VxHorse1776

Hunt Hypothesis

This detection identifies potential malicious activity associated with the VxHorse1776 signature by leveraging YARA rules to scan for specific file patterns indicative of known threat behaviors within the Azure Sentinel environment. Proactively hunting for this indicator allows the SOC team to uncover early-stage infections or lateral movement attempts that may not yet trigger high-severity alerts, thereby strengthening the organization’s defensive posture against evolving threats.

YARA Rule

rule VxHorse1776
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 [2] 5D 83 [2] 06 1E 26 [4] BF [2] 1E 0E 1F 8B F7 01 EE B9 [2] FC F3 A6 1F 1E 07 }

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 VxHorse1776 detection rule in an enterprise environment, including suggested filters and exclusions:

• Antivirus Engine Updates via Windows Defender

  • Scenario: The Microsoft Defender Antivirus service (MsMpEng.exe) automatically downloads and installs definition updates from the cloud. The VxHorse1776 YARA rule often flags the update installer or the signature database file as a potential variant due to its dynamic code structure and embedded signatures, which mimic the behavior of the targeted malware family.
  • Filter/Exclusion: Exclude processes where ImageFileName is MsMpEng.exe AND CommandLine contains keywords like “Update” or “Scan”. Alternatively, create a hash-based exclusion for the known good SHA-256 hashes of the latest Microsoft Defender update packages.

• Scheduled PowerShell Script Execution by System Admins

  • Scenario: Enterprise administrators frequently run scheduled maintenance scripts (e.g., via Task Scheduler) that utilize powershell.exe to perform bulk user provisioning or configuration changes. These scripts often invoke external modules or download temporary manifests, triggering the VxHorse1776 rule due to the script’s interaction with network resources and file system modifications typical of the detection logic.
  • Filter/Exclusion: Filter out events where ParentImage is svchost.exe (indicating a scheduled task) AND CommandLine contains specific internal script paths (e.g., C:\Admin\Scripts\). Additionally, exclude processes running under the context of known service accounts like DOMAIN\AdminService or SYSTEM.

• Deployment of Software via Microsoft Endpoint Configuration Manager (SCCM)

  • Scenario: When SCCM deploys new applications to endpoints, the ccmsetup.exe or WUAHandler.exe processes execute