SPECb2

Hunt Hypothesis

This detection identifies potential malicious activity by leveraging a specific YARA signature (SPECb2) to match known file characteristics against endpoint telemetry in Azure Sentinel. Proactive hunting for this indicator is essential to uncover early-stage threats that may evade standard alerting thresholds, allowing the SOC team to validate and contain low-severity anomalies before they escalate into broader incidents.

YARA Rule

rule SPECb2
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 57 51 53 E8 [4] 5D 8B C5 81 ED [4] 2B 85 [4] 83 E8 09 89 85 [4] 0F B6 }

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 3-5 specific false positive scenarios for the SPECb2 detection rule, including targeted filters and exclusions suitable for an enterprise environment:

• Scenario: Scheduled Antivirus Definition Updates via PowerShell

  • Context: Enterprise endpoints often run scheduled tasks (e.g., Task Scheduler or System Center Configuration Manager) that invoke powershell.exe to download and install the latest definition packs from vendors like Microsoft Defender for Endpoint or CrowdStrike Falcon. These scripts frequently execute complex logic involving file writes, registry modifications, and network calls that mimic the behavioral patterns SPECb2 is designed to catch.
  • Filter/Exclusion: Create a YARA exception or SIEM filter excluding alerts where the parent process is TaskScheduler.exe (or ccmexec.exe) AND the command line contains keywords such as “UpdateDefinition” or specific vendor hash signatures (e.g., Hash: 8a3f...).

• Scenario: CI/CD Pipeline Artifact Deployment by Azure DevOps Agents

  • Context: In a DevOps environment, build agents running on dedicated servers often use tools like Azure DevOps Agent Service or GitHub Actions Runner to deploy artifacts. These agents frequently spawn temporary processes that extract archives and execute configuration scripts using standard interpreters (like python.exe or node.exe) which trigger SPECb2 due to the rapid creation of new executables in temporary directories.
  • Filter/Exclusion: Exclude alerts originating from specific service accounts (e.g., NT SERVICE\AzureDevOpsAgentService) and restrict the detection scope to exclude paths within known build artifact directories, such as C:\ProgramData\Microsoft\Azure DevOps or D:\builds\temp.

• Scenario: Automated Reporting Tool Execution by Tableau Server

  • Context: Business Intelligence platforms