This note was attached to the Samba 2.2.8 release notes as it contained an important security fix. The information contained here applies to Samba installations in general.
In many installations of Samba the greatest threat comes for outside your immediate network. By default Samba will accept 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 only allow access to your server from a specific range of hosts. An example might be:
hosts allow = 127.0.0.1 192.168.2.0/24 192.168.3.0/24 hosts deny = 0.0.0.0/0
The above will only allow SMB connections from 'localhost' (your own computer) and from the two private networks 192.168.2 and 192.168.3. All other connections will be refused connections as soon as the client sends its first packet. The refusal will be marked as a 'not listening on called name' error.
By default Samba will accept connections on any network interface that it finds on your system. That means if you have a 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 behaviour using options like the following:
interfaces = eth* lo bind interfaces only = yes
This tells Samba to only listen for connections on interfaces with a name starting with 'eth' such as eth0, 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 a SMB connection to your host over a PPP interface called 'ppp0' then they will get a TCP connection refused reply. In that case no Samba code is run at all as the operating system has been told not to pass connections from that interface to any process.
Many people use a firewall to deny access to services that they don't want exposed outside their network. This can be a very good idea, although I would recommend using it in conjunction with the above methods so that you are protected even if your firewall is not active for some reason.
If you are setting up a firewall then you need to know what TCP and UDP ports to allow and block. Samba uses the following:
UDP/137 - used by nmbd UDP/138 - used by nmbd TCP/139 - used by smbd TCP/445 - used by smbd
The last one is important as many older firewall setups may not be aware of it, given that this port was only added to the protocol in recent years.
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 that you could use:
[ipc$]
hosts allow = 192.168.115.0/24 127.0.0.1
hosts deny = 0.0.0.0/0this would tell Samba that IPC$ connections are not allowed from anywhere but the two listed places (localhost and a local subnet). Connections to other shares would still be allowed. As the IPC$ share is the only share that is always accessible anonymously this provides some level of protection against attackers that do not know a username/password for your host.
If you use this method then clients will be given a 'access denied' reply when they try to access the IPC$ share. That means that those clients will not be able to browse shares, and may also be unable to access some other resources.
This is not recommended unless you cannot use one of the other methods listed above for some reason.
Please check regularly on http://www.samba.org/ for updates and important announcements. Occasionally security releases are made and it is highly recommended to upgrade Samba when a security vulnerability is discovered.