Cyber threats continue to grow in both volume and sophistication. Protecting your systems has never been more critical. Linux hardening is the process of reducing a system’s attack surface by applying security configurations. The goal is simple: minimize vulnerabilities and entry points that cybercriminals could exploit.
Several security frameworks provide baseline recommendations. However, they differ depending on the context:
- CIS Benchmarks (Center for Internet Security) provide detailed technical recommendations widely adopted in both private and public sectors.
- STIG (Security Technical Implementation Guides) are produced by the Defense Information Systems Agency (DISA) and mainly targeted at the U.S. government and military environments.
- NIST SP 800-123 provides a more generic framework for server hardening, regardless of the operating system.
What they all have in common is that they define OS-specific recommendations, which makes them difficult to apply consistently across a heterogeneous fleet.
1. Restrict application security rights
Controlling application rights and permissions is a fundamental measure, since it’s a native feature in the various Linux and Microsoft Windows OSs. It involves enabling security modules built into the Linux kernel, like AppArmor (for Debian systems) or SELinux (for Red Hat systems), and configuring them. These enable granular access control by defining rights per application (such as network access or file read and write permissions) and associating them with a security profile that restricts access to the OS.
2. Reduce the attack surface by disabling non-essential services for security purposes
The more software or components installed on an OS, the greater the potential attack surface. Limit, disable or remove access to your servers through non-essential services as much as possible.
These services could be graphical interfaces, print servers, email or web servers, for example, that could allow data to be exfiltrated.
3. For a secured configuration : remove risky application clients
Some applications, such as FTP (File Transfer Protocol), Telnet or RSH (Remote Shell) clients, allow you to exchange files and data across the network.
Depending on the application, data could be sent through a network unencrypted, making it highly vulnerable to sniffing attacks.
So, make sure that none of these clients are running on your machines unless they’re needed. Blocking these applications prevents attackers from accessing your system through them and ensures that your data is not compromised.
4. Audit your server configurations
Hardware and network configurations are often overlooked in hardening strategies, yet they represent a significant attack vector.
Also bear in mind to protect access to CPU activity on your machines in order to prevent side-channel attacks, which work by monitoring the system’s behavior (CPU activity in particular) in relation to certain user actions in order to extract information. Finally, make sure that your servers are scheduled to shut down if an incident is identified. Rudder platform has proven its great help on these purposes, with its continuous audit and remediation approach.
5. Enable logging — the first step toward compliance
Log management is essential in a hardening process, especially to carry out post-mortems analysis after an incident.
Make sure that auditd and journald are enabled and configured, and that the logs they generate are properly compressed.
At a minimum, verify auditd is active (systemctl status auditd ) and ensure audit rules monitor access to sensitive files such as /etc/passwd or /etc/sudoers
Auditd generates event logs in the system (date, time, type, number of access attempts, imports, exports and so on). Journald collects and centralizes all the system logs.
6. Harden security of SSH configuration
- you cannot log in as an administrator
- you can only log in using keys and not passwords
- these keys are secure enough.
7. Get privilege creep under control
Privilege creep is a major risk when it comes to directories, including symbolic links to higher privileged directories. User rights must be assigned on the principle of least privilege by default. But sometimes you need to give administrator rights temporarily, sudo does exactly just that.
This measure aims to ensure that the sudo package is properly installed and configured, and to verify that users who rely on it have the appropriate permissions and restrictions in place.
It also defines a maximum usage duration for this command and logs all actions performed with its privileges.
8. Control local user and group configuration
Get a grip on user permissions and rights, as well as access to related information. The file that stores user passwords, for example, needs to be encrypted and accessed only by the administrator.
This avoids user duplication and assigns a unique identifier to each user, making actions more traceable. Don’t forget to check that the machine has only one superuser and that user groups are set up correctly.
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9. Enforce a security policy for local user passwords
As well as managing users, you need to define secure authentication parameters for them.
Check that users cannot have empty passwords and require the password chosen to follow a specific protocol (minimum number of characters, non-alphanumeric characters, no old passwords, etc.). Finally, make sure that access is blocked in case of too many attempts with incorrect passwords.
10. Check for application updates
Vulnerabilities are discovered and released all the time. Failing to install patches and updates leaves you with a bigger target on your back. The waves of malware attacks that strike after patches are released show the need to be proactive and take action.
The goal with this hardening technique is to always keep an eye out to make sure your systems are up-to-date with the latest available version. Even though you need to perform regression testing before rolling out updates, excessive delays can prove to be a dangerous strategy.
That is exactly why Rudder offers a patch management module designed to detect available updates and automate remediation campaigns without ever losing control over regression testing.
Frequently asked questions about Linux hardening
What is Linux hardening?
Linux hardening is the process of reducing a Linux system’s attack surface by applying secure configurations, disabling unnecessary services, strengthening access controls, and maintaining up-to-date systems.
Is Linux hardening enough to secure a system?
No. Hardening is a preventive measure that reduces the attack surface, but it must be part of a broader defense-in-depth strategy that includes log monitoring, patch management, and intrusion detection.
How often should hardening rules be applied?
Configurations can drift over time due to updates, changes, or new users. Continuous compliance monitoring.
Which tools can automate Linux hardening?
Configuration management tools such as Ansible, Chef, or Puppet can deploy secure configurations. OpenSCAP supports CIS/STIG compliance auditing. Solutions like Rudder combine audit, automated remediation, and continuous compliance monitoring.
Manage and automate hardening with a single tool
Ensuring these 10 measures remain continuously enforced across all your Linux and don’t drift systems is demanding and time-consuming. This is even more true in environments subject to compliance requirements (such as CIS, NIST, or NIS2). The most effective way to achieve this is to rely on a tool that manages and secures your entire infrastructure.
That’s where Rudder comes in. Rudder not only deploys these rules on your systems, it also makes sure they are continuously applied from audit to remediation: see for yourself.
Don’t want to implement these measures yourself? Let our Policy and benchmark compliance solution handle it, it continuously ensures compliance with your standards and security frameworks (CIS Benchmarks, NIST, NIS2…). Rudder doesn’t just audit, it also automatically remediates when needed. And that is not all: we also offer a subscription-based patch management module, which identifies available updates and automates patch campaigns to keep your systems up to date
