Sysadmin Security

System security audit

Audit and improve the security of a Debian/Ubuntu Linux machine. Work through the checklist below interactively. Inspection is read-only and runs freely; every change is explained first, including how it could lock you out, and waits for confirmation.

Security checklist

1. User and access control

• Review user accounts: who should have access?
• Check for unused accounts.
• Verify sudo permissions.
• Review SSH keys.
• Check password policies.
• Disable root login over SSH.

2. SSH hardening

• Disable password authentication in favor of keys.
• Change the default SSH port (optional).
• Configure fail2ban.
• Limit SSH access by user or IP.
• Use SSH key passphrases.
• Review ~/.ssh/authorized_keys.

3. Firewall

• Enable UFW (Uncomplicated Firewall).
• Configure allowed ports.
• Block unnecessary services.
• Review current rules.
• Set a default-deny policy.

4. Updates and patches

• Check for system updates with apt update.
• Enable automatic security updates.
• Review update history.
• Check for end-of-life software.

5. Services and ports

• List running services with systemctl.
• Disable unnecessary services.
• Review open ports with ss -tlnp.
• Check listening services.
• Verify service permissions.

6. File permissions

• Check sensitive file permissions (/etc/passwd, /etc/shadow).
• Review home directory permissions.
• Check SUID/SGID files.
• Verify critical system files.

7. Logs and monitoring

• Review system logs with journalctl.
• Check authentication logs.
• Monitor failed login attempts.
• Set up log rotation.
• Consider log monitoring tools.

8. Application security

• Review installed packages with apt list --installed.
• Remove unnecessary software.
• Check for vulnerable packages.
• Update applications.
• Verify sources (PPAs, repositories).

9. Network security

• Review network interfaces.
• Check the routing table.
• Verify DNS settings.
• Review /etc/hosts.
• Check for suspicious connections.

10. Backup and recovery

• Verify backups exist.
• Test backup restoration.
• Document the recovery process.
• Secure backup storage.

Running the checks

Guide me through running each check and implementing hardening measures. Explain each step and the security benefit.

Safety first: explain the risks before any change that could lock me out, especially SSH, firewall, and account changes.

---

See full content at https://github.com/HermeticOrmus/linux-sysadmin-skills.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: HermeticOrmus
• Source: HermeticOrmus/linux-sysadmin-skills
• License: MIT

Install and usage instructions live in the source repository linked above.