In many installations of Samba, the greatest threat comes from outside your immediate network. By default, Samba accepts connections from any host, which means that if you run an insecure version of Samba on a host that is directly connected to the Internet, you can be especially vulnerable.

One of the simplest fixes in this case is to use the hosts allow and hosts deny options in the Samba smb.conf configuration file to allow access to your server only from a specific range of hosts. An example might be:

The above will allow SMB connections only from localhost (your own computer) and from the two private networks 192.168.2 and 192.168.3. All other connections will be refused as soon as the client sends its first packet. The refusal will be marked as not listening on called name error.

If you want to restrict access to your server to valid users only, then the following method may be of use. In the smb.conf [global] section put:

This restricts all server access either to the user jacko or to members of the system group smbusers.

By default, Samba accepts connections on any network interface that it finds on your system. That means if you have an ISDN line or a PPP connection to the Internet then Samba will accept connections on those links. This may not be what you want.

You can change this behavior using options like this:

This tells Samba to listen for connections only on interfaces with a name starting with eth such as eth0 or eth1, plus on the loopback interface called lo. The name you will need to use depends on what OS you are using. In the above, I used the common name for Ethernet adapters on Linux.

If you use the above and someone tries to make an SMB connection to your host over a PPP interface called ppp0, then [s]he will get a TCP connection refused reply. In that case, no Samba code is run at all, because the operating system has been told not to pass connections from that interface to any Samba process. However, the refusal helps a would-be cracker by confirming that the IP address provides valid active services.

A better response would be to ignore the connection (from, for example, ppp0) altogether. The advantage of ignoring the connection attempt, as compared with refusing it, is that it foils those who probe an interface with the sole intention of finding valid IP addresses for later use in exploitation or denial of service attacks. This method of dealing with potential malicious activity demands the use of appropriate firewall mechanisms.

Many people use a firewall to deny access to services they do not want exposed outside their network. This can be a good idea, although I recommend using it in conjunction with the above methods so you are protected even if your firewall is not active for some reason.

If you are setting up a firewall, you need to know what TCP and UDP ports to allow and block. Samba uses the following:

The last one is important because many older firewall setups may not be aware of it, given that this port was only added to the protocol in recent years.

When configuring a firewall, the high order ports (1024-65535) are often used for outgoing connections and therefore should be permitted through the firewall. It is prudent to block incoming packets on the high order ports except for established connections.

If the above methods are not suitable, then you could also place a more specific deny on the IPC$ share that is used in the recently discovered security hole. This allows you to offer access to other shares while denying access to IPC$ from potentially untrustworthy hosts.

To do this you could use:

This instructs Samba that IPC$ connections are not allowed from anywhere except the two listed network addresses (localhost and the 192.168.115 subnet). Connections to other shares are still allowed. Because the IPC$ share is the only share that is always accessible anonymously, this provides some level of protection against attackers who do not know a valid username/password for your host.

If you use this method, then clients will be given an `access denied' reply when they try to access the IPC$ share. Those clients will not be able to browse shares and may also be unable to access some other resources. This is not recommended unless for some reason you cannot use one of the other methods just discussed.

To configure NTLMv2 authentication, the following registry keys are worth knowing about:

		[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
		"lmcompatibilitylevel"=dword:00000003
		

The value 0x00000003 means to send NTLMv2 response only. Clients will use NTLMv2 authentication; use NTLMv2 session security if the server supports it. Domain controllers accept LM, NTLM, and NTLMv2 authentication.

		[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0]
		"NtlmMinClientSec"=dword:00080000
		

The value 0x00080000 means permit only NTLMv2 session security. If either NtlmMinClientSec or NtlmMinServerSec is set to 0x00080000, the connection will fail if NTLMv2 session security is negotiated.

This is a common problem. Linux vendors tend to install a default firewall. With the default firewall in place, only traffic on the loopback adapter (IP address 127.0.0.1) is allowed through the firewall.

The solution is either to remove the firewall (stop it) or modify the firewall script to allow SMB networking traffic through. See the Using a Firewall section.

“ We are unable to keep individual users from mapping to any other user's home directory once they have supplied a valid password! They only need to enter their own password. I have not found any method to configure Samba so that users may map only their own home directory. ”

“ User xyzzy can map his home directory. Once mapped, user xyzzy can also map anyone else's home directory. ”

This is not a security flaw, it is by design. Samba allows users to have exactly the same access to the UNIX file system as when they were logged on to the UNIX box, except that it only allows such views onto the file system as are allowed by the defined shares.

If your UNIX home directories are set up so that one user can happily cd into another user's directory and execute ls, the UNIX security solution is to change file permissions on the user's home directories so that the cd and ls are denied.

Samba tries very hard not to second guess the UNIX administrator's security policies and trusts the UNIX admin to set the policies and permissions he or she desires.

Samba allows the behavior you require. Simply put the only user = %S option in the [homes] share definition.

The only user works in conjunction with the users = list, so to get the behavior you require, add the line:

This is equivalent to adding

to the definition of the [homes] share, as recommended in the smb.conf man page.