PESpinv01Cyberbob

Hunt Hypothesis

This detection identifies potential CyberBob malware activity by matching file signatures against the PESpinv01Cyberbob YARA rule to uncover stealthy threats that may evade standard signature-based defenses. Proactive hunting for this behavior in Azure Sentinel is essential because low-severity detections often indicate early-stage compromises or benign false positives that require manual validation to prevent missed opportunities for threat intelligence enrichment and incident response optimization.

YARA Rule

rule PESpinv01Cyberbob
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 01 68 60 E8 00 00 00 00 8B 1C 24 83 C3 12 81 2B E8 B1 06 00 FE 4B FD 82 2C 24 5C CB 46 00 0B E4 74 9E 75 01 C7 81 73 04 D7 7A F7 2F 81 73 19 77 00 43 B7 F6 C3 6B B7 00 00 F9 FF E3 C9 C2 08 00 A3 68 72 01 FF 5D 33 C9 41 E2 17 EB 07 EA EB 01 EB EB 0D FF }
	$a1 = { EB 01 68 60 E8 00 00 00 00 8B 1C 24 83 C3 12 81 2B E8 B1 06 00 FE 4B FD 82 2C 24 5C CB 46 00 0B E4 74 9E 75 01 C7 81 73 04 D7 7A F7 2F 81 73 19 77 00 43 B7 F6 C3 6B B7 00 00 F9 FF E3 C9 C2 08 00 A3 68 72 01 FF 5D 33 C9 41 E2 17 EB 07 EA EB 01 EB EB 0D FF E8 01 00 00 00 EA 5A 83 EA 0B FF E2 8B 95 B3 28 40 00 8B 42 3C 03 C2 89 85 BD 28 40 00 41 C1 E1 07 8B 0C 01 03 CA 8B 59 10 03 DA 8B 1B 89 9D D1 28 40 00 53 8F 85 C4 27 40 00 BB ?? 00 00 00 B9 A5 08 00 00 8D BD 75 29 40 00 4F 30 1C 39 FE CB E2 F9 68 2D 01 00 00 59 8D BD AA 30 40 00 C0 0C 39 02 E2 FA E8 02 00 00 00 FF 15 5A 8D 85 07 4F 56 00 BB 54 13 0B 00 D1 E3 2B C3 FF E0 E8 01 00 00 00 68 E8 1A 00 00 00 8D 34 28 B8 [4] 2B C9 83 C9 15 0F A3 C8 0F 83 81 00 00 00 8D B4 0D C4 28 40 00 8B D6 B9 10 00 00 00 AC 84 C0 74 06 C0 4E FF 03 E2 F5 E8 00 00 00 00 59 81 C1 1D 00 00 00 52 51 C1 E9 05 23 D1 FF }

condition:
		$a0 at pe.entry_point or $a1 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the PESpinv01Cyberbob detection rule, including suggested filters and exclusions:

• Scenario: Scheduled Antivirus or EDR Scans

  • Context: Enterprise endpoint protection tools like CrowdStrike Falcon, Microsoft Defender for Endpoint, or Symantec Endpoint Protection often spawn background processes (e.g., C:\Program Files\CrowdStrike\csagent.exe or MsMpEng.exe) that utilize the same behavioral patterns or file signatures detected by the Cyberbob YARA rule during their daily scan cycles.
  • Suggested Filter: Exclude alerts where the parent process name matches known security agents and the execution time aligns with defined maintenance windows (e.g., 02:00–04:00 local time).
  • Exclusion Logic: IF ParentProcess IN ['csagent.exe', 'MsMpEng.exe'] AND Time BETWEEN 02:00 AND 04:00 THEN Suppress.

• Scenario: Automated Software Deployment via SCCM/Intune

  • Context: During mass software updates, tools like Microsoft Endpoint Configuration Manager (SCCM) or Microsoft Intune deploy installation packages that trigger the specific file creation or registry modification patterns identified by the rule. This often occurs on workstations during business hours when users are logged in.
  • Suggested Filter: Create a whitelist for known deployment service accounts and exclude events originating from the System user account running under the SCCM agent process (ccmexec.exe).
  • Exclusion Logic: IF UserAccount = 'SYSTEM' AND ParentProcess = 'ccmexec.exe' THEN Suppress.

• Scenario: Legacy Backup Agents on File Servers

  • Context: On-critical file