Hunt Hypothesis
This detection identifies potential malware activity by leveraging a specific YARA signature to match known malicious patterns within cloud workloads. A proactive hunt is essential in Azure Sentinel to uncover early-stage threats that may evade standard heuristic rules, ensuring rapid containment before adversaries establish persistence or lateral movement capabilities.
YARA Rule
rule VxNumberOne
{
meta:
author="malware-lu"
strings:
$a0 = { F9 07 3C 53 6D 69 6C 65 3E E8 }
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.
False Positive Guidance
Here are 4 specific false positive scenarios for the VxNumberOne detection rule in an enterprise environment, along with targeted exclusion strategies:
-
Scenario: Endpoint Protection Agent Self-Updates
- Context: The VxNumberOne signature often matches the executable behavior of major EDR agents (e.g., CrowdStrike Falcon or Microsoft Defender for Endpoint) when they perform background self-updates. These updates involve downloading new definition packs and replacing core binaries, which mimics the suspicious file creation patterns defined in the YARA rule.
- Filter/Exclusion: Add an exclusion based on the process image path to ignore known EDR update executables.
- Example:
Exclude if Process.ImagePath contains "C:\Program Files\CrowdStrike\fs_qa.exe" OR "C:\Windows\System32\MsMpEng.exe"
-
Scenario: Scheduled Antivirus Definition Refresh
- Context: Enterprise environments typically run scheduled tasks (via Windows Task Scheduler) to refresh antivirus definitions during off-hours. The YARA rule may flag the temporary extraction of definition archives by tools like Symantec Endpoint Protection or McAfee Total Protection as a potential threat deployment, generating high-volume alerts during maintenance windows.
- Filter/Exclusion: Implement a time-based exclusion window combined with a parent process filter for the specific scheduled task runner.
- Example:
Exclude if Event.Time is between 02:00 and 05:00 AND ParentProcess.Name == "TaskScheduler.exe" AND Process.CommandLine contains "Symantec"
-
Scenario: Software Deployment via SCCM/Intune
- Context: When IT administrators push software updates or new applications using Microsoft Endpoint Configuration Manager (SCCM) or Intune, the deployment engine extracts installation packages to a temporary directory. The VxNumberOne rule may