Chapter 15. Securing Samba

+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. +

+A new apprentice reported for duty to the Chief Engineer of a boiler house. He said, "Here I am, +if you will show me the boiler I'll start working on it." Then engineer replied, "You're leaning +on it!" +

+Security concerns are just like that: You need to know a little about the subject to appreciate +how obvious most of it really is. The challenge for most of us is to discover that first morsel +of knowledge with which we may unlock the secrets of the masters. +

Features and Benefits

+There are three level at which security principals must be observed in order to render a site +at least moderately secure. These are: the perimeter firewall, the configuration of the host +server that is running Samba, and Samba itself. +

+Samba permits a most flexible approach to network security. As far as possible Samba implements +the latest protocols to permit more secure MS Windows file and print operations. +

+Samba may be secured from connections that originate from outside the local network. This may be +done using host based protection (using samba's implementation of a technology +known as "tcpwrappers", or it may be done be using interface based exclusion +so that smbd will bind only to specifically permitted interfaces. It is also +possible to set specific share or resource based exclusions, eg: on the [IPC$] +auto-share. The [IPC$] share is used for browsing purposes as well as to establish +TCP/IP connections. +

+Another method by which Samba may be secured is by way of setting Access Control Entries in an Access +Control List on the shares themselves. This is discussed in the chapter on File, Directory and Share Access +Control. +

Technical Discussion of Protective Measures and Issues

+The key challenge of security is the fact that protective measures suffice at best +only to close the door on known exploits and breach techniques. Never assume that +because you have followed these few measures that the Samba server is now an impenetrable +fortress! Given the history of information systems so far, it is only a matter of time +before someone will find yet another vulnerability. +

Using host based protection

+ 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 as soon + as the client sends its first packet. The refusal will be marked as a + not listening on called name error. +

User based protection

+ 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: +

valid users = @smbusers, jacko

+ What this does is, it restricts all server access to either the user jacko + or to members of the system group smbusers. +

Using interface protection

+ 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 samba process. +

Using a firewall

+ 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. +

Using a IPC$ share deny

+ 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 = 0.0.0.0/0

+ this 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. +

NTLMv2 Security

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

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

+ 0x3 - 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
+

+ 0x80000 - NTLMv2 session security. If either NtlmMinClientSec or + NtlmMinServerSec is set to 0x80000, the connection will fail if NTLMv2 + session security is not negotiated. +

Upgrading Samba

+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. Check with your OS vendor for OS specific upgrades. +

Common Errors

+If all of samba and host platform configuration were really as intuitive as one might like then this +section would not be necessary. Security issues are often vexing for a support person to resolve, not +because of the complexity of the problem, but for reason that most administrators who post what turns +out to be a security problem request are totally convinced that the problem is with Samba. +

Smbclient works on localhost, but the network is dead

+ This is a very common problem. Red Hat Linux (as do others) will install a default firewall. + With the default firewall in place only traffic on the loopback adapter (IP address 127.0.0.1) + will be allowed through the firewall. +

+ The solution is either to remove the firewall (stop it) or to modify the firewall script to + allow SMB networking traffic through. See section above in this chapter. +

Why can users access home directories of other users?

+ “ + 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 that I can + use to configure samba to enforce that only a user may map 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 filesystem + as they would if they were logged onto the UNIX box, except + that it only allows such views onto the file system as are + allowed by the defined shares. +

+ This means that if your UNIX home directories are set up + such that one user can happily cd into another users + directory and do an ls, the UNIX security solution is to + change the UNIX file permissions on the users home directories + such that the cd and ls would be denied. +

+ Samba tries very hard not to second guess the UNIX administrators + security policies, and trusts the UNIX admin to set + the policies and permissions he or she desires. +

+ Samba does allow the setup you require when you have set the + only user = yes option on the share, is that you have not set the + valid users list for the share. +

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

users = %S

+ this is equivalent to: +

valid users = %S

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

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