<< Back to man.ChinaUnix.net

Chapter 17. Securing Samba

Andrew Tridgell

Samba Team

John H. Terpstra

Samba Team

May 26, 2003

Table of Contents

Introduction
Features and Benefits
Technical Discussion of Protective Measures and Issues
Using Host-Based Protection
User-Based Protection
Using Interface Protection
Using a Firewall
Using IPC$ Share-Based Denials
NTLMv2 Security
Upgrading Samba
Common Errors
Smbclient Works on Localhost, but the Network Is Dead
Why Can Users Access Home Directories of Other Users?

Introduction

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 File, Directory and Share Access Controls.

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 un-trustworthy 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.