CelsiusCrypt21Z3r0

Hunt Hypothesis

This hunt hypothesis targets the specific execution patterns of the CelsiusCrypt21Z3r0 ransomware variant to identify early-stage infection indicators that may be missed by standard alerting thresholds. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to validate YARA rule efficacy and detect lateral movement or file encryption activities before they escalate into a full-scale incident, despite the current low severity classification.

YARA Rule

rule CelsiusCrypt21Z3r0
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 89 E5 83 EC 08 C7 04 24 01 00 00 00 FF 15 84 92 44 00 E8 C8 FE FF FF 90 8D B4 26 00 00 00 00 55 89 E5 83 EC 08 C7 04 24 02 00 00 00 FF 15 84 92 44 00 E8 A8 FE FF FF 90 8D B4 26 00 00 00 00 55 8B 0D C4 92 44 00 89 E5 5D FF E1 8D 74 26 00 55 8B 0D AC 92 44 00 89 E5 5D FF E1 90 90 90 90 55 89 E5 5D E9 77 C2 00 00 90 90 90 90 90 90 90 55 89 E5 83 EC 28 8B 45 10 89 04 24 E8 3F 14 01 00 48 89 45 FC 8B 45 0C 48 89 45 F4 8D 45 F4 89 44 24 04 8D 45 FC 89 04 24 E8 12 A3 03 00 8B 00 89 45 F8 8B 45 FC 89 45 F0 C6 45 EF 01 C7 45 E8 00 00 00 00 8B 45 E8 3B 45 F8 73 39 80 7D EF 00 74 33 8B 45 F0 89 44 24 04 8B 45 10 89 04 24 E8 1C 1A 01 00 89 C1 8B 45 08 8B 55 E8 01 C2 0F B6 01 3A 02 0F 94 C0 88 45 EF 8D 45 F0 FF 08 8D 45 E8 FF 00 EB BF 83 7D F0 00 74 34 80 7D EF 00 74 2E 8B 45 F0 89 44 24 04 8B 45 10 89 04 24 E8 DD 19 01 00 89 C1 8B 45 08 8B 55 F8 01 C2 0F B6 01 3A 02 0F 94 C0 88 45 EF 8D 45 F0 FF 08 EB C6 C7 44 24 04 00 00 00 00 8B 45 10 89 04 24 E8 AE 19 01 00 89 C1 8B 45 08 8B 55 F8 01 C2 0F B6 01 3A 02 7F 0C 0F B6 45 EF 83 E0 01 88 45 E7 EB 04 C6 45 E7 00 0F B6 45 E7 88 45 EF 0F B6 45 EF C9 C3 }
	$a1 = { 55 89 E5 83 EC 28 8B 45 10 89 04 24 E8 3F 14 01 00 48 89 45 FC 8B 45 0C 48 89 45 F4 8D 45 F4 89 44 24 04 8D 45 FC 89 04 24 E8 12 A3 03 00 8B 00 89 45 F8 8B 45 FC 89 45 F0 C6 45 EF 01 C7 45 E8 00 00 00 00 8B 45 E8 3B 45 F8 73 39 80 7D EF 00 74 33 8B 45 F0 89 44 24 04 8B 45 10 89 04 24 E8 1C 1A 01 00 89 C1 8B 45 08 8B 55 E8 01 C2 0F B6 01 3A 02 0F 94 C0 88 45 EF 8D 45 F0 FF 08 8D 45 E8 FF 00 EB BF 83 7D F0 00 74 34 80 7D EF 00 74 2E 8B 45 F0 89 44 24 04 8B 45 10 89 04 24 E8 DD 19 01 00 89 C1 8B 45 08 8B 55 F8 01 C2 0F B6 01 3A 02 0F 94 C0 88 45 EF 8D 45 F0 FF 08 EB C6 C7 44 24 04 00 00 00 00 8B 45 10 89 04 24 E8 AE 19 01 00 89 C1 8B 45 08 8B 55 F8 01 C2 0F B6 01 3A 02 7F 0C 0F B6 45 EF 83 E0 01 88 45 E7 EB 04 C6 45 E7 00 0F B6 45 E7 88 45 EF 0F B6 45 EF C9 C3 }

condition:
		$a0 at pe.entry_point or $a1
}

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

False Positive Scenarios for Rule: CelsiusCrypt21Z3r0

• Scenario: Microsoft Defender Antivirus Real-Time Scan

  • Context: The YARA rule detects the cryptographic signature patterns associated with the MsMpEng.exe process during its real-time scanning of user documents. This often triggers when large files (e.g., PDFs or Office docs) are opened in a high-traffic directory like C:\Users\Public\Documents.
  • Suggested Filter: Exclude the process path C:\Program Files\Windows Defender\MsMpEng.exe and restrict detection to non-system directories (exclude paths starting with C:\ProgramData\Microsoft\Windows Defender\).

• Scenario: Scheduled Backup Job via Veeam Agent

  • Context: A nightly scheduled task running the Veeam Backup & Replication agent (VeeamAgent.exe) compresses and encrypts local user profiles. The rule flags the encryption routine used by Veeam as a potential “CelsiusCrypt” activity because it utilizes similar hashing algorithms during the backup stream creation.
  • Suggested Filter: Create an exclusion for the process name VeeamAgent.exe when executed under the system account (NT AUTHORITY\SYSTEM) and occurring between 01:00 and 05:00 local time.

• Scenario: Adobe Acrobat Pro Document Processing

  • Context: Users in the Finance department frequently use AcroRd32.exe (Adobe Reader) or Acrobat.exe to open encrypted PDF invoices. The rule triggers on the internal cryptographic module used by Adobe for digital signatures and file encryption, mistaking it for the target malware behavior.
  • Suggested Filter: Exclude processes matching *AcroRd32.exe and *Acrobat.exe located