AaseCrypterbysantasdad

Hunt Hypothesis

This detection identifies potential fileless or memory-based malware activity characterized by specific cryptographic patterns associated with the “AaseCrypterbysantasdad” signature within Azure Sentinel’s YARA engine. Proactive hunting for this behavior is essential to uncover stealthy initial access attempts that may bypass traditional network perimeter defenses, allowing the SOC team to validate early-stage compromises before they escalate into full-scale ransomware incidents.

YARA Rule

rule AaseCrypterbysantasdad
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 83 C4 F0 53 B8 A0 3E 00 10 E8 93 DE FF FF 68 F8 42 00 10 E8 79 DF FF FF 68 00 43 00 10 68 0C 43 00 10 E8 42 DF FF FF 50 E8 44 DF FF FF A3 98 66 00 10 83 3D 98 66 00 10 00 75 13 6A 00 68 18 43 00 10 68 1C 43 00 10 6A 00 E8 4B DF FF FF 68 2C 43 00 10 68 0C 43 [4] DF FF FF 50 E8 0E DF FF FF A3 94 66 00 10 83 3D 94 66 00 10 00 75 13 6A 00 68 18 43 00 10 68 38 43 00 10 6A 00 E8 15 DF FF FF 68 48 43 00 10 68 0C 43 00 10 E8 D6 DE FF FF 50 E8 D8 DE FF FF A3 A0 66 00 10 83 3D A0 66 00 10 00 75 13 6A 00 68 18 43 00 10 68 58 43 00 10 6A 00 E8 DF DE FF FF 68 6C 43 00 10 68 0C 43 00 10 E8 A0 DE FF FF 50 E8 A2 DE FF FF }

condition:
		$a0
}

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 AaseCrypterbysantasdad detection rule, including targeted filters and exclusions:

• Scenario: Microsoft Defender Antivirus Real-Time Scanning

  • Context: The YARA rule may trigger when MsMpEng.exe (Microsoft Defender) scans encrypted user documents in real-time. As the engine decrypts files on-the-fly to inspect them, the temporary memory structures or I/O operations can mimic the cryptographic behavior of the target malware.
  • Filter/Exclusion: Add a process exclusion for C:\Program Files\Windows Defender\MsMpEng.exe and its child processes. Alternatively, configure the rule to ignore events where the parent process is MsMpEng.exe.

• Scenario: Scheduled Backup Jobs via Veeam or Commvault

  • Context: Enterprise backup agents (e.g., VeeamAgent.exe or commvault_agent) frequently encrypt large datasets during nightly maintenance windows. The heavy cryptographic operations performed by these agents during the compression and encryption phase often match the signature logic of AaseCrypterbysantasdad.
  • Filter/Exclusion: Create a time-based exclusion for the rule between 01:00 AM and 05:00 AM on weekdays, or explicitly exclude processes named VeeamAgent.exe, vssrplc.exe, and commvault_agent from triggering this specific alert.

• Scenario: Office 365 ProPlus Auto-Update Mechanism

  • Context: When Microsoft Office updates itself in the background, the OfficeClickToRun.exe process often downloads and installs encrypted update packages. The installation routine involves temporary file creation with high entropy (encryption-like) that can confuse the YARA signature.
  • Filter/Exclusion: Exclude events where the command line