Misdat Backdoor

Hunt Hypothesis

The Misdat Backdoor detection identifies potential adversary persistence by leveraging a known malicious YARA signature, indicating possible unauthorized access or data exfiltration. SOC teams should proactively hunt for this behavior in Azure Sentinel to detect early-stage compromise and prevent lateral movement or data theft.

YARA Rule

rule Misdat_Backdoor
{
   
   meta:
        author = "Cylance SPEAR Team"
        /* Decode Function
        CODE:00406C71 8B 55 F4                  mov     edx, [ebp+var_C]
        CODE:00406C74 8A 54 1A FF               mov     dl, [edx+ebx-1]
        CODE:00406C78 8B 4D F8                  mov     ecx, [ebp+var_8]
        CODE:00406C7B C1 E9 08                  shr     ecx, 8
        CODE:00406C7E 32 D1                     xor     dl, cl
        CODE:00406C80 88 54 18 FF               mov     [eax+ebx-1], dl
        CODE:00406C84 8B 45 F4                  mov     eax, [ebp+var_C]
        CODE:00406C87 0F B6 44 18 FF            movzx   eax, byte ptr [eax+ebx-1]
        CODE:00406C8C 03 45 F8                  add     eax, [ebp+var_8]
        CODE:00406C8F 69 C0 D9 DB 00 00         imul    eax, 0DBD9h
        CODE:00406C95 05 3B DA 00 00            add     eax, 0DA3Bh
        CODE:00406C9A 89 45 F8                  mov     [ebp+var_8], eax
        CODE:00406C9D 43                        inc     ebx
        CODE:00406C9E 4E                        dec     esi
        CODE:00406C9F 75 C9                     jnz     short loc_406C6A
        */
    
    strings:
        $imul = {03 45 F8 69 C0 D9 DB 00 00 05 3B DA 00 00}
        $delphi = {50 45 00 00 4C 01 08 00 19 5E 42 2A}
        
    condition:
        $imul and $delphi
}

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: Legitimate scheduled job using schtasks.exe to run a maintenance script
  Filter/Exclusion: process.parent_process == "schtasks.exe" and process.command_line contains " /RU SYSTEM"

• Scenario: System update or patching using wuauclt.exe or msiexec.exe
  Filter/Exclusion: process.name == "wuauclt.exe" or process.name == "msiexec.exe"

• Scenario: Admin task using taskhost.exe to run a PowerShell script for log management
  Filter/Exclusion: process.parent_process == "taskhost.exe" and process.command_line contains "powershell.exe -Command"

• Scenario: Regular system monitoring tool like perfmon.exe or eventvwr.exe
  Filter/Exclusion: process.name == "perfmon.exe" or process.name == "eventvwr.exe"

• Scenario: Backup tool like VSSAdmin.exe or wbadmin.exe performing a scheduled backup
  Filter/Exclusion: process.name == "VSSAdmin.exe" or process.name == "wbadmin.exe" and process.command_line contains "backup"