10 Ways to Secure Your Samba Server on Linux

SMB (Server Message Block) protocol is a cornerstone of file and printer sharing in connected environments. However, the default configuration of Samba can pose significant security risks, leaving your network vulnerable to unauthorized access and cyberattacks.

If you’re hosting a Samba server, you need to be extra cautious with the configurations you have set in place. Here are 10 critical steps to ensure your SMB server remains secure and protected.

1. Enable Encryption for SMB Traffic

By default, SMB traffic is not encrypted. You can verify this by capturing network packets with tcpdump or Wireshark. It is paramount that you encrypt all traffic to prevent an attacker from intercepting and analyzing the traffic.

It’s recommended that you set up Transport Layer Security (TLS) to encrypt and secure your Linux Samba server’s traffic.

2. Implement Strict Access Controls and Permissions for Shared Resources

You should implement strict access controls and permissions to ensure that the connected users are not able to access unsolicited resources. Samba uses a central configuration file /etc/samba/smb.conf that allows you to define rules for access and permissions.

Using special syntax, you can define resources to share, users/groups to give access to those resources, and whether the resource(s) can be browsed, written onto, or read from. Here’s the sample syntax for declaring a resource and implementing access controls on it:

 [sambashare]
comment= Samba Example
path = /home/your_username/sambashare
browseable = yes
writable = yes
valid users = @groupname
 

In the above lines, we add a new share location with a path and with valid users, we restrict access to the share to only a single group. There are multiple other ways to define controls and access to a share. You can learn more about it from our dedicated guide on how to set up a network-shared folder on Linux with Samba.

3. Use Strong and Unique Passwords for SMB User Accounts

Enforcing robust password policies for SMB user accounts is a fundamental security best practice. As a system administrator, you should create or urge all users to create strong and unique passwords for their accounts.

You can also speed up this process by automatically generating strong passwords using tools. Optionally, you can also regularly rotate passwords to mitigate the risk of data leaks and unauthorized access.

4. Regularly Update Linux and Samba

The simplest form of passive defense against all sorts of cyberattacks is ensuring that you’re running updated versions of critical software. SMB is prone to vulnerabilities. It is always a lucrative target for attackers.

There have been multiple critical SMB vulnerabilities in the past that lead to complete system takeover or loss of confidential data. You must keep both your operating system and the critical services on it up to date.

5. Avoid Using SMBv1 Protocol

SMBv1 is an insecure protocol. It’s always recommended that whenever you use SMB, may it be on Windows or Linux, you should avoid using SMBv1 and only use SMBv2 and upwards. To disable the SMBv1 protocol, add this line to the configuration file:

 min protocol = SMB2 

This ensures that the minimum protocol level in use would be SMBv2.

6. Enforce Firewall Rules to Restrict Access to SMB Ports

Configure your network’s firewall to allow access to SMB ports, generally port 139 and port 445 only from trusted sources. This helps prevent unauthorized access and reduces the risk of SMB-based attacks from external threats.

You should also consider installing an IDS solution along with a dedicated firewall to have better control and logging of traffic. Unsure which firewall to use? You may find one that suits you from the list of the best free Linux firewalls to use.

7. Implement Network Segmentation to Isolate SMB Traffic From Untrusted Networks

Network segmentation is the technique of dividing a single monolithic model of a computer network into multiple subnets, each called a network segment. This is done to improve the security, performance, and manageability of the network.

To isolate SMB traffic from untrusted networks, you can create a separate network segment for SMB traffic and configure firewall rules to only allow SMB traffic to and from this segment. This allows you to manage and monitor SMB traffic in a focused way.

On Linux, you can use iptables or a similar networking tool to configure firewall rules to control the flow of traffic between network segments. You can create rules to allow SMB traffic to and from the SMB network segment while blocking all other traffic. This will effectively isolate SMB traffic from untrusted networks.

8. Monitor SMB Logs for Suspicious Activities and Security Incidents

Monitoring SMB logs for suspicious activities and security incidents is an important part of maintaining the security of your network. SMB logs contain information about SMB traffic, including file access, authentication, and other events. By regularly monitoring these logs, you can identify potential security threats and mitigate them.

On Linux, you can use the journalctl command and pipe its output to the grep command to view and analyze SMB logs.

 journalctl -u smbd.service 

This will display the logs for the smbd.service unit which is responsible for managing SMB traffic. You can use the -f option to follow the logs in real-time or use the -r option to view the most recent entries first.

To search the logs for specific events or patterns, pipe the output of the journalctl command to grep. For example, to search for failed authentication attempts, run:

 journalctl -u smbd.service | grep -i "authentication failure" 

This will display all log entries that contain the text “authentication failure,” allowing you to quickly identify any suspicious activity or brute-force attempts.

9. Limit the Use of Guest Access and Anonymous Connections

Enabling guest access allows users to connect to the Samba server without providing a username or password, while anonymous connections allow users to connect without providing any authentication information.

Both of these options can pose a security risk if not properly managed. It’s recommended you turn both of these off. To do that you need to add or modify a couple of lines in the Samba configuration file. Here’s what you need to add/modify in the global section of the smb.conf file:

 map to guest = never
restrict anonymous = 2 

10. Implement Host-Based Restrictions

By default, an exposed Samba server can be accessed by any host (IP address) without restrictions. By access, it is meant to establish a connection and not, literally access the resources.

To allow access to specific hosts, and deny to rest, you can make use of hosts allow and hosts deny options. Here’s the syntax to add to the configuration file to allow/deny hosts:

 hosts allow = 127.0.0.1 192.168.1.0/24
hosts deny = 0.0.0.0/0 

Here you’re commanding Samba to deny all connections except those of the local host and the 192.168.1.0/24 network. This is one of the fundamental ways to secure your SSH server too.

Now You Know How to Secure Your Samba Linux Server

Linux is great for hosting servers. However, whenever you’re dealing with servers, you have to tread carefully and be extra aware as Linux servers are always a lucrative target for threat actors.

It is paramount that you put sincere effort into fortifying your network and hardening your Linux servers. Besides properly configuring Samba, there are a few other measures you should take to ensure your Linux server is safe from the crosshair of adversaries.