Linux Server Security Mega-Checklist

If you're big on security, which you should when working in a production environment, this exhaustive list covers 50 important points to take care of when you set up a server running Linux but some of them can extend beyond just the operating system. We will cover those points in future articles on a one-by-one basis, looking at how to implement them properly.

1. Encrypt Data Communication: Utilize encryption protocols such as SSL/TLS for transmitting data over networks to prevent eavesdropping. Implement SSL/TLS for web servers using tools like Let's Encrypt or self-signed certificates. Ensure proper configuration of encryption ciphers and protocols to mitigate vulnerabilities like POODLE or BEAST.
2. Avoid Using Insecure Services: FTP, Telnet, Rlogin, and Rsh transmit data in plain text, making them vulnerable to interception. Instead, use secure alternatives like OpenSSH, SFTP, or FTPS, which add encryption (SSL or TLS) to FTP. Disable insecure services in server configurations to prevent accidental usage.
3. Minimize Software Installation: Limit installed software to essential packages to reduce the potential attack surface. Review and remove unnecessary packages using package managers like yum or apt. Regularly audit installed packages for vulnerabilities using tools like lynis or rpm -qa --last.
4. Isolate Network Services: Run each network service on separate servers or virtual machine instances to limit the impact of a compromise. Utilize network segmentation and VLANs to isolate critical services from each other and from user networks. Implement network access control lists (ACLs) to restrict traffic flow between different network segments.
5. Keep System Updated: Apply security patches regularly to the Linux kernel and installed software to address known vulnerabilities. Use package managers like yum, apt, or dpkg to update packages and apply security fixes. Subscribe to security mailing lists and CVE feeds to stay informed about new vulnerabilities.
6. Use Linux Security Extensions: Implement additional security measures like AppArmor/SELinux to enforce access controls and mitigate risks. Configure AppArmor/SELinux policies tailored to the specific requirements and applications running on the server. Regularly audit and review SELinux policies to ensure they align with security best practices.
7. Utilize SELinux: Employ SELinux to enforce Mandatory Access Control (MAC) policies and protect against malicious or flawed applications. Configure SELinux in targeted or enforcing mode depending on the server's security requirements and operational needs. Monitor SELinux logs and alerts for policy violations or unauthorized access attempts.
8. Enforce Strong Password Policies: Set up user accounts with strong passwords and implement policies for password aging and complexity. Enforce password complexity rules such as minimum length, character types, and prohibited words. Implement account lockout mechanisms to prevent brute-force attacks against user passwords.
9. Configure Password Aging: Set expiration dates for passwords and enforce regular password changes to enhance security. Configure password expiration policies based on risk assessment and compliance requirements. Notify users in advance of password expiration and provide guidance on creating strong passwords.
10. Prevent Password Reuse: Restrict users from reusing old passwords to prevent unauthorized access. Implement password history policies to prevent users from cycling through a set of previously used passwords. Educate users on the importance of using unique passwords for different systems and services.
11. Lock Accounts After Failures: Automatically lock user accounts after a specified number of login failures to deter brute-force attacks. Configure account lockout settings based on the organization's risk tolerance and security policies. Implement rate-limiting measures to prevent automated password guessing attacks.
12. Check for Empty Passwords: Ensure that no user accounts have empty passwords, as they pose a significant security risk. Periodically scan user account configurations and password databases for accounts with empty passwords. Prompt users to set strong passwords during initial account setup and password resets.
13. Audit UID 0 Accounts: Review accounts with UID 0 (root) to ensure they are legitimate and necessary. Monitor changes to root account permissions and access rights using auditing tools like auditd or AIDE. Restrict direct root access to the system and encourage the use of sudo for administrative tasks.
14. Disable Root Login: Prohibit direct root logins and use sudo for privilege escalation to enhance accountability and security. Configure sudoers file to grant specific privileges to individual users or groups based on the principle of least privilege. Monitor sudo logs for unauthorized or suspicious commands executed by privileged users.
15. Secure Physical Server Access: Implement physical security measures to protect server hardware and prevent unauthorized access. Secure server rooms or data centers with access controls such as biometric authentication, access badges, or keycards. Log and monitor physical access to server facilities to detect and respond to unauthorized entry attempts.
16. Disable Unnecessary Services: Turn off unnecessary network services and daemons to reduce the attack surface. Use tools like netstat or lsof to identify listening ports and associated services running on the server. Remove or disable services that are not required for the server's intended purpose to minimize potential security risks.
17. Identify Listening Ports: Monitor open network ports and associated services to detect potential vulnerabilities. Regularly scan server ports using network scanning tools like Nmap or Nessus to identify unauthorized services. Implement firewall rules to block or restrict access to unused ports and services.
18. Remove X Window Systems: Eliminate X11 and other unnecessary graphical interfaces to reduce the attack surface. Disable X11 forwarding in SSH configurations to prevent X Window System traffic from traversing insecure networks. Configure servers to boot into text mode instead of graphical mode to minimize resource usage and security risks.
19. Configure Firewall: Set up iptables or other firewall solutions to filter and control network traffic. Define firewall rules based on security policies and network architecture to allow only necessary connections. Regularly review and update firewall rules to adapt to changing threats and business requirements.
20. Harden Kernel Parameters: Adjust kernel parameters in /etc/sysctl.conf to enhance security and protect against various attacks. Configure kernel settings such as network stack parameters, process limits, and memory protections. Enable kernel security features like Address Space Layout Randomization (ASLR) and Kernel Page Table Isolation (KPTI) to mitigate memory-related vulnerabilities.
21. Partition Disk Separation: Partition disks to separate system files from user data and configure mount options for security. Use separate partitions for critical system directories (/, /var, /tmp) and user data to prevent resource exhaustion or unauthorized access. Implement file system encryption for sensitive data partitions to protect against unauthorized access in case of theft or data breaches.
22. Implement Disk Quotas: Enforce disk quotas for users to limit disk usage and prevent abuse. Set disk quota limits based on user roles, organizational policies, and available storage capacity. Regularly monitor disk usage and enforce quota violations to prevent users from exceeding their allocated storage space.
23. Disable IPv6 if Unused: Turn off IPv6 if not in use to reduce exposure to potential vulnerabilities. Disable IPv6 kernel modules and network services to prevent accidental IPv6 traffic leakage. Update network configurations and DNS settings to ensure compatibility and seamless fallback to IPv4.
24. Remove SUID/SGID Binaries: Identify and remove SUID/SGID binaries to prevent privilege escalation attacks. Use tools like find or rpm to search for files with SUID/SGID permissions and review their necessity. Replace or modify SUID/SGID binaries with safer alternatives or adjust file permissions to reduce security risks.
25. Secure World-Writable Files: Review and secure world-writable files to prevent unauthorized access and modifications. Identify world-writable directories and files using tools like find or ls and adjust permissions accordingly. Regularly audit file permissions and ownership to detect unauthorized changes or potential security misconfigurations.
26. Address Noowner Files: Identify and assign ownership to files without valid user or group associations. Use tools like find or stat to search for files with no owner or group and investigate their origin. Set appropriate ownership and permissions for orphaned files or directories to prevent security risks and file system corruption.
27. Centralized Authentication: Use centralized authentication systems like OpenLDAP for managing user accounts and credentials. Configure servers to authenticate users against LDAP or Active Directory servers to enforce centralized access control. Implement secure communication protocols (LDAPS, Kerberos) and authentication mechanisms (SSSD, PAM) for LDAP integration.
28. Implement Kerberos: Deploy Kerberos for secure authentication and encrypted communication across the network. Configure Kerberos realms and key distribution centers (KDCs) to authenticate users and services. Integrate Kerberos authentication with applications and services using GSSAPI or native Kerberos libraries.
29. Configure Logging and Auditing: Set up logging and auditing to monitor system activity and detect suspicious behavior. Configure syslog-ng or rsyslog to collect and centralize system logs from multiple servers. Enable auditing subsystems like auditd to track and record security-relevant events for forensic analysis and compliance.
30. Use Log Monitoring Tools: Employ tools like Logwatch or Logcheck to analyze system logs for security events and anomalies. Configure log monitoring rules and alerts based on security policies and threat intelligence feeds. Regularly review and analyze log reports to identify potential security incidents or unauthorized activities.
31. Enable System Auditing: Utilize auditd to track and record system events for forensic analysis and compliance purposes. Configure audit rules to monitor critical system files, directories, and system calls for unauthorized access or modifications. Regularly review audit logs and generate reports to demonstrate compliance with regulatory requirements and security standards.
32. Secure SSH Server: Configure SSH server settings to enhance security and protect against unauthorized access. Disable SSH protocol versions 1 and configure SSH protocol version 2 for improved security. Implement multi-factor authentication (MFA) or public key authentication for SSH access to enhance authentication security.
33. Install Intrusion Detection Systems: Deploy intrusion detection systems like fail2ban or denyhost to detect and respond to potential attacks. Configure IDS rules and filters to monitor network traffic and system logs for suspicious activities. Integrate IDS with security information and event management (SIEM) systems for centralized monitoring and analysis.
34. Disable External Device Ports: Turn off USB, Firewire, and Thunderbolt ports to prevent unauthorized data access and malware infection. Physically disable external device ports on server hardware or use BIOS/UEFI settings to block device enumeration. Educate users about the risks of using external storage devices and enforce security policies to restrict their usage.
35. Disable Unused Services: Stop and disable unnecessary services to reduce the attack surface and conserve system resources. Use systemctl or service commands to identify and disable unused or obsolete services running on the server. Regularly review and update service configurations to remove dependencies on deprecated or insecure protocols.
36. Implement IDS with fail2ban/denyhost: Utilize fail2ban or denyhost to block IP addresses exhibiting suspicious behavior and protect against brute-force attacks. Configure fail2ban filters to monitor system logs for authentication failures and ban malicious IP addresses automatically. Customize fail2ban or denyhost settings based on the organization's risk tolerance and security policies.
37. Secure Web Servers: Configure web servers (Apache, Nginx) with appropriate security settings and protections against common vulnerabilities. Enable HTTPS and configure SSL/TLS certificates to encrypt web traffic and protect sensitive information. Implement web application firewalls (WAFs) or intrusion detection/prevention systems (IDS/IPS) to detect and block malicious web traffic.
38. Protect Files and Email: Use encryption, permissions, and other security measures to safeguard sensitive data and communications. Encrypt file systems or individual files containing sensitive information using tools like GPG or OpenSSL. Implement email encryption protocols (S/MIME, PGP) and secure email gateways to protect email communications from interception or tampering.
39. Regular Backups: Implement regular backups to ensure data integrity and facilitate recovery in the event of a compromise or disaster. Configure automated backup schedules and backup rotation policies to ensure timely and consistent data protection. Store backup data securely off-site or in encrypted storage to prevent unauthorized access or data loss in case of theft or physical damage.
40. File Integrity Monitoring (FIM): Deploy file integrity monitoring tools like Tripwire or AIDE to detect unauthorized changes to critical system files and directories. Configure FIM to regularly scan system files and compare their current state with known baseline configurations to identify potential compromises.
41. Network Intrusion Prevention Systems (NIPS): Implement NIPS solutions like Snort or Suricata to analyze network traffic in real-time and block malicious activities such as port scanning, denial-of-service attacks, and exploits. Configure NIPS rulesets based on known attack signatures and behavioral anomalies to provide comprehensive network security.
42. Application Whitelisting: Utilize application whitelisting tools like AppArmor or SELinux to define a list of authorized applications and prevent unauthorized or malicious software from executing. Configure application profiles to restrict application capabilities and limit potential damage from exploited vulnerabilities.
43. Containerization and Microservices: Adopt containerization platforms like Docker or Kubernetes to isolate applications and services into lightweight, portable containers. Implement microservices architecture to break down monolithic applications into smaller, independently deployable components, reducing the attack surface and improving scalability.
44. Continuous Monitoring and Incident Response: Establish a comprehensive monitoring strategy using Security Information and Event Management (SIEM) systems to collect, analyze, and correlate security events across the infrastructure. Develop incident response plans and conduct regular tabletop exercises to test the effectiveness of response procedures and improve incident handling capabilities.
45. Privileged Access Management (PAM): Implement PAM solutions like FreeIPA or CyberArk to manage and secure privileged accounts, including root and administrative users. Enforce strict access controls, least privilege principles, and session recording for privileged accounts to prevent misuse and unauthorized access.
46. Security Information Sharing and Collaboration: Participate in threat intelligence sharing communities and information sharing and analysis centers (ISACs) to exchange actionable security intelligence and insights with peers and industry partners. Collaborate with cybersecurity organizations and government agencies to stay informed about emerging threats, vulnerabilities, and best practices.
47. Security Training and Awareness: Provide regular security training and awareness programs for employees, contractors, and third-party vendors to educate them about common security threats, social engineering tactics, and best practices for safeguarding sensitive information. Conduct phishing simulation exercises to test employees' susceptibility to phishing attacks and reinforce security awareness.
48. Secure Supply Chain Management: Establish security requirements and conduct thorough assessments of third-party vendors, suppliers, and service providers to ensure they adhere to industry-standard security practices. Implement secure coding practices, software dependency management, and supply chain integrity verification mechanisms to mitigate risks associated with software supply chain attacks.
49. Incident Forensics and Analysis: Develop incident forensics capabilities to investigate security incidents, breaches, and data breaches thoroughly. Preserve digital evidence using forensic techniques and tools to support legal proceedings, regulatory compliance, and post-incident analysis.
50. Secure DevOps Practices: Integrate security into the software development lifecycle (SDLC) by adopting DevSecOps practices and incorporating security testing and vulnerability assessments into the CI/CD pipeline. Automate security controls, code reviews, and vulnerability scanning to identify and remediate security vulnerabilities early in the development process.
51. Endpoint Security: Deploy endpoint security solutions such as antivirus, endpoint detection and response (EDR), and host-based intrusion prevention systems (HIPS) to protect against malware, ransomware, and advanced persistent threats (APTs). Implement endpoint encryption, application whitelisting, and device control policies to secure endpoints and prevent data breaches.