1 <chapter id="securing-samba">
6 <pubdate>May 26, 2003</pubdate>
9 <title>Securing Samba</title>
12 <title>Introduction</title>
14 This note was attached to the Samba 2.2.8 release notes as it contained an
15 important security fix. The information contained here applies to Samba
16 installations in general.
20 A new apprentice reported for duty to the Chief Engineer of a boiler house. He said, "Here I am,
21 if you will show me the boiler I'll start working on it." Then engineer replied, "You're leaning
26 Security concerns are just like that: You need to know a little about the subject to appreciate
27 how obvious most of it really is. The challenge for most of us is to discover that first morsel
28 of knowledge with which we may unlock the secrets of the masters.
34 <title>Features and Benefits</title>
37 There are three level at which security principals must be observed in order to render a site
38 at least moderately secure. These are: the perimeter firewall, the configuration of the host
39 server that is running Samba, and Samba itself.
43 Samba permits a most flexible approach to network security. As far as possible Samba implements
44 the latest protocols to permit more secure MS Windows file and print operations.
48 Samba may be secured from connections that originate from outside the local network. This may be
49 done using <emphasis>host based protection</emphasis> (using samba's implementation of a technology
50 known as "tcpwrappers", or it may be done be using <emphasis>interface based exclusion</emphasis>
51 so that &smbd; will bind only to specifically permitted interfaces. It is also
52 possible to set specific share or resource based exclusions, eg: on the <parameter>IPC$</parameter>
53 auto-share. The <parameter>IPC$</parameter> share is used for browsing purposes as well as to establish
58 Another method by which Samba may be secured is by way of setting Access Control Entries in an Access
59 Control List on the shares themselves. This is discussed in the chapter on File, Directory and Share Access
66 <title>Technical Discussion of Protective Measures and Issues</title>
69 The key challenge of security is the fact that protective measures suffice at best
70 only to close the door on known exploits and breach techniques. Never assume that
71 because you have followed these few measures that the Samba server is now an impenetrable
72 fortress! Given the history of information systems so far, it is only a matter of time
73 before someone will find yet another vulnerability.
77 <title>Using host based protection</title>
80 In many installations of Samba the greatest threat comes for outside
81 your immediate network. By default Samba will accept connections from
82 any host, which means that if you run an insecure version of Samba on
83 a host that is directly connected to the Internet you can be
84 especially vulnerable.
88 One of the simplest fixes in this case is to use the <parameter>hosts allow</parameter> and
89 <parameter>hosts deny</parameter> options in the Samba &smb.conf; configuration file to only
90 allow access to your server from a specific range of hosts. An example
94 <para><programlisting>
95 hosts allow = 127.0.0.1 192.168.2.0/24 192.168.3.0/24
96 hosts deny = 0.0.0.0/0
97 </programlisting></para>
100 The above will only allow SMB connections from 'localhost' (your own
101 computer) and from the two private networks 192.168.2 and
102 192.168.3. All other connections will be refused as soon
103 as the client sends its first packet. The refusal will be marked as a
104 <errorname>not listening on called name</errorname> error.
110 <title>User based protection</title>
113 If you want to restrict access to your server to valid users only then the following
114 method may be of use. In the &smb.conf; <parameter>[globals]</parameter> section put:
117 <para><programlisting>
118 valid users = @smbusers, jacko
119 </programlisting></para>
122 What this does is, it restricts all server access to either the user <emphasis>jacko</emphasis>
123 or to members of the system group <emphasis>smbusers</emphasis>.
130 <title>Using interface protection</title>
133 By default Samba will accept connections on any network interface that
134 it finds on your system. That means if you have a ISDN line or a PPP
135 connection to the Internet then Samba will accept connections on those
136 links. This may not be what you want.
140 You can change this behaviour using options like the following:
143 <para><programlisting>
145 bind interfaces only = yes
146 </programlisting></para>
149 This tells Samba to only listen for connections on interfaces with a
150 name starting with 'eth' such as eth0, eth1, plus on the loopback
151 interface called 'lo'. The name you will need to use depends on what
152 OS you are using, in the above I used the common name for Ethernet
157 If you use the above and someone tries to make a SMB connection to
158 your host over a PPP interface called 'ppp0' then they will get a TCP
159 connection refused reply. In that case no Samba code is run at all as
160 the operating system has been told not to pass connections from that
161 interface to any samba process.
167 <title>Using a firewall</title>
170 Many people use a firewall to deny access to services that they don't
171 want exposed outside their network. This can be a very good idea,
172 although I would recommend using it in conjunction with the above
173 methods so that you are protected even if your firewall is not active
178 If you are setting up a firewall then you need to know what TCP and
179 UDP ports to allow and block. Samba uses the following:
183 <member>UDP/137 - used by nmbd</member>
184 <member>UDP/138 - used by nmbd</member>
185 <member>TCP/139 - used by smbd</member>
186 <member>TCP/445 - used by smbd</member>
190 The last one is important as many older firewall setups may not be
191 aware of it, given that this port was only added to the protocol in
198 <title>Using a IPC$ share deny</title>
201 If the above methods are not suitable, then you could also place a
202 more specific deny on the IPC$ share that is used in the recently
203 discovered security hole. This allows you to offer access to other
204 shares while denying access to IPC$ from potentially untrustworthy
209 To do that you could use:
212 <para><programlisting>
214 hosts allow = 192.168.115.0/24 127.0.0.1
215 hosts deny = 0.0.0.0/0
216 </programlisting></para>
219 this would tell Samba that IPC$ connections are not allowed from
220 anywhere but the two listed places (localhost and a local
221 subnet). Connections to other shares would still be allowed. As the
222 IPC$ share is the only share that is always accessible anonymously
223 this provides some level of protection against attackers that do not
224 know a username/password for your host.
228 If you use this method then clients will be given a <errorname>access denied</errorname>
229 reply when they try to access the IPC$ share. That means that those
230 clients will not be able to browse shares, and may also be unable to
231 access some other resources.
235 This is not recommended unless you cannot use one of the other
236 methods listed above for some reason.
242 <title>NTLMv2 Security</title>
245 To configure NTLMv2 authentication the following registry keys are worth knowing about:
251 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
252 "lmcompatibilitylevel"=dword:00000003
254 0x3 - Send NTLMv2 response only. Clients will use NTLMv2 authentication,
255 use NTLMv2 session security if the server supports it. Domain
256 controllers accept LM, NTLM and NTLMv2 authentication.
258 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0]
259 "NtlmMinClientSec"=dword:00080000
261 0x80000 - NTLMv2 session security. If either NtlmMinClientSec or
262 NtlmMinServerSec is set to 0x80000, the connection will fail if NTLMv2
263 session security is not negotiated.
270 <title>Upgrading Samba</title>
273 Please check regularly on <ulink url="http://www.samba.org/">http://www.samba.org/</ulink> for updates and
274 important announcements. Occasionally security releases are made and
275 it is highly recommended to upgrade Samba when a security vulnerability
282 <title>Common Errors</title>
285 If all of samba and host platform configuration were really as intuitive as one might like then this
286 section would not be necessary. Security issues are often vexing for a support person to resolve, not
287 because of the complexity of the problem, but for reason that most administrators who post what turns
288 out to be a security problem request are totally convinced that the problem is with Samba.
292 <title>Smbclient works on localhost, but the network is dead</title>
295 This is a very common problem. Red Hat Linux (as do others) will install a default firewall.
296 With the default firewall in place only traffic on the loopback adapter (IP address 127.0.0.1)
297 will be allowed through the firewall.
301 The solution is either to remove the firewall (stop it) or to modify the firewall script to
302 allow SMB networking traffic through. See section above in this chapter.
308 <title>Why can users access home directories of other users?</title>
312 We are unable to keep individual users from mapping to any other user's
313 home directory once they have supplied a valid password! They only need
314 to enter their own password. I have not found *any* method that I can
315 use to configure samba to enforce that only a user may map their own
321 User xyzzy can map his home directory. Once mapped user xyzzy can also map
322 *anyone* else's home directory!
326 This is not a security flaw, it is by design. Samba allows
327 users to have *exactly* the same access to the UNIX filesystem
328 as they would if they were logged onto the UNIX box, except
329 that it only allows such views onto the file system as are
330 allowed by the defined shares.
334 This means that if your UNIX home directories are set up
335 such that one user can happily cd into another users
336 directory and do an ls, the UNIX security solution is to
337 change the UNIX file permissions on the users home directories
338 such that the cd and ls would be denied.
342 Samba tries very hard not to second guess the UNIX administrators
343 security policies, and trusts the UNIX admin to set
344 the policies and permissions he or she desires.
348 Samba does allow the setup you require when you have set the
349 <parameter>only user = yes</parameter> option on the share, is that you have not set the
350 valid users list for the share.
354 Note that only user works in conjunction with the users= list,
355 so to get the behavior you require, add the line :
359 this is equivalent to:
363 to the definition of the <parameter>[homes]</parameter> share, as recommended in
364 the &smb.conf; man page.