VxSlowload

Hunt Hypothesis

This rule detects the presence of VxSlowload, a malicious component often used for initial access or persistence, by identifying its specific code patterns within memory or disk artifacts. Proactively hunting for this signature allows the SOC team to identify compromised hosts early in the kill chain, enabling rapid containment before the adversary can establish deeper footholds or execute subsequent post-exploitation actions.

YARA Rule

rule VxSlowload
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 03 D6 B4 40 CD 21 B8 02 42 33 D2 33 C9 CD 21 8B D6 B9 78 01 }

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

• Scenario: A legacy Java application (e.g., an internal ERP system or middleware like WebSphere) performs a slow, chunked download of a large configuration file or log archive from a file server over HTTP/HTTPS. The VxSlowload heuristic often flags consistent, low-speed network reads that mimic slow-loading malware behavior.
  • Filter/Exclusion: Exclude processes named java.exe or jre.exe when the destination path contains \config\, \logs\, or \temp\ and the source IP is within the internal trusted subnet (e.g., 10.0.0.0/8).
• Scenario: An IT administrator manually copies a large ISO image or database backup file from a network share to a local drive using Windows Explorer or robocopy with a throttled bandwidth setting to avoid saturating the network during business hours.
  • Filter/Exclusion: Exclude processes explorer.exe, robocopy.exe, or copy.exe if the source path starts with \\ (UNC path) and the file extension is .iso, .bak, or .zip, provided the user account belongs to the IT_Admins or Backup_Ops group.
• Scenario: A scheduled maintenance job runs a Python or PowerShell script that downloads a model update or dataset from an internal artifact repository (e.g., Nexus, Artifactory, or a private S3 bucket) using a library like requests or Invoke-WebRequest with a default timeout or progress bar, resulting in a slow, steady stream of data.
  • Filter/Exclusion: Exclude processes python.exe, pwsh.exe, or powershell.exe if the command line contains keywords like download, fetch, or update and the destination directory is under C:\ProgramData\ or `D