Identify Dexter POSGrabber

Hunt Hypothesis

The hypothesis is that the detection rule identifies potential Dexter POSGrabber activity by monitoring for suspicious file execution or data exfiltration patterns commonly associated with this malware. SOC teams should proactively hunt for this behavior in Azure Sentinel to detect and mitigate early-stage POS data theft attempts that may evade traditional detection methods.

YARA Rule

rule dexter_strings
{
   
    meta:
        author = "Brian Wallace @botnet_hunter"
        author_email = "[email protected]"
        date = "2014-09-10"
        description = "Identify Dexter POSGrabber"
   
    strings:
        $s1 = "UpdateMutex:"
        $s2 = "response="
        $s3 = "page="
        $s4 = "scanin:"
   
    condition:
        all of them
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 4 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: Scheduled System Maintenance Task
  Description: A legitimate scheduled task runs a script that temporarily accesses system files, which could be mistaken for Dexter POSGrabber activity.
  Filter/Exclusion: Exclude processes associated with Task Scheduler or schtasks.exe with known maintenance scripts.

• Scenario: Database Backup Job
  Description: A database backup tool like MySQLDump or pg_dump may access database files, triggering the same file access patterns as Dexter POSGrabber.
  Filter/Exclusion: Exclude processes related to mysqldump, pg_dump, or any known backup tools with matching file paths.

• Scenario: Admin User Performing File Integrity Check
  Description: An admin user may manually check file integrity using tools like Tripwire or AIDE, which could trigger the same file access behavior.
  Filter/Exclusion: Exclude processes initiated by admin users with elevated privileges and associated with integrity checking tools.

• Scenario: Log File Rotation by Log Management Tool
  Description: Tools like Logrotate or Splunk may rotate or access log files, which could be flagged as suspicious file access.
  Filter/Exclusion: Exclude processes related to logrotate, splunk, or any log management tools with known log rotation behavior.

• Scenario: Antivirus or EDR Scanning Activity
  Description: Antivirus tools like Kaspersky, Bitdefender, or EDR platforms may scan files, leading to similar file access patterns.
  Filter/Exclusion: Exclude processes associated with known antivirus or EDR tools, such as kavsvc.exe, bitdefender.exe, or microsoft defender.