Stuxnet Sample - file maindll.decrypted.unpacked.dll_

Hunt Hypothesis

The detection identifies potential Stuxnet malware activity through the presence of a suspiciously named DLL file, indicating possible adversary use of advanced persistent threats. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate early-stage malware execution and lateral movement attempts.

YARA Rule

rule Stuxnet_maindll_decrypted_unpacked 
{

    meta:
        description = "Stuxnet Sample - file maindll.decrypted.unpacked.dll_"
        author = "Florian Roth"
        reference = "Internal Research"
        date = "2016-07-09"
        hash1 = "4c3d7b38339d7b8adf73eaf85f0eb9fab4420585c6ab6950ebd360428af11712"

    strings:
        $s1 = "%SystemRoot%\\system32\\Drivers\\mrxsmb.sys;%SystemRoot%\\system32\\Drivers\\*.sys" fullword wide
        $s2 = "<Actions Context=\"%s\"><Exec><Command>%s</Command><Arguments>%s,#%u</Arguments></Exec></Actions>" fullword wide
        $s3 = "%SystemRoot%\\inf\\oem7A.PNF" fullword wide
        $s4 = "%SystemRoot%\\inf\\mdmcpq3.PNF" fullword wide
        $s5 = "%SystemRoot%\\inf\\oem6C.PNF" fullword wide
        $s6 = "@abf varbinary(4096) EXEC @hr = sp_OACreate 'ADODB.Stream', @aods OUT IF @hr <> 0 GOTO endq EXEC @hr = sp_OASetProperty @" wide
        $s7 = "STORAGE#Volume#1&19f7e59c&0&" fullword wide
        $s8 = "view MCPVREADVARPERCON as select VARIABLEID,VARIABLETYPEID,FORMATFITTING,SCALEID,VARIABLENAME,ADDRESSPARAMETER,PROTOKOLL,MAXLIMI" ascii

    condition:
         6 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 8 string patterns in its detection logic.

References

• Internal Research
• Source Rule

False Positive Guidance

• Scenario: Legitimate Antivirus Scan Using VirusTotal Integration
  Description: A security tool like VirusTotal or Malwarebytes performs a scan and unpacks a DLL as part of its analysis.
  Filter/Exclusion: Exclude files with the VirusTotal or Malwarebytes process names, or filter by file source (e.g., C:\Program Files\Malwarebytes\).

• Scenario: Scheduled System Update or Patch Deployment
  Description: A Windows Update or Microsoft Endpoint Manager (MEM) task unpacks a DLL as part of a system update.
  Filter/Exclusion: Exclude files with Windows Update or Microsoft Endpoint Manager process names, or filter by file path (e.g., C:\Windows\Temp\ or C:\Program Files\WindowsUpdate\).

• Scenario: Enterprise Security Tool Unpacking for Analysis
  Description: A tool like Procmon (Process Monitor) or Wireshark (with packet capture) unpacks a DLL during analysis.
  Filter/Exclusion: Exclude files with Procmon or Wireshark process names, or filter by file path (e.g., C:\Windows\System32\ or C:\Program Files\Procmon\).

• Scenario: Admin Task to Decompress a DLL for Debugging
  Description: A system administrator uses 7-Zip or WinRAR to decompress a DLL for debugging or analysis.
  Filter/Exclusion: Exclude files with 7-Zip or WinRAR process names, or filter by file path (e.g., C:\Temp\ or C:\Users\Administrator\Desktop\).

• **Scenario: Legitimate DLL Packing for