Linux

Below a couple of ideas, some building on what has already been stated. It's all detailed here:

• https://gofoss.net/server-hardening-basics/
• https://gofoss.net/server-hardening-advanced/

Feedback really welcomed, as there's always something to be learned in server security :)

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General hardening:

• set up a firewall (ufw)
• make sure your system time is correct (ntp)
• enable unattended upgrades
• limit privileged access (sudo)
• hide process information (/proc)
• enforce strict password policy (pam, login.defs)
• enforce stricter permissions (umask)
• close all unused ports (check with nmap)
• install a malware scanner (lmd)
• install an antivirus (clamav)
• disable core dumps
• disable unused kernel modules
• add legal banner

SSH:

• change the port
• limit the nb of login attempts
• limit access to admin users
• enable access logs
• forbid remote access to root
• use auth keys with instead of password auth
• disconnect after inactivity period
• remove short encryption keys

MySQL (if applicable):

• run a hardening script
• disable remote access
• prevent unauthorised access to local files
• create separate users with limited privileges for each app

Apache (if applicable):

• enable security modules
• hide http headers
• set up modsecurity, a web app firewall

PHP (if applicable):

• hide php version in headers
• disable remote code execution
• disable potentially harmful functions
• limit script runtime & memory allocation

Network security (sysctl):

• ip spoofing protection
• ignore icmp broadcasts & redirects
• disable source paket routing
• block syn attacks
• log martians
• ignore pings

i personally don't recommend fail2ban: it's a good way to lock yourself out of your own server but will probably not protect you from any attacks.

for protection unattended upgrades (like you have) and disabling SSH password auth (PasswordAuthentication no in /etc/ssh/sshd_config) is the best you can do.

also i've never run a lemmy instance but make sure the database isn't reachable from the internet, only from localhost :)

The linux upskill challenge guides users through setting up a server from the beginning and shows you how to set up a firewall with ufw as well as disabling root login via ssh.

disable root login over ssh, and use public keys auth if possible.

fail2ban is good, but needs to be configured properly.

there's probably lots more, but that's a start.

Don't allow root ssh access, you could also change the ssh port to one that's not 22. Also you could disallow ssh password login and allow key-based authentication only.