ICrypt10byBuGGz

Hunt Hypothesis

This hunt hypothesis targets the presence of the ICrypt10 ransomware variant by leveraging a specific YARA signature to identify its unique code patterns within Azure Sentinel workloads. Proactive hunting for this threat is essential to detect early-stage infections before encryption begins, allowing the SOC team to isolate affected assets and mitigate potential data loss despite the rule’s current low severity classification.

YARA Rule

rule ICrypt10byBuGGz
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 83 C4 EC 53 56 57 33 C0 89 45 EC B8 70 3B 00 10 E8 3C FA FF FF 33 C0 55 68 6C 3C 00 10 64 FF 30 64 89 20 6A 0A 68 7C 3C 00 10 A1 50 56 00 10 50 E8 D8 FA FF FF 8B D8 53 A1 50 56 00 10 50 E8 0A FB FF FF 8B F8 53 A1 50 56 00 10 50 E8 D4 FA FF FF 8B D8 53 E8 D4 FA FF FF 8B F0 85 F6 74 26 8B D7 4A B8 64 56 00 10 E8 25 F6 FF FF B8 64 56 00 10 E8 13 F6 FF FF 8B CF 8B D6 E8 E6 FA FF FF 53 E8 90 FA FF FF 8D 4D EC BA 8C 3C 00 10 A1 64 56 00 10 E8 16 FB FF FF 8B 55 EC B8 64 56 00 10 E8 C5 F4 FF FF B8 64 56 00 10 E8 DB F5 FF FF E8 56 FC FF FF 33 C0 5A 59 59 64 89 10 68 73 3C 00 10 8D 45 EC E8 4D F4 FF FF C3 E9 E3 EE FF FF EB F0 5F 5E 5B E8 4D F3 FF FF 00 53 45 54 [4] 00 FF FF FF FF 08 00 00 00 76 6F 74 72 65 63 6C 65 }

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 5 specific false positive scenarios for the ICrypt10byBuGGz detection rule, including targeted filters and exclusions:

• Scenario: Microsoft Office Background Telemetry & Auto-Save

  • Context: The YARA rule likely flags encryption routines within WINWORD.EXE or EXCEL.EXE when they perform background auto-save operations that utilize local temporary file locking and encryption. This is common during large document edits where the application encrypts cache files before committing to disk.
  • Filter/Exclusion: Exclude process paths containing C:\Program Files\Microsoft Office\root\Office16\ and specifically target executable names WINWORD.EXE, EXCEL.EXE, and OUTLOOK.EXE. Additionally, filter out events where the command line contains arguments related to “AutoRecover” or “TempFile”.

• Scenario: Scheduled Antivirus Real-Time Scanning

  • Context: Enterprise Endpoint Detection and Response (EDR) agents like CrowdStrike Falcon or Microsoft Defender for Endpoint often trigger this rule when their real-time protection engine scans encrypted archives (.zip, .7z) in shared network folders. The scanning process involves reading and decrypting file streams that match the YARA signature’s entropy thresholds.
  • Filter/Exclusion: Create a whitelist exclusion for processes named FalconSensor.exe, MsMpEng.exe, or DefenderService.exe. Filter events where the parent process is one of these security agents and the action type is “Scan” rather than “Execution”.

• Scenario: Backup Agent Encryption Jobs (Veeam/Acronis)

  • Context: Scheduled backup jobs running via tools like Veeam Backup & Replication or Acronis Cyber Protect frequently encrypt data streams before writing them to the repository. The rule