VIRUSIWormBagle

Hunt Hypothesis

This hypothesis targets the presence of the Bagle worm, a legacy Windows file-sharing worm that propagates via network shares and email attachments, often indicating a compromised host or an unpatched system in the environment. Proactively hunting for this signature allows the SOC to identify dormant or persistent infections that may have been missed by real-time detections, ensuring that known low-severity threats are contained before they can leverage lateral movement or establish footholds within the Azure network.

YARA Rule

rule VIRUSIWormBagle
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 6A 00 E8 95 01 00 00 E8 9F E6 FF FF 83 3D 03 50 40 00 00 75 14 68 C8 AF 00 00 E8 01 E1 FF FF 05 88 13 00 00 A3 03 50 40 00 68 5C 57 40 00 68 F6 30 40 00 FF 35 03 50 40 00 E8 B0 EA FF FF E8 3A FC FF FF 83 3D 54 57 40 00 00 74 05 E8 F3 FA FF FF 68 E8 03 00 }

condition:
		$a0
}

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

• Legacy Application Maintenance via Antivirus Exclusions:

  • Scenario: IT operations maintain a legacy line-of-business application (e.g., an old ERP module or custom Java-based tool) that was originally developed in the early 2000s. To prevent false positives from the AV engine during routine updates or database backups, the bin or lib directories of this application are added to the antivirus exclusion list. If the YARA rule is scanned against these excluded files during a full integrity check or if the exclusion is misconfigured, the rule may trigger on the specific byte patterns of the legacy code that resemble the Bagle worm’s header or payload.
  • Filter/Exclusion: Exclude file paths matching C:\Program Files\LegacyApp\* or specific service executables like LegacyService.exe from the YARA scan scope, or ensure the rule is only applied to files modified within the last 30 days.

• Scheduled Backup Job with Embedded Signatures:

  • Scenario: A nightly scheduled task (e.g., BackupLegacyData in Task Scheduler) runs a custom PowerShell script that compresses and encrypts backup archives. The compression algorithm or the encryption header used by the specific backup tool (e.g., an older version of WinRAR or a proprietary in-house tool) contains a byte sequence that matches the YARA pattern for VIRUSIWormBagle. This occurs because the rule is overly broad in its matching criteria and catches the structural header of the archive rather than the actual worm payload.
  • Filter/Exclusion: Exclude files with extensions .zip, .rar, .7z, or .bak located in the designated backup directory (e.g., \\fileserver\backups\) from the YARA detection logic, or refine the YARA rule to require a minimum file size (e.g., > 5