ExeBundlev30smallloader

Hunt Hypothesis

This rule detects the presence of a small, bundled executable loader that may be used by adversaries to stage initial access payloads or facilitate fileless execution techniques. Proactively hunting for this artifact in Azure Sentinel allows the SOC team to identify low-severity staging components before they are leveraged to deploy more complex malware or establish persistence within the environment.

YARA Rule

rule ExeBundlev30smallloader
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 00 00 00 00 60 BE 00 F0 40 00 8D BE 00 20 FF FF 57 83 CD FF EB 10 90 90 90 90 90 90 8A 06 46 88 07 47 01 DB 75 07 8B 1E 83 EE FC 11 DB 72 ED B8 01 00 00 00 01 DB 75 07 8B 1E 83 EE FC 11 }

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: Execution of a self-extracting archive (SFX) created by 7-Zip or WinRAR for software distribution.
  • Filter/Exclusion: Exclude files with extensions .sfx, .7z, or .rar located in standard software installation directories (e.g., C:\Program Files\, C:\Temp\) or where the parent process is 7z.exe or WinRAR.exe.
• Scenario: Running a small, packed installer for a specific enterprise application (e.g., Adobe Creative Cloud, Oracle Java, or .NET Framework) that uses a compact loader stub to extract the main payload.
  • Filter/Exclusion: Allowlist specific known-good hashes (SHA-256) for common installers or exclude processes with names like setup.exe, install.exe, or installer.exe originating from trusted vendor directories.
• Scenario: Execution of a lightweight CLI tool or utility (e.g., curl.exe, wget.exe, or a custom internal PowerShell wrapper compiled to EXE) that is small in size (< 100KB) and may match the “small loader” heuristic due to its minimal code footprint.
  • Filter/Exclusion: Exclude executables smaller than 50KB if the parent process is a known administrative tool (e.g., powershell.exe, cmd.exe, wmic.exe) or if the file path resides in C:\Windows\System32\ or C:\Program Files (x86)\Common Files\.
• Scenario: A scheduled task or service startup that launches a small helper executable for a monitoring agent (e.g., Datadog, New Relic, or Splunk) which acts as a bootstrap loader for the main agent service.
  • Filter/Exclusion: Exclude processes associated with known