Server Hardening Skill

This skill provides comprehensive workflows, checklists, and automated scripts for hardening Linux servers according to industry best practices and CIS Benchmarks.

When to Use This Skill

Use this skill when:

• Setting up new production servers
• Improving security posture of existing servers
• Implementing CIS Benchmark compliance
• Preparing for security audits
• Responding to security incidents
• Establishing security baselines
• Configuring security monitoring

Core Principles

1. Defense in Depth

Multiple layers of security:

• Network security (firewall, IDS/IPS)
• Host security (hardening, patching)
• Application security (WAF, input validation)
• Access control (authentication, authorization)
• Monitoring (logging, alerting)

2. Principle of Least Privilege

Always apply:

• Minimal user permissions
• Minimal service permissions
• Minimal network access
• Minimal file system access

3. Zero Trust

Assume breach:

• Never trust, always verify
• Verify every request
• Log every action
• Monitor continuously

Hardening Workflow

Phase 1: Pre-Hardening Assessment

┌─────────────────────────────────────────────────────────────┐
│ Pre-Hardening Checklist                                     │
├─────────────────────────────────────────────────────────────┤
│ □ Document current configuration                            │
│ □ Create full system backup                                 │
│ □ Document running services                                 │
│ □ Document network connections                              │
│ □ Document user accounts                                    │
│ □ Identify compliance requirements                          │
│ □ Schedule maintenance window                               │
│ □ Prepare rollback plan                                     │
└─────────────────────────────────────────────────────────────┘

Phase 2: Security Baseline

Establish baseline before hardening:

#!/bin/bash
# Security baseline assessment

# System information
echo "=== System Information ==="
uname -a
hostnamectl
cat /etc/os-release

# Running services
echo -e "\n=== Running Services ==="
systemctl list-units --type=service --state=running

# Open ports
echo -e "\n=== Open Ports ==="
ss -tulpn

# User accounts
echo -e "\n=== User Accounts ==="
cat /etc/passwd | grep -v nologin

# Sudo users
echo -e "\n=== Sudo Users ==="
getent group sudo

# Installed packages
echo -e "\n=== Security Packages ==="
dpkg -l | grep -E "(fail2ban|ufw|rkhunter|chkrootkit)" || echo "Not installed"

# Last logins
echo -e "\n=== Last Logins ==="
last -10

# Failed logins
echo -e "\n=== Failed Logins ==="
grep "Failed password" /var/log/auth.log | tail -10 || echo "No failed logins"

# Disk usage
echo -e "\n=== Disk Usage ==="
df -h

# Pending updates
echo -e "\n=== Pending Updates ==="
apt list --upgradable 2>/dev/null | head -20 || yum check-update

Phase 3: Hardening Implementation

Follow the hardening checklist:

┌─────────────────────────────────────────────────────────────┐
│ Server Hardening Checklist                                  │
├─────────────────────────────────────────────────────────────┤
│ SYSTEM UPDATES                                              │
│ □ Update all packages to latest versions                    │
│ □ Configure automatic security updates                      │
│ □ Remove unnecessary packages                               │
├─────────────────────────────────────────────────────────────┤
│ USER MANAGEMENT                                             │
│ □ Remove/disable unused accounts                            │
│ □ Enforce strong password policy                            │
│ □ Configure sudo access                                     │
│ □ Disable root login                                        │
├─────────────────────────────────────────────────────────────┤
│ SSH HARDENING                                               │
│ □ Change default SSH port                                   │
│ □ Disable root login                                        │
│ □ Disable password authentication                           │
│ □ Configure key-based authentication                        │
│ □ Limit users/groups                                        │
│ □ Configure idle timeout                                    │
├─────────────────────────────────────────────────────────────┤
│ FIREWALL CONFIGURATION                                      │
│ □ Enable UFW/firewalld                                      │
│ □ Default deny incoming                                     │
│ □ Allow only required ports                                 │
│ □ Configure rate limiting                                   │
├─────────────────────────────────────────────────────────────┤
│ FILE SYSTEM SECURITY                                        │
│ □ Set proper permissions on sensitive files                 │
│ □ Mount /tmp, /var/tmp with noexec                          │
│ □ Enable audit logging                                      │
│ □ Configure log rotation                                    │
├─────────────────────────────────────────────────────────────┤
│ NETWORK SECURITY                                            │
│ □ Disable IPv6 if not used                                  │
│ □ Disable IP forwarding                                     │
│ □ Configure kernel security parameters                      │
│ □ Enable SYN flood protection                               │
├─────────────────────────────────────────────────────────────┤
│ MONITORING & LOGGING                                        │
│ □ Install and configure Fail2ban                            │
│ □ Configure centralized logging                             │
│ □ Enable audit daemon                                       │
│ □ Configure log aggregation                                 │
├─────────────────────────────────────────────────────────────┤
│ SECURITY TOOLS                                              │
□ Install intrusion detection (rkhunter, chkrootkit)          │
□ Install vulnerability scanner (lynis)                       │
□ Configure malware scanning                                  │
└─────────────────────────────────────────────────────────────┘

Implementation Scripts

Automated Hardening Script

See resources/security-checklist.md for the complete automated hardening script.

Manual Hardening Steps

Step 1: System Updates

# Update system
apt-get update && apt-get upgrade -y
apt-get dist-upgrade -y

# Remove unused packages
apt-get autoremove -y
apt-get autoclean

# Install security tools
apt-get install -y fail2ban rkhunter chkrootkit lynis unattended-upgrades

Step 2: User Management

# Create admin user
adduser admin
usermod -aG sudo admin

# Set password policy
cat >> /etc/pam.d/common-password << EOF
password requisite pam_pwquality.so retry=3 minlen=14 dcredit=-1 ucredit=-1 ocredit=-1 lcredit=-1
EOF

# Lock unused accounts
usermod -L games
usermod -L lists
usermod -L news

Step 3: SSH Hardening

# Backup SSH config
cp /etc/ssh/sshd_config /etc/ssh/sshd_config.backup

# Configure SSH
cat > /etc/ssh/sshd_config << 'EOF'
Port 2222
Protocol 2
PermitRootLogin no
PasswordAuthentication no
PubkeyAuthentication yes
MaxAuthTries 3
X11Forwarding no
ClientAliveInterval 300
ClientAliveCountMax 2
EOF

# Restart SSH
systemctl restart sshd

Step 4: Firewall Configuration

# Configure UFW
ufw --force reset
ufw default deny incoming
ufw default allow outgoing

# Allow SSH (custom port)
ufw allow 2222/tcp comment 'SSH'

# Allow HTTP/HTTPS
ufw allow 80/tcp comment 'HTTP'
ufw allow 443/tcp comment 'HTTPS'

# Enable UFW
ufw --force enable

Phase 4: Validation

Verify hardening was successful:

#!/bin/bash
# Post-hardening validation

echo "=== SSH Configuration ==="
sshd -T | grep -E "^(port|permitrootlogin|passwordauthentication)"

echo -e "\n=== Firewall Status ==="
ufw status verbose

echo -e "\n=== Running Services ==="
systemctl list-units --type=service --state=running | wc -l
echo "services running"

echo -e "\n=== Open Ports ==="
ss -tulpn | grep LISTEN

echo -e "\n=== Security Tools Status ==="
systemctl status fail2ban --no-pager
systemctl status auditd --no-pager

echo -e "\n=== Lynis Security Audit ==="
lynis audit system 2>&1 | grep -E "(Warnings|Suggestions)" | head -20

Phase 5: Ongoing Maintenance

Regular security maintenance:

# Daily Tasks
- Review security logs
- Check failed login attempts
- Verify backup completion

# Weekly Tasks
- Review system updates
- Check intrusion detection alerts
- Audit user accounts

# Monthly Tasks
- Run vulnerability scan (lynis)
- Review firewall rules
- Update security baseline
- Test backup restoration

# Quarterly Tasks
- Full security audit
- Review and rotate credentials
- Update hardening scripts
- Compliance review

CIS Benchmark Compliance

Key CIS Controls

1. Account Policies:

# Password expiration
chage -M 90 -m 7 -W 14 username

# Lock inactive accounts
usermod -L -e 1 username

2. Audit Configuration:

# Enable auditd
systemctl enable auditd
systemctl start auditd

# Configure audit rules
cat >> /etc/audit/rules.d/hardening.rules << 'EOF'
# Monitor user/group changes
-w /etc/passwd -p wa -k identity
-w /etc/group -p wa -k identity
-w /etc/shadow -p wa -k identity

# Monitor sudo
-w /etc/sudoers -p wa -k sudoers

# Monitor SSH
-w /etc/ssh/sshd_config -p wa -k sshd

# Monitor authentication
-w /var/log/auth.log -p wa -k auth_log
EOF

3. File Permissions:

# Secure sensitive files
chmod 644 /etc/passwd
chmod 640 /etc/shadow
chmod 640 /etc/group
chmod 600 /etc/ssh/sshd_config

# Find world-writable files
find / -type f -perm -002 2>/dev/null

# Find SUID binaries
find / -type f -perm -4000 2>/dev/null

Security Monitoring

Log Monitoring

#!/bin/bash
# Security log monitoring script

LOG_FILE="/var/log/auth.log"

echo "=== Failed Login Attempts (Last 24h) ==="
grep "Failed password" $LOG_FILE | since "24 hours ago" | wc -l

echo -e "\n=== Top IPs with Failed Logins ==="
grep "Failed password" $LOG_FILE | awk '{print $(NF-3)}' | sort | uniq -c | sort -rn | head -10

echo -e "\n=== Successful Logins (Last 24h) ==="
grep "Accepted" $LOG_FILE | since "24 hours ago" | wc -l

echo -e "\n=== Sudo Usage (Last 24h) ==="
grep "sudo:" $LOG_FILE | since "24 hours ago" | wc -l

echo -e "\n=== New User Accounts ==="
grep "useradd" $LOG_FILE | tail -10

echo -e "\n=== SSH Key Changes ==="
grep "authorized_keys" $LOG_FILE | tail -10

Intrusion Detection

#!/bin/bash
# Check for signs of compromise

echo "=== Checking for Rootkits ==="
rkhunter --check --skip-keypress

echo -e "\n=== Checking for Malware ==="
chkrootkit

echo -e "\n=== Checking for Suspicious Processes ==="
ps auxf | grep -v grep | grep -E "(nc|netcat|nmap|masscan)" || echo "No suspicious processes"

echo -e "\n=== Checking for Unusual Network Connections ==="
netstat -antp | grep -E "(ESTABLISHED|LISTEN)" | grep -v "127.0.0.1"

echo -e "\n=== Checking for Modified Binaries ==="
debsums -s 2>/dev/null || rpm -Va 2>/dev/null || echo "Package verification not available"

echo -e "\n=== Checking Cron Jobs ==="
cat /etc/crontab
ls -la /etc/cron.*

Incident Response

Security Incident Checklist

┌─────────────────────────────────────────────────────────────┐
│ Security Incident Response                                  │
├─────────────────────────────────────────────────────────────┤
│ IMMEDIATE ACTIONS                                           │
│ □ Isolate affected system                                   │
│ □ Preserve evidence (logs, memory dump)                     │
│ □ Document timeline                                         │
│ □ Notify security team                                      │
├─────────────────────────────────────────────────────────────┤
│ INVESTIGATION                                               │
│ □ Identify attack vector                                    │
│ □ Determine scope of compromise                             │
│ □ Check for lateral movement                                │
│ □ Identify affected data                                    │
├─────────────────────────────────────────────────────────────┤
│ CONTAINMENT                                                 │
│ □ Block attacker IPs                                        │
│ □ Rotate compromised credentials                            │
│ □ Patch vulnerabilities                                     │
│ □ Remove malware/backdoors                                  │
├─────────────────────────────────────────────────────────────┤
│ RECOVERY                                                    │
│ □ Restore from clean backup                                 │
│ □ Verify system integrity                                   │
│ □ Re-enable services                                        │
│ □ Monitor for re-infection                                  │
├─────────────────────────────────────────────────────────────┤
│ POST-INCIDENT                                               │
│ □ Document lessons learned                                  │
│ □ Update security controls                                  │
│ □ Report to stakeholders                                    │
│ □ Update incident response plan                             │
└─────────────────────────────────────────────────────────────┘

Implementation Resources

Refer to the following resources in this skill for detailed implementations:

• `resources/cis-benchmarks.md`: Complete CIS Benchmark implementation guide
• `resources/security-checklist.md`: Comprehensive security checklist with scripts
• `resources/hardening-scripts/`: Automated hardening scripts for different distributions

Compliance Frameworks

Supported Frameworks

Anti-Patterns

Avoid these hardening mistakes:

❌ Hardening without backup (always backup first) ❌ Applying all controls at once (phase the rollout) ❌ Not testing after hardening (verify functionality) ❌ Ignoring application requirements (consider app needs) ❌ One-time hardening (continuous process) ❌ No documentation (document all changes) ❌ Skipping validation (verify with scans) ❌ Ignoring updates (regular patching required)

Success Metrics

Measure hardening effectiveness:

Tool Usage

Recommended Tools

Security Scanning:

• Lynis (system audit)
• OpenVAS (vulnerability scan)
• rkhunter (rootkit detection)
• chkrootkit (rootkit detection)

Monitoring:

• OSSEC (HIDS)
• Wazuh (SIEM)
• Fail2ban (intrusion prevention)
• Auditd (audit logging)

Compliance:

• CIS-CAT (CIS benchmark)
• OpenSCAP (compliance scanning)
• InSpec (compliance as code)