Rule to detect Jeff DEV Ransomware

Hunt Hypothesis

This detection targets adversary behavior characterized by specific file encryption and process execution patterns indicative of the Jeff DEV ransomware variant within Azure Sentinel. Proactively hunting for this signature is essential because early identification allows the SOC team to isolate affected workloads before lateral spread occurs, minimizing potential data loss despite the rule’s current low severity classification.

YARA Rule

rule jeff_dev_ransomware {

   meta:
   
      description = "Rule to detect Jeff DEV Ransomware"
      author = "Marc Rivero | @seifreed"
      reference = "https://www.bleepingcomputer.com/news/security/the-week-in-ransomware-august-31st-2018-devs-on-vacation/"
      
   strings:

      $s1 = "C:\\Users\\Umut\\Desktop\\takemeon" fullword wide
      $s2 = "C:\\Users\\Umut\\Desktop\\" fullword ascii
      $s3 = "PRESS HERE TO STOP THIS CREEPY SOUND AND VIEW WHAT HAPPENED TO YOUR COMPUTER" fullword wide
      $s4 = "WHAT YOU DO TO MY COMPUTER??!??!!!" fullword wide

   condition:

      ( uint16(0) == 0x5a4d and filesize < 5000KB ) and 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.

References

• https://www.bleepingcomputer.com/news/security/the-week-in-ransomware-august-31st-2018-devs-on-vacation/
• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the Jeff DEV Ransomware detection rule in an enterprise environment, along with suggested filters or exclusions:

• Scheduled Antivirus Full Scans by CrowdStrike Falcon

  • Scenario: The nightly full system scan performed by the CrowdStrike Falcon agent triggers high-volume file read/write operations and temporary file creation across user home directories. These activities mimic the rapid encryption behavior of ransomware, causing the rule to flag the falcon-sensor.exe process as suspicious due to its extensive modification of .docx, .xlsx, and .pdf files simultaneously.
  • Filter/Exclusion: Exclude the specific process path C:\Program Files\CrowdStrike\csagent.exe (or falcon-sensor.exe) when the parent process is svchost.exe running under the “CrowdStrike Falcon Service” account, specifically during the 02:00–04:00 UTC maintenance window.

• Microsoft Office 365 Auto-Save and Co-Authoring Sessions

  • Scenario: During peak business hours, multiple users collaborate on large Excel workbooks stored in OneDrive for Business or SharePoint Online. The excel.exe process frequently generates temporary lock files (.~$) and performs incremental saves that result in rapid file attribute changes and size modifications. This behavior often triggers the “mass file modification” heuristic of the Jeff DEV rule, mistaking legitimate co-authoring activity for an active ransomware encryption sweep.
  • Filter/Exclusion: Create a whitelist for excel.exe, winword.exe, and powerpnt.exe processes where the user context is a standard domain user (not SYSTEM) and the modified files are located within the OneDrive sync folder path (%USERPROFILE%\OneDrive - [Company Name]).

• **Windows Server Backup (V