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
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
bash# 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
bash# 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
bash# 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
bash# 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
bash# 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
| Concept | Description |
|---|---|
| /var/log/auth.log | Primary authentication log on Debian/Ubuntu systems |
| /var/log/secure | Primary authentication log on RHEL/CentOS systems |
| wtmp/btmp | Binary logs recording successful and failed login sessions |
| .bash_history | User command history file (can be cleared by attackers) |
| crontab | Scheduled task system commonly used for persistence |
| authorized_keys | SSH public keys granting passwordless access to an account |
| SUID bit | File permission allowing execution as the file owner (privilege escalation vector) |
| LD_PRELOAD | Environment variable that loads a shared library before all others (hooking technique) |
Tools & Systems
| Tool | Purpose |
|---|---|
| chkrootkit | Rootkit detection scanner for Linux systems |
| rkhunter | Rootkit Hunter - checks for rootkits, backdoors, and local exploits |
| AIDE | Advanced Intrusion Detection Environment - file integrity monitor |
| auditd | Linux audit framework for system call and file access monitoring |
| last/lastb | Parse wtmp/btmp for login and failed login history |
| Plaso/log2timeline | Super-timeline creation including Linux artifacts |
| osquery | SQL-based system querying for live forensic investigation |
| Velociraptor | Endpoint agent with Linux artifact collection capabilities |
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/
