Bublik Trojan Downloader

Hunt Hypothesis

The Bublik Trojan Downloader is likely being used to establish a foothold in the network by downloading additional malicious payloads. SOC teams should proactively hunt for this behavior in Azure Sentinel to detect and mitigate early-stage adversary activity before it leads to data exfiltration or lateral movement.

YARA Rule

rule Bublik
{
   
    meta:
        author="Kevin Falcoz"
        date="29/09/2013"
        description="Bublik Trojan Downloader"
        
    strings:
        $signature1={63 6F 6E 73 6F 6C 61 73}
        $signature2={63 6C 55 6E 00 69 6E 66 6F 2E 69 6E 69}
        
    condition:
        $signature1 and $signature2
}

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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: Scheduled System Maintenance Task
  Description: A legitimate system maintenance task, such as Windows Update or disk cleanup, may trigger the rule due to similar network behavior.
  Filter/Exclusion: Check for process.name containing “wuauserv” or “cleanmgr.exe” and exclude traffic initiated by scheduled tasks with task.name matching known maintenance tasks.

• Scenario: Admin Performing Remote Desktop Protocol (RDP) Session
  Description: An administrator using RDP to access a remote server may trigger the rule if the connection is misinterpreted as a malicious download.
  Filter/Exclusion: Filter by process.name containing “mstsc.exe” or “rdpclip.exe” and check for user.name matching known admin accounts.

• Scenario: Software Update Deployment via Microsoft Endpoint Manager (MEM)
  Description: A legitimate software update deployment using Microsoft Endpoint Manager may trigger the rule due to similar network patterns.
  Filter/Exclusion: Check for process.name containing “msiexec.exe” or “setup.exe” and verify the source IP against known MEM update servers.

• Scenario: Legitimate File Transfer via PowerShell Script
  Description: A PowerShell script used for automated file transfers or configuration management may trigger the rule due to similar download behavior.
  Filter/Exclusion: Filter by process.name containing “powershell.exe” and check for script.name or command_line indicating known legitimate automation scripts.

• Scenario: Antivirus or Endpoint Protection Software Performing Quarantine Actions
  Description: Antivirus software may trigger the rule when moving files to quarantine or performing on-demand scans.
  Filter/Exclusion: Check for process.name containing “mpcmdrun.exe” or “avastui.exe” and verify the file hash against known safe files.