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.

Key Log Files and Locations

Log File	Path	Contents
auth.log / secure	/var/log/auth.log (Debian) or /var/log/secure (RHEL)	Authentication, sudo, SSH, PAM
syslog / messages	/var/log/syslog (Debian) or /var/log/messages (RHEL)	General system messages
kern.log	/var/log/kern.log	Kernel messages, USB events, driver loads
lastlog	/var/log/lastlog	Last login per user (binary)
wtmp	/var/log/wtmp	Login/logout records (binary, read with `last`)
btmp	/var/log/btmp	Failed login attempts (binary, read with `lastb`)
faillog	/var/log/faillog	Failed login counter (binary)
cron.log	/var/log/cron or /var/log/syslog	Scheduled task execution
audit.log	/var/log/audit/audit.log	Linux Audit Framework events
journal	/var/log/journal/ or /run/log/journal/	systemd binary journal
dpkg.log	/var/log/dpkg.log	Package installation/removal (Debian)
yum.log	/var/log/yum.log	Package installation/removal (RHEL)

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()

Verification Criteria

Confirm successful execution by validating:

[ ] All prerequisite tools and access requirements are satisfied
[ ] Each workflow step completed without errors
[ ] Output matches expected format and contains expected data
[ ] No security warnings or misconfigurations detected
[ ] Results are documented and evidence is preserved for audit

Compliance Framework Mapping

This skill supports compliance evidence collection across multiple frameworks:

SOC 2: CC7.3 (Incident Identification), CC7.4 (Incident Response)
ISO 27001: A.16.1 (Security Incident Management), A.12.4 (Logging)
NIST 800-53: AU-6 (Audit Review), IR-4 (Incident Handling), AU-9 (Audit Protection)
NIST CSF: RS.AN (Analysis), RS.RP (Response Planning)

Claw GRC Tip: When this skill is executed by a registered agent, compliance evidence is automatically captured and mapped to the relevant controls in your active frameworks.

Deploying This Skill with Claw GRC

Agent Execution

# Install via CLI
npx claw-grc skills add performing-linux-log-forensics-investigation

# Or load dynamically via MCP
grc.load_skill("performing-linux-log-forensics-investigation")

Audit Trail Integration

When executed through Claw GRC, every step of this skill generates tamper-evident audit records:

SHA-256 chain hashing ensures no step can be modified after execution
Evidence artifacts (configs, scan results, logs) are automatically attached to relevant controls
Trust score impact — successful execution increases your agent's trust score

Continuous Compliance

Schedule this skill for recurring execution to maintain continuous compliance posture. Claw GRC monitors for drift and alerts when re-execution is needed.

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

Performing Linux Log Forensics Investigation

Overview

Key Log Files and Locations

Log File	Path	Contents
auth.log / secure	/var/log/auth.log (Debian) or /var/log/secure (RHEL)	Authentication, sudo, SSH, PAM
syslog / messages	/var/log/syslog (Debian) or /var/log/messages (RHEL)	General system messages
kern.log	/var/log/kern.log	Kernel messages, USB events, driver loads
lastlog	/var/log/lastlog	Last login per user (binary)
wtmp	/var/log/wtmp	Login/logout records (binary, read with `last`)
btmp	/var/log/btmp	Failed login attempts (binary, read with `lastb`)
faillog	/var/log/faillog	Failed login counter (binary)
cron.log	/var/log/cron or /var/log/syslog	Scheduled task execution
audit.log	/var/log/audit/audit.log	Linux Audit Framework events
journal	/var/log/journal/ or /run/log/journal/	systemd binary journal
dpkg.log	/var/log/dpkg.log	Package installation/removal (Debian)
yum.log	/var/log/yum.log	Package installation/removal (RHEL)

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()

Verification Criteria

Confirm successful execution by validating:

[ ] All prerequisite tools and access requirements are satisfied
[ ] Each workflow step completed without errors
[ ] Output matches expected format and contains expected data
[ ] No security warnings or misconfigurations detected
[ ] Results are documented and evidence is preserved for audit

Compliance Framework Mapping

This skill supports compliance evidence collection across multiple frameworks:

SOC 2: CC7.3 (Incident Identification), CC7.4 (Incident Response)
ISO 27001: A.16.1 (Security Incident Management), A.12.4 (Logging)
NIST 800-53: AU-6 (Audit Review), IR-4 (Incident Handling), AU-9 (Audit Protection)
NIST CSF: RS.AN (Analysis), RS.RP (Response Planning)

Claw GRC Tip: When this skill is executed by a registered agent, compliance evidence is automatically captured and mapped to the relevant controls in your active frameworks.

Deploying This Skill with Claw GRC

Agent Execution

# Install via CLI
npx claw-grc skills add performing-linux-log-forensics-investigation

# Or load dynamically via MCP
grc.load_skill("performing-linux-log-forensics-investigation")

Audit Trail Integration

When executed through Claw GRC, every step of this skill generates tamper-evident audit records:

SHA-256 chain hashing ensures no step can be modified after execution
Evidence artifacts (configs, scan results, logs) are automatically attached to relevant controls
Trust score impact — successful execution increases your agent's trust score

Continuous Compliance

Schedule this skill for recurring execution to maintain continuous compliance posture. Claw GRC monitors for drift and alerts when re-execution is needed.

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

Performing Linux Log Forensics Investigation

Overview

Key Log Files and Locations

Analysis Techniques

Authentication Log Analysis

Systemd Journal Analysis

Linux Audit Framework Analysis

Cron Job Investigation

Python Forensic Log Parser

Verification Criteria

Compliance Framework Mapping

Deploying This Skill with Claw GRC

Agent Execution

Audit Trail Integration

Continuous Compliance

References

Use with Claw GRC Agents

Tags

Related Skills

Analyzing Linux System Artifacts

Analyzing Linux Kernel Rootkits

Performing Memory Forensics with Volatility3

Performing Timeline Reconstruction with Plaso

Acquiring Disk Image with dd and dcfldd

Analyzing Browser Forensics with Hindsight

Performing Linux Log Forensics Investigation

Overview

Key Log Files and Locations

Analysis Techniques

Authentication Log Analysis

Systemd Journal Analysis

Linux Audit Framework Analysis

Cron Job Investigation

Python Forensic Log Parser

Verification Criteria

Compliance Framework Mapping

Deploying This Skill with Claw GRC

Agent Execution

Audit Trail Integration

Continuous Compliance

References

Use with Claw GRC Agents

Tags

Related Skills

Analyzing Linux System Artifacts

Analyzing Linux Kernel Rootkits

Performing Memory Forensics with Volatility3

Performing Timeline Reconstruction with Plaso

Acquiring Disk Image with dd and dcfldd

Analyzing Browser Forensics with Hindsight