securing-samba.xml   [plain text]

<chapter id="securing-samba">

<chapterinfo>
	&author.tridge;
	&author.jht;
	<pubdate>May 26, 2003</pubdate>
</chapterinfo>

<title>Securing Samba</title>

<sect1>
<title>Introduction</title>
<para>
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.
</para>

<para>
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!"
</para>

<para>
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.
</para>

</sect1>

<sect1>
<title>Features and Benefits</title>

<para>
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.
</para>

<para>
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.
</para>

<para>
Samba may be secured from connections that originate from outside the local network. This may be
done using <emphasis>host based protection</emphasis> (using samba's implementation of a technology
known as "tcpwrappers", or it may be done be using <emphasis>interface based exclusion</emphasis>
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 <smbconfsection>[IPC$]</smbconfsection>
auto-share. The <smbconfsection>[IPC$]</smbconfsection> share is used for browsing purposes as well as to establish
TCP/IP connections.
</para>

<para>
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.
</para>

</sect1>

<sect1>
<title>Technical Discussion of Protective Measures and Issues</title>

<para>
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.
</para>

	<sect2>
	<title>Using host based protection</title>

	<para>
	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.
	</para>

	<para>
	One of the simplest fixes in this case is to use the <smbconfoption><name>hosts allow</name></smbconfoption> and
	<smbconfoption><name>hosts deny</name></smbconfoption> 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:
	</para>

	<para><smbconfblock>
<smbconfoption><name>hosts allow</name><value>127.0.0.1 192.168.2.0/24 192.168.3.0/24</value></smbconfoption>
<smbconfoption><name>hosts deny</name><value>0.0.0.0/0</value></smbconfoption>
	</smbconfblock></para>

	<para>
	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
	<errorname>not listening on called name</errorname> error.
	</para>

	</sect2>

	<sect2>
	<title>User based protection</title>

	<para>
	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; <smbconfsection>[global]</smbconfsection> section put:
	</para>

	<para><smbconfblock>
<smbconfoption><name>valid users</name><value>@smbusers, jacko</value></smbconfoption>
	</smbconfblock></para>

	<para>
	What this does is, it restricts all server access to either the user <emphasis>jacko</emphasis>
	or to members of the system group <emphasis>smbusers</emphasis>.
	</para>

	</sect2>

	<sect2>

	<title>Using interface protection</title>

	<para>
	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.
	</para>

	<para>
	You can change this behaviour using options like the following:
	</para>

	<para><smbconfblock>
<smbconfoption><name>interfaces</name><value>eth* lo</value></smbconfoption>
<smbconfoption><name>bind interfaces only</name><value>yes</value></smbconfoption>
	</smbconfblock></para>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	</sect2>

	<sect2>
	<title>Using a firewall</title>

	<para>
	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.
	</para>

	<para>
	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:
	</para>

	<simplelist>
		<member>UDP/137 - used by nmbd</member>
		<member>UDP/138 - used by nmbd</member>
		<member>TCP/139 - used by smbd</member>
		<member>TCP/445 - used by smbd</member>
	</simplelist>

	<para>
	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. 
	</para>

	</sect2>

	<sect2>
	<title>Using a IPC$ share deny</title>

	<para>
	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.
	</para>

	<para>
	To do that you could use:
	</para>

	<para><smbconfblock>
<smbconfsection>[ipc$]</smbconfsection>
<smbconfoption><name>hosts allow</name><value>192.168.115.0/24 127.0.0.1</value></smbconfoption>
<smbconfoption><name>hosts deny</name><value>0.0.0.0/0</value></smbconfoption>
	</smbconfblock></para>

	<para>
	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.
	</para>

	<para>
	If you use this method then clients will be given a <errorname>access denied</errorname>
	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. 
	</para>

	<para>
	This is not recommended unless you cannot use one of the other
	methods listed above for some reason.
	</para>

	</sect2>

	<sect2>
	<title>NTLMv2 Security</title>

	<para>
	To configure NTLMv2 authentication the following registry keys are worth knowing about:
	</para>

	<para>
		<screen>
		[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
		"lmcompatibilitylevel"=dword:00000003
		</screen>
	</para>

	<para>
		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.
	</para>

	<para>
		<screen>
		[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0]
		"NtlmMinClientSec"=dword:00080000
		</screen>
	</para>

	<para>
		0x80000 - NTLMv2 session security. If either NtlmMinClientSec or
		NtlmMinServerSec is set to 0x80000, the connection will fail if NTLMv2
		session security is not negotiated.
	</para>
	</sect2>
</sect1>

<sect1>
<title>Upgrading Samba</title>

<para>
Please check regularly on <ulink noescape="1" url="http://www.samba.org/">http://www.samba.org/</ulink> 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.
</para>

</sect1>

<sect1>
<title>Common Errors</title>

<para>
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.
</para>

	<sect2>
	<title>Smbclient works on localhost, but the network is dead</title>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	</sect2>

	<sect2>
	<title>Why can users access home directories of other users?</title>

	<para>
	<quote>
	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.
	</quote>
	</para>

	<para><quote>
	User xyzzy can map his home directory. Once mapped user xyzzy can also map
	*anyone* else's home directory!
	</quote></para>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	<para>
	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.
	</para>

	<para>
	Samba does allow the setup you require when you have set the
	<smbconfoption><name>only user</name><value>yes</value></smbconfoption> option on the share, is that you have not set the
	valid users list for the share.
	</para>

	<para>
	Note that only user works in conjunction with the users= list,
	so to get the behavior you require, add the line :
	<smbconfblock>
<smbconfoption><name>users</name><value>%S</value></smbconfoption>
</smbconfblock>
	this is equivalent to:
	<smbconfblock>
<smbconfoption><name>valid users</name><value>%S</value></smbconfoption>
	</smbconfblock>
	to the definition of the <smbconfsection>[homes]</smbconfsection> share, as recommended in
	the &smb.conf; man page.
	</para>
	</sect2>

</sect1>
</chapter>