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So I’ve recently had to lock down a public-facing CentOS server. Always a fun process, as I’m sure you know. When all was said and done, I created a quick checklist for my next Linux server hardening project. I’m of course keeping it general; everyone’s purpose, environment, and security standards are different. Hope you find it useful!

Linux Server Hardening Checklist

Documentation
  • Write down all relevant machine details – hostname, IP address, MAC address, OS version
    • Store in your relevant database
The Basics
  • Update the system (yum, apt, etc)
  • Set up disk encryption
  • Disable USB and peripheral devices
  • Create a non-root user for daily use
    • Remove any unused accounts
    • Disable shell or elevated access for standard/built-in users
  • Disable logon as root
  • Disable all unnecessary running services (init.d and xinetd)
  • Uninstall/disable all unnecessary or insecure apps (ftp, telnet, X11)
  • Set up and configure a firewall
  • Use an antivirus and IDS/IPS
  • Schedule backup of log files and lock down directory storage
  • Separate disk partitions – /usr, /home, /var & /var/tmp, /tmp
  • Run only one network service per system
Security Policies and Standards
  • Enable SELinux
  • Use complex passwords for all accounts
    • Enable a strong policy (minimum length, blend of character types, etc)
    • Use a strong hashing algorithm like SHA512
    • Create a “lock account after X failed login attempts” policy
    • Set up password aging and expiration
    • Restrict use of previous passwords
  • Make sure all accounts have a password set
    • awk -F: '($2 == "") {print}' /etc/shadow
  • Verify no non-root account have a UID set to 0 (full permissions to machine)
    • awk -F: '($3 == "0") {print}' /etc/passwd
  • Enable disk usage quotas
  • Lock down SSH
    • Use public/private keypairs
    • Prohibit logins as root
    • Don’t allow logins using a password
    • Disable either IPv4 or IPv6 depending on what’s not used
    • Use an IP whitelist to control who can use SSH
    • Enable 2FA
  • Set chmod 0700 for all cron tasks so only the root account can see them
  • Delete symlinks and disable their creation (more info here)
  • Encrypt communication – SSH, VPNs, rsync, PGP, SSL, SFTP, GPG
  • Make sure no files have no owner specified
    • find /dir -xdev \( -nouser -o -nogroup \) -print
  • Verify no files are world-writeable
    • find /dir -xdev -type d \( -perm -0002 -a ! -perm -1000 \) -print
  • Configure auditd
  • Configure regular backups
Other Useful Tools
  • Fail2ban (link) – a great tool for automatically banning suspicious IP addresses
  • ClamAV (link) – an open-source antivirus engine
  • Lynis (link) – open-source auditing tool for Linux

Am I missing anything? Let me know in the comments! Otherwise, if you want some customized help with your hardening projects, give us a call.

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