Security hardening is the process of reducing the attack surface of your server infrastructure. In this guide we cover the top 10 practices: (1) Disable unnecessary services, (2) Apply the principle of least privilege, (3) Enable UFW / firewall rules, (4) Install fail2ban, (5) Configure SSH key-only authentication, (6) Enable SELinux / AppArmor, (7) Schedule automatic security updates, (8) Centralise audit logging, (9) Encrypt data at rest and in transit, (10) Run regular vulnerability scans. Each practice is explained with concrete configuration examples for both Ubuntu and CentOS environments.