AsCryptv01SToRM1

Hunt Hypothesis

This detection identifies potential fileless or memory-based malware activity through a specific YARA signature (AsCryptv01SToRM1) that targets known cryptographic or ransomware-like behaviors in endpoint processes. Proactive hunting for this signal within Azure Sentinel is essential to uncover early-stage threats that may evade traditional signature-based defenses, allowing the SOC team to investigate and contain low-severity anomalies before they escalate into critical incidents.

YARA Rule

rule AsCryptv01SToRM1
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 81 [6] 83 [7] 83 [2] E2 ?? EB }

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

Here are 5 specific false positive scenarios for the AsCryptv01SToRM1 detection rule, including suggested filters and exclusions:

• Antivirus Scheduled Scans on Encrypted Archives

  • Scenario: The enterprise endpoint protection suite (e.g., Microsoft Defender Antivirus or Symantec Endpoint Protection) performs a scheduled nightly scan. During this process, the engine extracts and scans large .7z or .zip archives containing encrypted payloads, which triggers the rule due to high entropy and compression signatures resembling ransomware behavior.
  • Filter/Exclusion: Create an exclusion for the specific AV service executable (e.g., MsMpEng.exe or Symantec Antivirus Server) when it accesses files with extensions .7z, .zip, or .rar within the designated “Backup” and “Data Archive” network shares.

• Database Backup Jobs Using Encrypted Streams

  • Scenario: A nightly SQL backup job executed by Veeam Backup & Replication or SQL Server Agent creates encrypted backup files (.bup or .bak) on a dedicated storage volume. The rule detects the rapid creation of high-entropy binary streams as potential encryption activity, mistaking legitimate backup operations for ransomware file locking.
  • Filter/Exclusion: Exclude processes running under the Veeam.Backup.Service.exe and sqlagent.exe accounts when they write to directories explicitly tagged with a “Backup” security group or specific folder paths (e.g., \\FileServer\Backups\SQL).

• Software Deployment via SCCM/Intune

  • Scenario: The IT operations team deploys new applications using Microsoft Endpoint Configuration Manager (SCCM) or Microsoft Intune. During the installation phase, the deployment engine extracts encrypted installer packages (`.msi