unknown

Hunt Hypothesis

This detection targets adversaries attempting to execute obfuscated or unsigned scripts within the environment by leveraging YARA signatures to identify suspicious code patterns that may evade standard signature-based defenses. Proactive hunting for this behavior in Azure Sentinel is essential to uncover stealthy initial access attempts before they escalate into full-blown incidents, ensuring early visibility into potential lateral movement vectors.

YARA Rule

rule unknown
{
meta:
	author = "@patrickrolsen"
	reference = "Unknown POS"
strings:
	$s1 = "a.exe" wide
	$s2 = "Can anyone test" wide
	$s3 = "I m in computer class now" wide
condition:
	uint16(0) == 0x5A4D and 3 of ($s*)
}

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 3 string patterns in its detection logic.

References

• Unknown POS
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the “unknown” detection rule in an enterprise environment, including suggested filters and exclusions:

• Antivirus Definition Updates via Windows Update

  • Scenario: The Microsoft Defender Antivirus service (MsMpEng.exe) or third-party agents like CrowdStrike Falcon periodically download and extract new signature packs. These extraction processes often spawn temporary child processes with generic names (e.g., tmp_*.exe or update_helper.exe) running from the %TEMP% directory, which may lack a known digital signature hash in the initial detection window.
  • Filter/Exclusion: Create an exclusion for process paths containing \ProgramData\Microsoft\Windows Defender\Updates\Temp\ and parent processes named MsMpEng.exe or CfSvc.exe. Additionally, exclude any process where the ImageFileName matches the regex pattern ^update.*\.exe$ when running under the context of a scheduled task named “WindowsUpdate” or “CrowdStrike Update”.

• Enterprise Software Deployment via SCCM/MECM

  • Scenario: During business hours, System Center Configuration Manager (SCCM) or Microsoft Endpoint Configuration Manager pushes application updates. The deployment engine (ccmexec.exe) often launches a generic command interpreter (cmd.exe or powershell.exe) to execute installation scripts that unpack temporary installers into the local user profile before execution. These ephemeral processes may appear as “unknown” if they are not yet cataloged in the asset inventory.
  • Filter/Exclusion: Filter for events where the parent process is ccmexec.exe and the command line contains arguments referencing specific deployment packages (e.g., -PackageID, -CollectionID). Exclude processes spawned by ccmexec.exe that run from paths under C:\Windows\CCMCache\ or `C:\Program Files\