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 levels at which security principals must be observed in order to render a site -at least moderately secure. They 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 smbd will bind only to specifically permitted interfaces. It is also -possible to set specific share or resource-based exclusions, for example 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 setting Access Control Entries (ACEs) in an Access -Control List (ACL) on the shares themselves. This is discussed in . -

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 from 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 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

- This 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 behavior using options like this: -

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 an 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 they do not - want exposed outside their network. This can be a good idea, - although I recommend using it in conjunction with the above - methods so you are protected even if your firewall is not active - for some reason. -

- If you are setting up a firewall, 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 IPC$ Share-Based Denials

- 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 this you could use: -

[IPC$]

hosts allow = 192.168.115.0/24 127.0.0.1

hosts deny = 0.0.0.0/0

- This instructs Samba that IPC$ connections are not allowed from - anywhere except from the two listed network addresses (localhost and the 192.168.115 - subnet). Connections to other shares are still 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 valid username/password for your host. -

- If you use this method, then clients will be given an `access denied' - reply when they try to access the IPC$ share. 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
-

- The value 0x00000003 means 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
-

- The value 0x00080000 means permit only NTLMv2 session security. If either NtlmMinClientSec or - NtlmMinServerSec is set to 0x00080000, 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 them to be, 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 the 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 common problem. Red Hat Linux (and others) installs a default firewall. - With the default firewall in place, only traffic on the loopback adapter (IP address 127.0.0.1) - is allowed through the firewall. -

- The solution is either to remove the firewall (stop it) or 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 to configure - Samba so that users may map only 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 file system as when 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. -

- If your UNIX home directories are set up so that one user can happily cd - into another users directory and execute ls, the UNIX security solution is to change file - permissions on the user's home directories such that the cd and ls are 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 allows the behavior you require. Simply put the only user = %S - option in the [homes] share definition. -

- The 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 adding -

valid users = %S

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

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