ChSfxsmallv11

Hunt Hypothesis

This detection identifies small executable files with specific structural characteristics that may indicate early-stage malware or benign utilities masquerading as legitimate software. Proactive hunting for these artifacts in Azure Sentinel is essential to uncover stealthy threats that often evade standard signature-based defenses due to their minimal footprint and low severity classification.

YARA Rule

rule ChSfxsmallv11
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BA [2] E8 [2] 8B EC 83 EC ?? 8C C8 BB [2] B1 ?? D3 EB 03 C3 8E D8 05 [2] 89 }

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 ChSfxsmallv11 detection rule in an enterprise environment, including suggested filters and exclusions:

• Scenario: Automated Software Deployment via Microsoft Endpoint Configuration Manager (SCCM)

  • Context: SCCM agents frequently execute small executable payloads during scheduled maintenance windows to push updates or install new applications. The ChSfxsmallv11 rule may flag these legitimate, small-sized installer executables as suspicious due to their size and execution pattern.
  • Filter/Exclusion: Exclude processes where the parent process is ccmsetup.exe (SCCM Setup) or WUAHandler.exe running under the context of the SYSTEM account on domain-joined endpoints during defined maintenance windows (e.g., 02:00–06:00).

• Scenario: Scheduled Antivirus Definition Updates via CrowdStrike Falcon

  • Context: The CrowdStrike Falcon sensor periodically downloads and installs small definition update packages. These updates often manifest as small, transient executables that match the YARA signature’s size constraints, triggering a false alert before the main agent service fully registers them.
  • Filter/Exclusion: Create an exclusion for file paths containing C:\ProgramData\CrowdStrike\ and process names matching csfalcon.exe or csccs.exe, specifically when the file hash matches known CrowdStrike update signatures stored in a whitelist database.

• Scenario: PowerShell Script Execution by DevOps CI/CD Pipelines (Azure DevOps)

  • Context: Azure DevOps agents on build servers often invoke small, compiled helper scripts or utility tools to manage artifact deployment. These tools are typically under 50KB and execute briefly, which aligns with the “small” heuristic of the ChSfxsmallv11 rule.