Server Audit

Server Audit

Security auditing and hardening for Linux systems. Covers SSH, firewalls, ports, users, and general hardening.

Quick Start

Run a Quick Security Audit

Check the essentials in order:

# 1. Open ports
ss -tlnp

# 2. SSH config
grep -E "^(PasswordAuthentication|PermitRootLogin|PubkeyAuthentication)" /etc/ssh/sshd_config

# 3. Firewall status
sudo ufw status verbose   # Ubuntu/Debian
sudo firewall-cmd --list-all  # RHEL/Arch (firewalld)
sudo iptables -L -n        # Raw iptables

# 4. Failed login attempts (last 24h)
journalctl -u sshd --since "24 hours ago" | grep -c "Failed password"

# 5. Users with login shells
grep -v "nologin\|false" /etc/passwd | cut -d: -f1

SSH Hardening

Audit Checklist

Recommended SSH Config

# /etc/ssh/sshd_config — hardened
PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
MaxAuthTries 3
ClientAliveInterval 300
ClientAliveCountMax 2
X11Forwarding no
AllowTcpForwarding no

After SSH Changes

# Validate config before restarting
sudo sshd -t

# Restart SSH
sudo systemctl restart sshd

Firewall Audit

UFW (Ubuntu/Debian)

# Status
sudo ufw status verbose

# List rules with numbers
sudo ufw status numbered

# Check default policies
sudo ufw show raw | head -20

Expected defaults:

• Incoming: DENY
• Outgoing: ALLOW
• Only required ports open (22/SSH, 80/443 if web server, etc.)

Firewalld (Arch/RHEL)

# Active zone and rules
sudo firewall-cmd --list-all

# List all zones
sudo firewall-cmd --get-active-zones

# Check specific port
sudo firewall-cmd --query-port=8080/tcp

Iptables (Raw)

# List all rules
sudo iptables -L -n -v

# Check for wide-open rules
sudo iptables -L -n | grep "0.0.0.0/0.*ACCEPT"

Open Port Audit

Discover Listening Ports

# All listening TCP ports with process names
ss -tlnp

# All listening UDP ports
ss -ulnp

# Combined with process info (requires root for all)
sudo ss -tlnp

Evaluate Open Ports

Scan from External Perspective

# Scan a host from this machine
nmap -sT -O <target-ip>

# Quick scan common ports
nmap -F <target-ip>

# Scan specific port range
nmap -p 1-1024 <target-ip>

User and Permission Audit

Check Users

# Users with login shells (potential interactive users)
grep -v "nologin\|false" /etc/passwd | cut -d: -f1

# Users with UID 0 (root-level)
awk -F: '$3 == 0 {print $1}' /etc/passwd

# Users in sudo group
getent group sudo | cut -d: -f4
getent group wheel | cut -d: -f4  # Arch/RHEL

Check Permissions

# World-writable files (security risk)
find / -type f -perm -o+w 2>/dev/null | head -20

# SUID binaries (potential privilege escalation)
find / -type f -perm -4000 2>/dev/null

# Check home directory permissions
ls -la /home/

Check Sudoers

# View sudoers (never edit directly, use visudo)
sudo cat /etc/sudoers
sudo ls -la /etc/sudoers.d/

Fail2ban Integration

Check Status

# Fail2ban status
sudo fail2ban-client status

# SSH jail specifically
sudo fail2ban-client status sshd

# Banned IPs
sudo fail2ban-client get sshd banip --with-time

Recommended Jail Config

# /etc/fail2ban/jail.local
[sshd]
enabled = true
port = ssh
filter = sshd
logpath = /var/log/auth.log
maxretry = 5
bantime = 3600
findtime = 600

Severity Ratings

When reporting audit findings, rate each item:

Audit Report Format

When delivering an audit, structure it as:

1. Executive Summary — Overall health rating, critical findings count
2. Critical Findings — Items that need immediate attention
3. Recommendations — Prioritized list with severity, fix commands, and reasoning
4. What's Good — Positive findings (acknowledge what's already well-configured)

Kai skill — Security auditing and server hardening