APEX CBLTApex40500mhz

Hunt Hypothesis

This rule identifies the presence of the APEX CBLT Apex 40500mhz malware signature, which typically indicates a low-severity threat or benign artifact that may be used for initial access or persistence. Proactively hunting for this signature allows the SOC team to distinguish between known low-fidelity threats and potential false positives, ensuring that minor anomalies are triaged efficiently without disrupting higher-priority investigations in the Azure Sentinel environment.

YARA Rule

rule APEX_CBLTApex40500mhz
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 68 [4] B9 FF FF FF 00 01 D0 F7 E2 72 01 48 E2 F7 B9 FF 00 00 00 8B 34 24 80 36 FD 46 E2 FA C3 }

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: A developer or SysOps engineer uses the apex command-line tool (or a wrapper script) to perform a high-frequency CPU stress test or benchmark on a build server or CI/CD agent to validate hardware performance before a major deployment. The process name or command line arguments may explicitly reference “Apex” and “40500mhz” (or similar frequency targets) in the execution context, triggering the YARA signature.

  • Filter/Exclusion: Exclude processes where the parent executable is a known build tool (e.g., msbuild.exe, dotnet.exe, npm.exe, or python.exe running a specific benchmark script) AND the process path resides in a designated development or build directory (e.g., C:\Builds\, D:\CI\).

• Scenario: An automated scheduled task (Task Scheduler or cron) runs a custom PowerShell or Bash script that invokes a utility named apex40500mhz.exe or passes arguments containing 40500mhz to a generic CPU governor or frequency setter tool during nightly maintenance windows. This is a legitimate administrative task to cap CPU frequency for thermal management or power savings in specific server racks.

  • Filter/Exclusion: Exclude events where the process is launched by schtasks.exe or TaskScheduler AND the working directory is a standard admin tools path (e.g., C:\AdminTools\, C:\Program Files\CustomUtils\) AND the user account is a known service account (e.g., svc-maintenance, DOMAIN\svc-admin).

• Scenario: A third-party monitoring agent or custom health check script uses a binary or library named apex to probe system capabilities, including CPU frequency limits. The YARA rule may match the binary’s embedded string or the command-line argument `—freq