The following are some system hardening recommendation from security auditor for default configuration of Debian GNU/Linux. You may apply if appropriate.
/etc/login.defs: Enforce changing password after limited days
/etc/ssh/sshd_config: Disable root login as many brute force attack target to 'root' user
/etc/pam.d/common-password: Enforce using strong password (Requires libpam-cracklib)
password required pam_unix.so nullok obscure md5 remember=11 min=6
/etc/issue, /etc/issue.net: Set warning message and remove server identity such as OS version
WARNING: This system is restricted access to authorized person only.