Clearing Logs

Clearing Logs: A Comprehensive Guide for CEH System Hacking

Why Clearing Logs is Important

In the context of system hacking, clearing logs is a critical technique used by attackers to remove evidence of their unauthorized activities. After successfully compromising a system, a skilled attacker will attempt to eliminate all traces of the intrusion by deleting or modifying system logs.

Log files record system events, user activities, and security incidents. These digital footprints can reveal detailed information about who accessed a system, what actions they performed, and when these actions occurred. For cybersecurity professionals, understanding how attackers clear logs is essential for both offensive security testing and defensive countermeasures.

What is Log Clearing?

Log clearing refers to the process of removing, altering, or corrupting log entries to hide evidence of unauthorized access or malicious activities. Operating systems, applications, and network devices maintain various types of logs:

- Event logs: Record system events, application activities, and security incidents
- Access logs: Track who accessed resources and when
- Audit logs: Document changes to system configurations and security settings
- Network logs: Record network traffic and connections

How Log Clearing Works

Attackers typically employ several techniques to clear logs:

1. Direct Log Deletion: Simply deleting log files using administrative privileges

2. Log Editing: Selectively removing incriminating entries while leaving the rest intact

3. Log File Manipulation: Changing timestamps or corrupting log file integrity

4. Log Saturation: Flooding logs with irrelevant events to obscure malicious activities

5. Using Specialized Tools: Employing tools like Clear_Event_Viewer_Logs.exe, Auditpol, or Meterpreter's "clearev" command

Common Log Clearing Techniques on Different Systems

Windows Systems:
- Using Wevtutil.exe to clear event logs: wevtutil cl System
- Using PowerShell: Clear-EventLog -LogName Security
- Clearing Event Viewer logs via GUI
- Modifying registry settings for logs

Linux Systems:
- Removing log files: rm /var/log/auth.log
- Truncating logs: cat /dev/null > /var/log/auth.log
- Using shred to securely delete logs: shred -zu /var/log/auth.log
- Editing log entries with text editors

Web Servers:
- Clearing Apache access logs: echo "" > /var/log/apache2/access.log
- Modifying IIS logs through file system access

Countermeasures Against Log Clearing

Security professionals implement several defenses against log clearing:

1. Remote Log Servers: Sending copies of logs to separate, secure servers

2. Write-Once Media: Storing logs on non-rewritable media

3. Log Monitoring Tools: Using SIEM solutions to detect log tampering

4. File Integrity Monitoring: Implementing tools that alert when log files change

5. Restricted Permissions: Limiting who can access and modify log files

Exam Tips: Answering Questions on Clearing Logs

When tackling CEH exam questions about log clearing:

1. Understand Attack Progression: Remember that log clearing typically happens after an attacker has gained administrative access and completed their primary objectives.

2. Know the Commands: Memorize common log clearing commands for different operating systems (wevtutil, event viewer, echo, cat /dev/null, etc.).

3. Identify the Best Tool: Learn which tools are appropriate for specific scenarios—e.g., Meterpreter's "clearev" command for remote clearing during penetration tests.

4. Recognize Log Types: Be familiar with different log types (Security, System, Application in Windows; auth.log, syslog in Linux) and how they're stored.

5. Focus on Countermeasures: Pay special attention to questions asking about defending against log clearing—remote logging and SIEM solutions are often correct answers.

6. Consider the Context: For scenario-based questions, think about whether the described activity represents an attacker covering tracks or legitimate system administration.

7. Remember Evidence Preservation: In incident response scenarios, preserving logs before analysis is crucial.

8. Understand Detection Signs: Know indicators that logs may have been tampered with (missing time periods, unusual gaps, inconsistent entries).

By mastering these concepts and techniques, you'll be well-prepared to answer exam questions related to log clearing and demonstrate your understanding of this important aspect of system hacking.