Analyzing Linux System Artifacts

Analyzing Linux System Artifacts

When to Use

• When investigating a compromised Linux server or workstation
• For identifying persistence mechanisms (cron, systemd, SSH keys)
• When tracing user activity through shell history and authentication logs
• During incident response to determine the scope of a Linux-based breach
• For detecting rootkits, backdoors, and unauthorized modifications

Detection Gaps & Validation

• Most-missed artifacts: beyond auth.log/.bash_history, collect /var/log/journal/ (binary systemd logs that survive when text logs are rotated/deleted), wtmp+btmp+lastlog, /etc/systemd/system/* and user ~/.config/systemd/user/ timers, /etc/cron.* and at jobs, every ~/.ssh/authorized_keys, /etc/ld.so.preload, and /var/log/audit/audit.log. Persistence increasingly hides in systemd timers and .service units, not crontab.
• Anti-forensics that defeats this analysis: .bash_history is trivially cleared (history -c, unset HISTFILE, ln -s /dev/null ~/.bash_history) - a missing or gapped history is itself an indicator, not "no activity." Cross-check against auditd execve records, journald, and last/wtmp. Timestamps can be touch-stomped; compare against inode change time (stat ctime), which user tools cannot set directly.
• Cleared/rotated logs: a truncated or missing auth.log/secure is suspicious - recover prior entries from journald, rotated .gz archives, or unallocated space, and look for the logrotate/wtmp gap that brackets the intrusion.
• Validate with a second source: corroborate a suspect SSH login (auth.log) with the matching wtmp session, the source IP in lastlog, the authorized_keys mtime, and any sudo/execve in auditd before attributing activity to an account.
• Interpretation pitfalls (false positives): legitimate admin/automation (Ansible, cron, package updates) produces wget/curl/chmod/useradd activity, and sanctioned UID-0 secondary accounts or NOPASSWD sudoers exist. Confirm /etc/timezone and check clock skew/NTP before timelining.

Prerequisites

• Forensic image or live access to the Linux system (read-only)
• Understanding of Linux file system hierarchy (FHS)
• Knowledge of common Linux logging locations (/var/log/)
• Tools: chkrootkit, rkhunter, AIDE, auditd logs
• Familiarity with systemd, cron, and PAM configurations
• Root access for complete artifact collection

Workflow

Step 1: Mount and Collect System Artifacts

# Mount forensic image read-only
mount -o ro,loop,offset=$((2048*512)) /cases/case-2024-001/images/linux_evidence.dd /mnt/evidence

# Create collection directories
mkdir -p /cases/case-2024-001/linux/{logs,config,users,persistence,network}

# Collect authentication logs
cp /mnt/evidence/var/log/auth.log* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/secure* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/syslog* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/kern.log* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/audit/audit.log* /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/wtmp /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/btmp /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/lastlog /cases/case-2024-001/linux/logs/
cp /mnt/evidence/var/log/faillog /cases/case-2024-001/linux/logs/

# Collect user artifacts
for user_dir in /mnt/evidence/home/*/; do
    username=$(basename "$user_dir")
    mkdir -p /cases/case-2024-001/linux/users/$username
    cp "$user_dir"/.bash_history /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.zsh_history /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp -r "$user_dir"/.ssh/ /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.bashrc /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.profile /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.viminfo /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.wget-hsts /cases/case-2024-001/linux/users/$username/ 2>/dev/null
    cp "$user_dir"/.python_history /cases/case-2024-001/linux/users/$username/ 2>/dev/null
done

# Collect root user artifacts
cp /mnt/evidence/root/.bash_history /cases/case-2024-001/linux/users/root/ 2>/dev/null
cp -r /mnt/evidence/root/.ssh/ /cases/case-2024-001/linux/users/root/ 2>/dev/null

# Collect system configuration
cp /mnt/evidence/etc/passwd /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/shadow /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/group /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/sudoers /cases/case-2024-001/linux/config/
cp -r /mnt/evidence/etc/sudoers.d/ /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/hosts /cases/case-2024-001/linux/config/
cp /mnt/evidence/etc/resolv.conf /cases/case-2024-001/linux/config/
cp -r /mnt/evidence/etc/ssh/ /cases/case-2024-001/linux/config/

Step 2: Analyze User Accounts and Authentication

# Analyze user accounts for anomalies
python3 << 'PYEOF'
print("=== USER ACCOUNT ANALYSIS ===\n")

# Parse /etc/passwd
with open('/cases/case-2024-001/linux/config/passwd') as f:
    for line in f:
        parts = line.strip().split(':')
        if len(parts) >= 7:
            username, _, uid, gid, comment, home, shell = parts[0], parts[1], int(parts[2]), int(parts[3]), parts[4], parts[5], parts[6]

            # Flag accounts with UID 0 (root equivalent)
            if uid == 0 and username != 'root':
                print(f"  ALERT: UID 0 account: {username} (shell: {shell})")

            # Flag accounts with login shells that shouldn't have them
            if shell not in ('/bin/false', '/usr/sbin/nologin', '/bin/sync') and uid >= 1000:
                print(f"  User: {username} (UID:{uid}, Shell:{shell}, Home:{home})")

            # Flag system accounts with login shells
            if uid < 1000 and uid > 0 and shell in ('/bin/bash', '/bin/sh', '/bin/zsh'):
                print(f"  WARNING: System account with shell: {username} (UID:{uid}, Shell:{shell})")

# Parse /etc/shadow for account status
print("\n=== PASSWORD STATUS ===")
with open('/cases/case-2024-001/linux/config/shadow') as f:
    for line in f:
        parts = line.strip().split(':')
        if len(parts) >= 3:
            username = parts[0]
            pwd_hash = parts[1]
            last_change = parts[2]

            if pwd_hash and pwd_hash not in ('*', '!', '!!', ''):
                hash_type = 'Unknown'
                if pwd_hash.startswith('$6$'): hash_type = 'SHA-512'
                elif pwd_hash.startswith('$5$'): hash_type = 'SHA-256'
                elif pwd_hash.startswith('$y$'): hash_type = 'yescrypt'
                elif pwd_hash.startswith('$1$'): hash_type = 'MD5 (WEAK)'
                print(f"  {username}: {hash_type} hash, last changed: day {last_change}")
PYEOF

# Analyze login history
last -f /cases/case-2024-001/linux/logs/wtmp > /cases/case-2024-001/linux/analysis/login_history.txt
lastb -f /cases/case-2024-001/linux/logs/btmp > /cases/case-2024-001/linux/analysis/failed_logins.txt 2>/dev/null

Step 3: Examine Persistence Mechanisms

# Check cron jobs for all users
echo "=== CRON JOBS ===" > /cases/case-2024-001/linux/persistence/cron_analysis.txt

# System cron
for cronfile in /mnt/evidence/etc/crontab /mnt/evidence/etc/cron.d/*; do
    echo "--- $cronfile ---" >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
    cat "$cronfile" 2>/dev/null >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
    echo "" >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
done

# User cron tabs
for cronfile in /mnt/evidence/var/spool/cron/crontabs/*; do
    echo "--- User crontab: $(basename $cronfile) ---" >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
    cat "$cronfile" 2>/dev/null >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
    echo "" >> /cases/case-2024-001/linux/persistence/cron_analysis.txt
done

# Check systemd services for persistence
echo "=== SYSTEMD SERVICES ===" > /cases/case-2024-001/linux/persistence/systemd_analysis.txt
find /mnt/evidence/etc/systemd/system/ -name "*.service" -newer /mnt/evidence/etc/os-release \
   >> /cases/case-2024-001/linux/persistence/systemd_analysis.txt

for svc in /mnt/evidence/etc/systemd/system/*.service; do
    echo "--- $(basename $svc) ---" >> /cases/case-2024-001/linux/persistence/systemd_analysis.txt
    cat "$svc" >> /cases/case-2024-001/linux/persistence/systemd_analysis.txt
    echo "" >> /cases/case-2024-001/linux/persistence/systemd_analysis.txt
done

# Check authorized SSH keys (backdoor detection)
echo "=== SSH AUTHORIZED KEYS ===" > /cases/case-2024-001/linux/persistence/ssh_keys.txt
find /mnt/evidence/home/ /mnt/evidence/root/ -name "authorized_keys" -exec sh -c \
   'echo "--- {} ---"; cat {}; echo ""' \; >> /cases/case-2024-001/linux/persistence/ssh_keys.txt

# Check rc.local and init scripts
cat /mnt/evidence/etc/rc.local 2>/dev/null > /cases/case-2024-001/linux/persistence/rc_local.txt

# Check /etc/profile.d/ for login-triggered scripts
ls -la /mnt/evidence/etc/profile.d/ > /cases/case-2024-001/linux/persistence/profile_scripts.txt

# Check for LD_PRELOAD hijacking
grep -r "LD_PRELOAD" /mnt/evidence/etc/ 2>/dev/null > /cases/case-2024-001/linux/persistence/ld_preload.txt
cat /mnt/evidence/etc/ld.so.preload 2>/dev/null >> /cases/case-2024-001/linux/persistence/ld_preload.txt

Step 4: Analyze Shell History and Command Execution

# Analyze bash history for each user
python3 << 'PYEOF'
import os, glob

print("=== SHELL HISTORY ANALYSIS ===\n")

suspicious_commands = [
    'wget', 'curl', 'nc ', 'ncat', 'netcat', 'python -c', 'python3 -c',
    'perl -e', 'base64', 'chmod 777', 'chmod +s', '/dev/tcp', '/dev/udp',
    'nmap', 'masscan', 'hydra', 'john', 'hashcat', 'passwd', 'useradd',
    'iptables -F', 'ufw disable', 'history -c', 'rm -rf /', 'dd if=',
    'crontab', 'at ', 'systemctl enable', 'ssh-keygen', 'scp ', 'rsync',
    'tar czf', 'zip -r', 'openssl enc', 'gpg --encrypt', 'shred',
    'chattr', 'setfacl', 'awk', '/tmp/', '/dev/shm/'
]

for hist_file in glob.glob('/cases/case-2024-001/linux/users/*/.bash_history'):
    username = hist_file.split('/')[-2]
    print(f"User: {username}")

    with open(hist_file, 'r', errors='ignore') as f:
        lines = f.readlines()

    print(f"  Total commands: {len(lines)}")
    flagged = []
    for i, line in enumerate(lines):
        line = line.strip()
        for cmd in suspicious_commands:
            if cmd in line.lower():
                flagged.append((i+1, line))
                break

    if flagged:
        print(f"  Suspicious commands: {len(flagged)}")
        for lineno, cmd in flagged:
            print(f"    Line {lineno}: {cmd[:120]}")
    print()
PYEOF

Step 5: Check for Rootkits and Modified Binaries

# Check for known rootkit indicators
# Compare system binary hashes against known-good
find /mnt/evidence/usr/bin/ /mnt/evidence/usr/sbin/ /mnt/evidence/bin/ /mnt/evidence/sbin/ \
   -type f -executable -exec sha256sum {} \; > /cases/case-2024-001/linux/analysis/binary_hashes.txt

# Check for SUID/SGID binaries (potential privilege escalation)
find /mnt/evidence/ -perm -4000 -type f 2>/dev/null > /cases/case-2024-001/linux/analysis/suid_files.txt
find /mnt/evidence/ -perm -2000 -type f 2>/dev/null > /cases/case-2024-001/linux/analysis/sgid_files.txt

# Check for suspicious files in /tmp and /dev/shm
find /mnt/evidence/tmp/ /mnt/evidence/dev/shm/ -type f 2>/dev/null \
   -exec file {} \; > /cases/case-2024-001/linux/analysis/tmp_files.txt

# Check for hidden files and directories
find /mnt/evidence/ -name ".*" -not -path "*/\." -type f 2>/dev/null | \
   head -100 > /cases/case-2024-001/linux/analysis/hidden_files.txt

# Check kernel modules
ls -la /mnt/evidence/lib/modules/$(ls /mnt/evidence/lib/modules/ | head -1)/extra/ 2>/dev/null \
   > /cases/case-2024-001/linux/analysis/extra_modules.txt

# Check for modified PAM configuration (authentication backdoors)
diff /mnt/evidence/etc/pam.d/ /cases/baseline/pam.d/ 2>/dev/null \
   > /cases/case-2024-001/linux/analysis/pam_changes.txt

Key Concepts

Tools & Systems

Common Scenarios

Scenario 1: SSH Brute Force Followed by Compromise Analyze auth.log for failed SSH attempts followed by success, identify the attacking IP, check .bash_history for post-compromise commands, examine authorized_keys for added backdoor keys, check crontab for persistence, review network connections.

Scenario 2: Web Server Compromise via Application Vulnerability Examine web server access and error logs for exploitation attempts, check /tmp and /dev/shm for webshells, analyze the web server user's activity (www-data), check for privilege escalation via SUID binaries or kernel exploits, review outbound connections.

Scenario 3: Insider Threat on Database Server Analyze the suspect user's bash_history for database dump commands, check for large tar/zip files in home directory or /tmp, examine scp/rsync commands for data transfer, review cron jobs for automated exfiltration, check USB device logs.

Scenario 4: Crypto-Miner on Cloud Instance Check for high-CPU processes in /proc (live) or systemd service files, examine crontab entries for miner restart scripts, check /tmp for mining binaries, analyze network connections for mining pool communications, review authorized_keys for attacker access.

Output Format

Linux Forensics Summary:
  System: webserver01 (Ubuntu 22.04 LTS)
  Hostname: webserver01.corp.local
  Kernel: 5.15.0-91-generic

  User Accounts:
    Total: 25 (3 with UID 0 - 1 ANOMALOUS)
    Interactive shells: 8 users
    Recently created: admin2 (created 2024-01-15)

  Authentication Events:
    Successful SSH logins: 456
    Failed SSH attempts: 12,345 (from 23 unique IPs)
    Sudo executions: 89

  Persistence Mechanisms Found:
    Cron jobs: 3 suspicious (reverse shell, miner restart)
    Systemd services: 1 unknown (update-checker.service)
    SSH keys: 2 unauthorized keys in root authorized_keys
    rc.local: Modified with download cradle

  Suspicious Activity:
    - bash_history contains wget to pastebin URL
    - SUID binary /tmp/.hidden/escalate found
    - /dev/shm/ contains compiled ELF binary
    - LD_PRELOAD in /etc/ld.so.preload pointing to /lib/.hidden.so

  Report: /cases/case-2024-001/linux/analysis/