Performing Linux Log Forensics Investigation

Performing Linux Log Forensics Investigation

Overview

Linux systems maintain extensive logs that serve as primary evidence sources in forensic investigations. Unlike Windows Event Logs, Linux logs are typically plain-text files stored in /var/log/ and binary journal files managed by systemd-journald. Key forensic logs include auth.log (authentication events, sudo usage, SSH sessions), syslog (system-wide messages), kern.log (kernel events), and application-specific logs. The Linux Audit framework (auditd) provides detailed security event logging comparable to Windows Security Event Logs. Forensic analysis of these logs enables investigators to reconstruct user sessions, identify unauthorized access, detect privilege escalation, trace lateral movement, and establish comprehensive event timelines.

When to Use

• When conducting security assessments that involve performing linux log forensics investigation
• When following incident response procedures for related security events
• When performing scheduled security testing or auditing activities
• When validating security controls through hands-on testing

Detection Gaps & Validation

What this analysis misses most often, and how to avoid wrong conclusions:

• Cleared/truncated logs read as "quiet," not tampered. A near-empty /var/log/auth.log, a gap in sequential syslog timestamps, or a log that restarts mid-day is evidence of tampering, not absence of activity. Cross-check file mtime/ctime vs. the last log line, and confirm logrotate timing in /etc/logrotate.d/.
• journald gaps the text logs hide. If journalctl --verify reports FAIL, or journalctl --list-boots is missing boots present in wtmp, the binary journal was edited or rotated. Always pull both text logs AND /var/log/journal/* — an attacker who truncates auth.log may forget the journal (or vice versa).
• wtmp/btmp/lastlog are trivially wiped. last/lastb showing no entries does not mean no logins. Carve the raw binary for utmp records, and corroborate logins against sshd Accepted lines, ~/.bash_history, and SUDO entries — these rarely all get cleaned consistently.
• Validate by cross-corroboration, not a single source. A real SSH session should appear in auth.log (Accepted), journald (-u sshd), wtmp (last), and process/file timestamps. A login present in one but absent from the others flags either tampering or a forged entry.
• Interpretation false positives. High Failed password counts are usually internet background brute-force, not the actual intrusion — find the matching Accepted line. sudo COMMAND entries reflect intent, not success; confirm with the command's side effects. Timezone drift between syslog (local, no year) and journald (UTC) corrupts timelines — normalize before correlating.

Prerequisites

• Familiarity with digital forensics concepts and tools
• Access to a test or lab environment for safe execution
• Python 3.8+ with required dependencies installed
• Appropriate authorization for any testing activities

Key Log Files and Locations

Analysis Techniques

Authentication Log Analysis

# Find all successful SSH logins
grep "Accepted" /var/log/auth.log

# Find failed SSH login attempts
grep "Failed password" /var/log/auth.log

# Extract unique source IPs from failed logins
grep "Failed password" /var/log/auth.log | grep -oP '\d+\.\d+\.\d+\.\d+' | sort -u

# Find sudo command execution
grep "sudo:" /var/log/auth.log | grep "COMMAND"

# Detect brute force patterns (>10 failures from same IP)
grep "Failed password" /var/log/auth.log | awk '{print $(NF-3)}' | sort | uniq -c | sort -rn | head -20

# Find account creation events
grep "useradd\|adduser" /var/log/auth.log

# Detect SSH key authentication
grep "Accepted publickey" /var/log/auth.log

Systemd Journal Analysis

# Export journal in JSON format for forensic processing
journalctl --output=json --no-pager > journal_export.json

# Filter by time range
journalctl --since "2025-02-01" --until "2025-02-15" --output=json > timerange.json

# Filter by unit/service
journalctl -u sshd --output=json > sshd_journal.json

# Show kernel messages (USB events, module loads)
journalctl -k --output=json > kernel_journal.json

# Filter by priority (0=emerg to 7=debug)
journalctl -p err --output=json > errors.json

# Boot-specific logs
journalctl -b 0 --output=json > current_boot.json
journalctl --list-boots  # List all recorded boot sessions

Linux Audit Framework Analysis

# Search audit log for specific event types
ausearch -m USER_AUTH --start today

# Search for file access events
ausearch -f /etc/shadow

# Search for process execution
ausearch -m EXECVE --start "02/01/2025" --end "02/28/2025"

# Generate report of login events
aureport --login --start "02/01/2025"

# Generate summary of failed authentications
aureport --auth --failed

# Search for specific user activity
ausearch -ua 1001  # By UID
ausearch -ua username  # By username

Cron Job Investigation

# Check system-wide crontab
cat /etc/crontab

# Check user crontabs
ls -la /var/spool/cron/crontabs/

# Review cron execution logs
grep "CRON" /var/log/syslog

# Check for at/batch jobs
ls -la /var/spool/at/
atq

Python Forensic Log Parser

import re
import json
import sys
import os
from datetime import datetime
from collections import defaultdict


class LinuxLogForensicAnalyzer:
    """Analyze Linux system logs for forensic investigation."""

    def __init__(self, log_dir: str, output_dir: str):
        self.log_dir = log_dir
        self.output_dir = output_dir
        os.makedirs(output_dir, exist_ok=True)

    def parse_auth_log(self, auth_log_path: str) -> dict:
        """Parse auth.log for authentication events."""
        events = {
            "successful_logins": [],
            "failed_logins": [],
            "sudo_commands": [],
            "account_changes": [],
            "ssh_sessions": []
        }

        ssh_accepted = re.compile(
            r'(\w+\s+\d+\s+[\d:]+)\s+(\S+)\s+sshd\[\d+\]:\s+Accepted\s+(\S+)\s+for\s+(\S+)\s+from\s+([\d.]+)'
        )
        ssh_failed = re.compile(
            r'(\w+\s+\d+\s+[\d:]+)\s+(\S+)\s+sshd\[\d+\]:\s+Failed\s+password\s+for\s+(\S*)\s+from\s+([\d.]+)'
        )
        sudo_cmd = re.compile(
            r'(\w+\s+\d+\s+[\d:]+)\s+(\S+)\s+sudo:\s+(\S+)\s+:.*COMMAND=(.*)'
        )
        useradd = re.compile(
            r'(\w+\s+\d+\s+[\d:]+)\s+(\S+)\s+useradd\[\d+\]:\s+new user: name=(\S+)'
        )

        with open(auth_log_path, "r", errors="replace") as f:
            for line in f:
                m = ssh_accepted.search(line)
                if m:
                    events["successful_logins"].append({
                        "timestamp": m.group(1), "host": m.group(2),
                        "method": m.group(3), "user": m.group(4), "source_ip": m.group(5)
                    })
                    continue

                m = ssh_failed.search(line)
                if m:
                    events["failed_logins"].append({
                        "timestamp": m.group(1), "host": m.group(2),
                        "user": m.group(3), "source_ip": m.group(4)
                    })
                    continue

                m = sudo_cmd.search(line)
                if m:
                    events["sudo_commands"].append({
                        "timestamp": m.group(1), "host": m.group(2),
                        "user": m.group(3), "command": m.group(4).strip()
                    })
                    continue

                m = useradd.search(line)
                if m:
                    events["account_changes"].append({
                        "timestamp": m.group(1), "host": m.group(2),
                        "new_user": m.group(3)
                    })

        return events

    def detect_brute_force(self, auth_events: dict, threshold: int = 10) -> list:
        """Detect brute force attempts from auth log data."""
        ip_failures = defaultdict(int)
        for event in auth_events.get("failed_logins", []):
            ip_failures[event["source_ip"]] += 1

        brute_force = []
        for ip, count in ip_failures.items():
            if count >= threshold:
                brute_force.append({"source_ip": ip, "failed_attempts": count})

        return sorted(brute_force, key=lambda x: x["failed_attempts"], reverse=True)

    def generate_report(self, auth_log_path: str) -> str:
        """Generate comprehensive forensic analysis report."""
        auth_events = self.parse_auth_log(auth_log_path)
        brute_force = self.detect_brute_force(auth_events)

        report = {
            "analysis_timestamp": datetime.now().isoformat(),
            "log_source": auth_log_path,
            "summary": {
                "successful_logins": len(auth_events["successful_logins"]),
                "failed_logins": len(auth_events["failed_logins"]),
                "sudo_commands": len(auth_events["sudo_commands"]),
                "account_changes": len(auth_events["account_changes"]),
                "brute_force_sources": len(brute_force)
            },
            "brute_force_detected": brute_force,
            "auth_events": auth_events
        }

        report_path = os.path.join(self.output_dir, "linux_log_forensics.json")
        with open(report_path, "w") as f:
            json.dump(report, f, indent=2)

        print(f"[*] Successful logins: {report['summary']['successful_logins']}")
        print(f"[*] Failed logins: {report['summary']['failed_logins']}")
        print(f"[*] Sudo commands: {report['summary']['sudo_commands']}")
        print(f"[*] Brute force sources: {report['summary']['brute_force_sources']}")
        return report_path


def main():
    if len(sys.argv) < 3:
        print("Usage: python process.py <auth_log_path> <output_dir>")
        sys.exit(1)
    analyzer = LinuxLogForensicAnalyzer(os.path.dirname(sys.argv[1]), sys.argv[2])
    analyzer.generate_report(sys.argv[1])


if __name__ == "__main__":
    main()

References

• Linux Forensics In Depth: https://amr-git-dot.github.io/forensic%20investigation/Linux_Forensics/
• SANS Practical Linux Forensics: https://nostarch.com/linuxforensics
• HackTricks Linux Forensics: https://book.hacktricks.xyz/generic-methodologies-and-resources/basic-forensic-methodology/linux-forensics
• Log Sources for Digital Forensics: https://letsdefend.io/blog/log-sources-for-digital-forensics-windows-and-linux