Detect unusual file system activity

Detecting unusual file system activity is crucial for identifying potential security breaches, malware behavior, or insider threats. Unusual file system activity can indicate unauthorized access, data exfiltration, ransomware encryption, or other malicious actions. Here’s a comprehensive guide on how to detect such activity effectively:

1. Understand Normal File System Behavior

Before detecting anomalies, establish a baseline of typical file system operations, including:

• Commonly accessed files and directories

• Regular file creation, modification, and deletion patterns

• Typical user and process activity on the file system

2. Use File Integrity Monitoring (FIM) Tools

FIM tools track changes to files and directories, alerting when unexpected modifications occur. Examples include:

• Tripwire

• OSSEC

• AIDE
  These tools record cryptographic hashes of files and notify administrators if files are added, deleted, or altered unexpectedly.

3. Monitor File Access Patterns

Track how files are accessed to identify abnormal behavior, such as:

• Excessive file read/write operations in a short time

• Access to sensitive files by unauthorized users or processes

• Unusual file locking or sharing patterns

4. Analyze File Creation and Deletion

Suspicious spikes in file creation or deletion can indicate malware activity or data tampering. Watch for:

• Sudden mass file creations, especially in critical directories

• Unexpected deletions of logs or backup files

5. Detect Ransomware Activity

Ransomware often renames or encrypts large numbers of files rapidly. Indicators include:

• Bulk renaming or encryption of files with unusual extensions

• Rapid file write operations followed by deletion of original files

• Creation of ransom notes in multiple directories

6. Use System and Security Logs

Operating system logs and security audit logs can reveal unauthorized file system access:

• Windows Event Logs (e.g., Security, System)

• Linux audit logs (auditd)
  Analyze these logs for suspicious file access events, permission changes, or attempts to escalate privileges.

7. Employ Behavior-Based Detection

Use advanced endpoint detection and response (EDR) tools or SIEM (Security Information and Event Management) solutions that apply behavioral analytics to identify deviations from normal file system usage.

8. Automate Alerts for Critical File Changes

Set up automated alerts for critical files, such as:

• Configuration files

• System binaries

• Security policy files

9. Correlate File System Activity with Network and Process Data

Cross-reference unusual file system changes with network connections and running processes to identify potentially malicious behavior.

10. Regular Audits and Forensics

Perform regular audits and forensic analysis of file system activity to detect stealthy attacks or long-term compromises.

Implementing a combination of these strategies helps in timely detection of unusual file system activity, reducing risk and enabling rapid incident response.