WARNINGTROJANXiaoHui

Hunt Hypothesis

This detection identifies the presence of the XiaoHui trojan on endpoints by leveraging a specific YARA signature to flag potentially compromised systems with low-severity indicators. A proactive hunt is essential in Azure Sentinel to uncover early-stage infections that may evade standard AV alerts, allowing analysts to investigate lateral movement or data exfiltration before the adversary establishes persistence.

YARA Rule

rule WARNINGTROJANXiaoHui
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 9C E8 00 00 00 00 5D B8 ?? 85 40 00 2D ?? 85 40 00 }

condition:
		$a0 at pe.entry_point
}

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 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the WARNINGTROJANXiaoHui detection rule, including suggested filters and exclusions:

• Scenario: Antivirus Engine Scanning of Large Archives

  • Context: The enterprise endpoint protection suite (e.g., CrowdStrike Falcon or Microsoft Defender) performs a scheduled deep scan on large .zip or .7z archives containing thousands of documents. The YARA rule flags the scanning process itself as suspicious because it involves high I/O activity and temporary file extraction that mimics trojan behavior.
  • Filter/Exclusion: Create an exclusion for the AV engine’s primary executable (e.g., C:\Program Files\CrowdStrike\csagent.exe) when the parent process is running a scheduled scan task, or exclude specific high-volume archive directories like D:\Backups\Daily_Archives.

• Scenario: Deployment of Internal Development Tools

  • Context: The DevOps team uses Ansible or PowerShell DSC to push configuration updates to workstations. These tools often download and execute temporary scripts from an internal artifact repository (e.g., Azure Artifacts). The rule triggers because the downloaded script exhibits behavior patterns similar to the XiaoHui trojan, specifically regarding network connections and registry modifications during installation.
  • Filter/Exclusion: Exclude processes spawned by ansible-runner.exe or powershell.exe when the command line contains specific internal tags (e.g., -Tag "DevOps_Deployment"), or whitelist the internal artifact repository IP range in the detection logic.

• Scenario: Execution of Legacy Line-of-Business Applications

  • Context: A legacy financial reporting tool, SAP BusinessObjects, runs nightly batch jobs that generate PDF reports and email them via an embedded SMTP client. The rule flags the report