IMuler code tricks

Hunt Hypothesis

Adversaries may use IMuler code tricks to obfuscate malicious activities, making detection through traditional methods challenging. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate potential advanced persistent threats that evade standard detection mechanisms.

YARA Rule

rule IMulerCode : IMuler Family 
{
    meta:
        description = "IMuler code tricks"
        author = "Seth Hardy"
        last_modified = "2014-06-16"
        
    strings:
        // Load these function strings 4 characters at a time. These check the first two blocks:
        $L4_tmpSpotlight = { C7 ?? 2F 74 6D 70 C7 ?? 04 2F 53 70 6F }
        $L4_TMPAAABBB = { C7 ?? ?? ?? ?? ?? 54 4D 50 41 C7 ?? ?? ?? ?? ?? 41 41 42 42 }
        $L4_FILEAGENTVer = { C7 ?? 46 49 4C 45 C7 ?? 04 41 47 45 4E }
        $L4_TMP0M34JDF8 = { C7 ?? ?? ?? ?? ?? 54 4D 50 30 C7 ?? ?? ?? ?? ?? 4D 33 34 4A }
        $L4_tmpmdworker = { C7 ?? 2F 74 6D 70 C7 ?? 04 2F 2E 6D 64 }
        
    condition:
        any of ($L4*)
}

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 5 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Scenario: Scheduled System Maintenance Job
  Description: A legitimate scheduled task runs a script that uses imuler as part of a system cleanup or configuration update.
  Filter/Exclusion: process.parent_process_name:"Task Scheduler" or process.command_line:"schtasks.exe"

• Scenario: Admin Performing Code Analysis with Ghidra
  Description: A security administrator uses Ghidra to analyze a binary that contains imuler as part of its disassembly or reverse engineering process.
  Filter/Exclusion: process.parent_process_name:"ghidra" or "java.exe" and process.user:"Administrator"

• Scenario: CI/CD Pipeline Artifact Extraction
  Description: A CI/CD pipeline (e.g., Jenkins, GitLab CI) extracts or processes artifacts that include imuler as part of a build or deployment step.
  Filter/Exclusion: process.parent_process_name:"jenkins.exe" or "gitlab-runner" and process.command_line:"*ci*"

• Scenario: Legitimate Use in a Security Tool for Code Obfuscation
  Description: A security tool (e.g., RetDec, IDA Pro) uses imuler as part of its code obfuscation or deobfuscation process.
  Filter/Exclusion: process.parent_process_name:"idaq.exe" or "retdec-cli" and process.command_line:"*deobfuscate*"

• Scenario: User-Initiated File Conversion with Converters
  Description: A user runs a file conversion tool (e.g., imuler as part of a custom script) to convert file formats in a legitimate business process.
  Filter/Exclusion: process.user:"User123" and process.command_line:"*convert*" or `process.parent