOptionalFeature -Online -FeatureName SMB1Protocol

SMBv1 can also be deactivated via Powershell (restart required):

Disable-WindowsOptionalFeature -Online -FeatureName SMB1Protocol

Server

The same commands apply to servers. A restart should be scheduled in advance.

Step 2 - Optionally activate SMB signing on the client side

In the next step, SMB signing is optionally activated on the client side. A global group policy can be set for this purpose. The corresponding guidelines can be found here:

Computerkonfiguration       ->
 Windows-Einstellungen      -> 
  Sicherheitseinstellungen  ->
   Lokale Richtlinien       ->
    Sicherheitsoptionen

Alternatively, the settings can also be set in the registry.

Windows Registry Editor Version 5.00
 
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters]
"EnableSecuritySignature"=dword:00000001
"RequireSecuritySignature"=dword:00000000

Step 3 - Force SMB signing on the server side

Option 1: via group policy

Computerkonfiguration       ->
 Windows-Einstellungen      -> 
  Sicherheitseinstellungen  ->
   Lokale Richtlinien       ->
    Sicherheitsoptionen

Option 2: via Powershell

Retrieve status

Get-SmbClientConfiguration | select RequireSecuritySignature, EnableSecuritySignature
Get-SmbServerConfiguration | select RequireSecuritySignature, EnableSecuritySignature

Enable force

Set-SmbClientConfiguration -EnableSecuritySignature $true
Set-SmbServerConfiguration -EnableSecuritySignature $true
Set-SmbServerConfiguration -RequireSecuritySignature $true
Set-SmbClientConfiguration -RequireSecuritySignature $true

References

^{https://learn.microsoft.com/de-de/troubleshoot/windows-server/networking/overview-server-message-block-signing}

^{https://en.hackndo.com/ntlm-relay/}

SMB Hardening