Chapter 24. 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. -

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. -

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. +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!"

-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. +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.

-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. +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.

-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. +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.

-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 +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 [globals] 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.
+		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
+		[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

+ 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. -

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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 admininstrators 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* elses 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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