How to Detect and Respond to Unauthorized Access to Critical Files

Unauthorized access to critical files can lead to data breaches, tampering, or other malicious activity. Detecting and responding effectively requires monitoring file activity logs, correlating user behavior, and enforcing robust controls.

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1. Review File Access Logs

What to Look For:

• File Access Events:

  • Event ID 4663: Captures attempts to access objects (files, directories).

  • Look for:

    • Users or accounts accessing files they shouldn’t have permissions for.

    • Access during unusual hours or outside business operations.

Red Flags: Access by unauthorized users or repeated access attempts from unexpected systems.

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2. Monitor for Unusual File Modifications

What to Look For:

• Modification and Deletion Events:

  • Event ID 4660: Logs object deletions.

  • Event ID 4656: Tracks handle requests for objects, indicating potential file tampering.

• Patterns:

  • Sudden or frequent modifications to critical files.

  • Unexpected deletions of sensitive data.

Red Flags: High-frequency edits, unexpected deletions, or changes by non-administrative users.

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3. Investigate Access by System or Service Accounts

What to Look For:

• Service Account Activity:

  • File access by system or service accounts not typically associated with the targeted files.

• Attack Techniques:

  • Attackers often use compromised service accounts to bypass detection.

Red Flags: Unusual activity by accounts that don’t typically interact with critical files or directories.

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4. Cross-Reference with User Activity Logs

What to Do:

• Correlate File Access and User Activity:

  • Event ID 4624: Logs successful logons.

  • Event ID 4672: Tracks special privileges assigned during logons.

• Patterns:

  • File access following privileged logon sessions.

  • Logons from unexpected devices or geolocations.

Red Flags: File access coinciding with privileged logons by users who don’t typically require such access.

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5. Analyze File Share Access Logs

What to Look For:

• Shared Resource Activity:

  • Event ID 5140: Logs network share access.

  • Look for:

    • Access to critical shared folders by unauthorized accounts.

    • Unusual patterns such as bulk access to files.

Red Flags: Unexpected or unauthorized access to shared folders containing sensitive data.

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6. Immediate Response

What to Do:

• Revoke Access:

  • Immediately revoke access for the unauthorized user or account to prevent further actions.

• Isolate the System:

  • Disconnect the affected system from the network if suspicious activity continues.

• Review Actions Taken:

  • Investigate all actions performed by the account, including file modifications, deletions, and access attempts.

• Notify Stakeholders:

  • Inform the security team, management, and relevant departments to manage the incident appropriately.

• Enhance Security Controls:

  • Implement stricter access controls, such as:

    • Role-based access control (RBAC).

    • Least privilege principles.

    • Multi-factor authentication (MFA).

  • Enforce regular audits of access permissions.

Post-Incident Steps:

• Audit and Monitor:

  • Increase monitoring of critical files and systems to detect similar activity in the future.

• Train Employees:

  • Educate users about secure file handling practices and recognizing signs of unauthorized activity.

• Conduct a Root Cause Analysis:

  • Determine how the unauthorized access occurred and address vulnerabilities, such as weak passwords or overly permissive access policies.

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Conclusion

Detecting unauthorized access to critical files requires vigilant monitoring of file access, modification, and user activity logs. Swift response and proactive controls ensure that such incidents are mitigated and prevented effectively.