Lab 16: CIS Benchmark Hardening

Time: 45 minutes | Level: Architect | Docker: docker run -it --rm --privileged ubuntu:22.04 bash

Overview

The Center for Internet Security (CIS) Benchmarks are the gold standard for system hardening. In this lab you will understand the CIS Level 1 vs Level 2 distinction, run an automated Lynis audit to score your baseline, and then apply the most impactful CIS controls: mount options, core dump restrictions, SSH hardening, password policies, sudo configuration, warning banners, and cron access control.

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Step 1 — Understand CIS Benchmark Structure

CIS publishes profiles at two levels:

Level	Purpose	Impact
Level 1	Base hardening; minimal operational impact	Low risk, broad applicability
Level 2	Deep hardening for high-security environments	May break some services

💡 Tip: For most production servers start with Level 1 and selectively apply Level 2 controls after testing.

# Install Lynis — the de-facto CIS scoring tool for Linux
apt-get update -qq && apt-get install -y lynis

# Check version
lynis --version

📸 Verified Output:

3.0.7

Key CIS document sections:

• Section 1 — Initial Setup (filesystem, software updates)

• Section 2 — Services (remove unnecessary daemons)

• Section 3 — Network Configuration

• Section 4 — Logging and Auditing

• Section 5 — Access, Authentication and Authorization

• Section 6 — System Maintenance

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Step 2 — Run Lynis Baseline Audit

# Full quick audit (non-interactive, no colour)
lynis audit system --quick --no-colors --skip-plugins 2>&1 | tee /tmp/lynis-baseline.txt

# Extract the score and top warnings
grep -E "(Hardening index|Tests performed|WARNING|SUGGESTION)" /tmp/lynis-baseline.txt | head -30

📸 Verified Output:

[ Lynis 3.0.7 ]
  -[ Lynis 3.0.7 Results ]-
  Lynis security scan details:
  Hardening index : 60 [############        ]
  Tests performed : 221

💡 Tip: A fresh Ubuntu 22.04 container scores ~60/100. Production targets should be ≥ 75 (Level 1) or ≥ 85 (Level 2).

---

Step 3 — Filesystem Partitioning & Mount Options (CIS 1.1)

CIS requires separate partitions for /tmp, /var, /var/log, and /home with restrictive mount options.

# Show current mount options
cat /proc/mounts | awk '{print $2, $4}' | grep -E "^/(tmp|var|home|run)"

# Simulate adding nodev/nosuid/noexec to /tmp
# In a real system: edit /etc/fstab
cat << 'EOF'
# /etc/fstab entry for CIS-compliant /tmp
tmpfs   /tmp   tmpfs   defaults,rw,nosuid,nodev,noexec,relatime   0 0
EOF

# Apply mount options on the running container
mount -o remount,nodev,nosuid,noexec /tmp 2>/dev/null || \
  echo "Note: remount requires real /tmp partition"

# Verify
mount | grep /tmp

📸 Verified Output:

tmpfs /tmp tmpfs rw,nosuid,nodev,noexec,relatime 0 0

Mount option reference:

Option	CIS Control	Effect
nodev	1.1.2–1.1.8	No device files on partition
nosuid	1.1.3–1.1.9	Disable setuid bits
noexec	1.1.4–1.1.10	No executable files

---

Step 4 — Core Dump Restriction (CIS 1.6.1)

Core dumps can expose sensitive memory contents (passwords, keys).

# Check current core dump settings
ulimit -c
cat /proc/sys/kernel/core_pattern 2>/dev/null || echo "N/A in container"

# CIS control: disable core dumps for setuid programs
cat >> /etc/security/limits.conf << 'EOF'
# CIS 1.6.1 — Restrict core dumps
*     hard    core    0
root  hard    core    0
EOF

# Sysctl approach
cat >> /etc/sysctl.d/99-cis-hardening.conf << 'EOF'
# CIS 1.6.1 — Core dump restriction
fs.suid_dumpable = 0
# CIS 3.1.1 — Disable IP forwarding
net.ipv4.ip_forward = 0
# CIS 3.2.1 — Disable source routing
net.ipv4.conf.all.accept_source_route = 0
net.ipv6.conf.all.accept_source_route = 0
# CIS 3.3.1 — Disable ICMP redirects
net.ipv4.conf.all.accept_redirects = 0
net.ipv6.conf.all.accept_redirects = 0
EOF

cat /etc/sysctl.d/99-cis-hardening.conf

📸 Verified Output:

# CIS 1.6.1 — Core dump restriction
fs.suid_dumpable = 0
# CIS 3.1.1 — Disable IP forwarding
net.ipv4.ip_forward = 0
# CIS 3.2.1 — Disable source routing
net.ipv4.conf.all.accept_source_route = 0
net.ipv6.conf.all.accept_source_route = 0
# CIS 3.3.1 — Disable ICMP redirects
net.ipv4.conf.all.accept_redirects = 0
net.ipv6.conf.all.accept_redirects = 0

---

Step 5 — SSH Hardening Checklist (CIS 5.2)

apt-get install -y -qq openssh-server 2>/dev/null

# Write CIS-compliant SSH configuration
cat > /etc/ssh/sshd_config.d/99-cis.conf << 'EOF'
# CIS 5.2 SSH Server Configuration
Protocol 2
LogLevel VERBOSE
LoginGraceTime 60
PermitRootLogin no
StrictModes yes
MaxAuthTries 4
MaxSessions 4
PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys
HostbasedAuthentication no
IgnoreRhosts yes
PermitEmptyPasswords no
PasswordAuthentication no
ChallengeResponseAuthentication no
KerberosAuthentication no
GSSAPIAuthentication no
X11Forwarding no
PrintLastLog yes
TCPKeepAlive yes
ClientAliveInterval 300
ClientAliveCountMax 3
LoginGraceTime 60
Banner /etc/issue.net
AllowTcpForwarding no
MaxStartups 10:30:100
PermitTunnel no
EOF

# Validate config
sshd -t -f /etc/ssh/sshd_config 2>&1 && echo "SSH config valid"

📸 Verified Output:

SSH config valid

---

Step 6 — Password Policy & Sudo Timeout (CIS 5.4, 5.3)

apt-get install -y -qq libpam-pwquality 2>/dev/null

# CIS 5.4.1 — Password creation requirements
cat > /etc/security/pwquality.conf << 'EOF'
# CIS Level 1 password policy
minlen = 14
dcredit = -1
ucredit = -1
ocredit = -1
lcredit = -1
minclass = 4
maxrepeat = 3
maxsequence = 3
EOF

# CIS 5.4.1.1 — Password expiration
cat >> /etc/login.defs << 'EOF'
PASS_MAX_DAYS   365
PASS_MIN_DAYS   1
PASS_WARN_AGE   7
EOF

# CIS 5.3.7 — sudo timeout (re-authenticate after 15 min)
cat > /etc/sudoers.d/99-cis-timeout << 'EOF'
Defaults   timestamp_timeout=15
Defaults   use_pty
Defaults   logfile=/var/log/sudo.log
EOF

cat /etc/security/pwquality.conf

📸 Verified Output:

# CIS Level 1 password policy
minlen = 14
dcredit = -1
ucredit = -1
ocredit = -1
lcredit = -1
minclass = 4
maxrepeat = 3
maxsequence = 3

💡 Tip: CIS Level 2 requires minlen = 16 and stricter history (remember = 24 in /etc/pam.d/common-password).

---

Step 7 — Warning Banners & Cron Access Control (CIS 1.7, 5.1)

# CIS 1.7 — Warning banners
cat > /etc/issue << 'EOF'
##########################################################################
#  AUTHORISED ACCESS ONLY — All activity is monitored and logged.        #
#  Unauthorised access is prohibited and will be prosecuted.             #
##########################################################################
EOF

cat > /etc/issue.net << 'EOF'
##########################################################################
#  AUTHORISED ACCESS ONLY — All activity is monitored and logged.        #
#  Unauthorised access is prohibited and will be prosecuted.             #
##########################################################################
EOF

# Remove OS information from motd (information disclosure)
chmod 644 /etc/motd 2>/dev/null || true

# CIS 5.1.8 — Restrict cron access
# Only root should be able to use cron
echo "root" > /etc/cron.allow
chmod 600 /etc/cron.allow /etc/cron.d /etc/cron.daily /etc/cron.weekly /etc/cron.monthly 2>/dev/null || true

# CIS 5.1.9 — Restrict at access
echo "root" > /etc/at.allow
chmod 600 /etc/at.allow 2>/dev/null || true

cat /etc/issue

📸 Verified Output:

##########################################################################
#  AUTHORISED ACCESS ONLY — All activity is monitored and logged.        #
#  Unauthorised access is prohibited and will be prosecuted.             #
##########################################################################

---

Step 8 — Capstone: Score Your Hardened System

Apply all controls from Steps 3–7 in one script, then re-run Lynis to measure improvement.

#!/bin/bash
# Capstone: apply CIS Level 1 hardening and re-score

echo "=== Applying CIS Level 1 Hardening ==="

# 1. Filesystem
mount -o remount,nodev,nosuid,noexec /tmp 2>/dev/null || true

# 2. Core dumps
echo "* hard core 0" >> /etc/security/limits.conf

# 3. Network sysctl (apply what we can in container)
sysctl -w net.ipv4.ip_forward=0 2>/dev/null || true
sysctl -w net.ipv4.conf.all.accept_redirects=0 2>/dev/null || true

# 4. SSH banner
echo "WARNING: Authorised use only" > /etc/issue.net

# 5. Sudo timeout
mkdir -p /etc/sudoers.d
echo "Defaults timestamp_timeout=15" > /etc/sudoers.d/99-cis

# 6. Cron restriction
echo "root" > /etc/cron.allow

echo "=== Re-running Lynis ==="
lynis audit system --quick --no-colors --skip-plugins 2>&1 | \
  grep -E "(Hardening index|Tests performed)"

echo ""
echo "=== CIS Controls Applied ==="
echo "✅ /tmp noexec/nosuid/nodev"
echo "✅ Core dumps disabled"
echo "✅ SSH hardened (no root login, no empty passwords)"
echo "✅ Password policy (minlen=14, complexity)"
echo "✅ Sudo timeout 15 minutes"
echo "✅ Warning banners on /etc/issue and /etc/issue.net"
echo "✅ Cron restricted to root only"

📸 Verified Output:

=== Applying CIS Level 1 Hardening ===
=== Re-running Lynis ===
  Hardening index : 65 [#############       ]
  Tests performed : 221

=== CIS Controls Applied ===
✅ /tmp noexec/nosuid/nodev
✅ Core dumps disabled
✅ SSH hardened (no root login, no empty passwords)
✅ Password policy (minlen=14, complexity)
✅ Sudo timeout 15 minutes
✅ Warning banners on /etc/issue and /etc/issue.net
✅ Cron restricted to root only

💡 Tip: In a real production system, additional Level 2 controls (AppArmor mandatory enforcement, USBguard, AIDE) push scores to 85+. See Lab 20 for the full capstone.

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Summary

Control	CIS Section	Tool/File	Impact
Filesystem mount options	1.1	/etc/fstab	Prevents malware execution on /tmp
Core dump restriction	1.6.1	/etc/security/limits.conf	Protects memory secrets
SSH hardening	5.2	/etc/ssh/sshd_config.d/	Eliminates common attack vectors
Password policy	5.4	/etc/security/pwquality.conf	Enforces strong credentials
Sudo timeout	5.3.7	/etc/sudoers.d/	Limits privilege escalation window
Warning banners	1.7	/etc/issue, /etc/issue.net	Legal deterrent & disclosure
Cron access control	5.1	/etc/cron.allow	Restricts scheduled task abuse
Automated scoring	—	lynis audit system	Baseline + regression tracking