FlyCrypter10ut1lz

Hunt Hypothesis

This hypothesis targets the presence of the FlyCrypter ransomware variant, which typically encrypts files and appends a specific extension to extort victims. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify early-stage infections or dormant payloads on endpoints before the encryption process begins, minimizing potential data loss and operational downtime.

YARA Rule

rule FlyCrypter10ut1lz
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 53 56 57 55 BB 2C [2] 44 BE 00 30 44 44 BF 20 [2] 44 80 7B 28 00 75 16 83 3F 00 74 11 8B 17 89 D0 33 D2 89 17 8B E8 FF D5 83 3F 00 75 EF 83 3D 04 30 44 44 00 74 06 FF 15 58 30 44 44 80 7B 28 02 75 0A 83 3E 00 75 05 33 C0 89 43 0C FF 15 20 30 44 44 80 7B 28 01 76 05 83 3E 00 74 22 8B 43 10 85 C0 74 1B FF 15 18 30 44 44 8B 53 10 8B 42 10 3B 42 04 74 0A 85 C0 74 06 50 E8 2F FA FF FF FF 15 24 30 44 44 80 7B 28 01 75 03 FF 53 24 80 7B 28 00 74 05 E8 35 FF FF FF 83 3B 00 75 17 83 3D 10 [2] 44 00 74 06 FF 15 10 [2] 44 8B 06 50 E8 51 FA FF FF 8B 03 56 8B F0 8B FB B9 0B 00 00 00 F3 A5 5E E9 73 FF FF FF 5D 5F 5E 5B C3 A3 00 30 44 44 E8 26 FF FF FF C3 }
	$a1 = { 55 8B EC 83 C4 F0 53 B8 18 22 44 44 E8 7F F7 FF FF E8 0A F1 FF FF B8 09 00 00 00 E8 5C F1 FF FF 8B D8 85 DB 75 05 E8 85 FD FF FF 83 FB 01 75 05 E8 7B FD FF FF 83 FB 02 75 05 E8 D1 FD FF FF 83 FB 03 75 05 E8 87 FE FF FF 83 FB 04 75 05 E8 5D FD FF FF 83 FB 05 75 05 E8 B3 FD FF FF 83 FB 06 75 05 E8 69 FE FF FF 83 FB 07 75 05 E8 5F FE FF FF 83 FB 08 75 05 E8 95 FD FF FF 83 FB 09 75 05 E8 4B FE FF FF 5B E8 9D F2 FF FF 90 }

condition:
		$a0 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

• Scenario: A developer or DevOps engineer executes a custom Python or Go-based encryption utility (e.g., aes_encrypt.py or go_encrypt.exe) to secure configuration files or database dumps before archiving them to cold storage. The binary may contain strings like “FlyCrypt” or specific algorithm identifiers that match the YARA rule’s pattern.
  • Filter/Exclusion: Exclude processes where the parent process is a known scripting interpreter (e.g., python.exe, go.exe, node.exe) or where the command line contains arguments like --encrypt, --archive, or --backup.
• Scenario: An IT administrator uses a legitimate file encryption tool like AxCrypt or 7-Zip with AES-256 encryption to share sensitive documents with external partners. The executable or its helper libraries may trigger the rule if the YARA signature matches common cryptographic library strings or specific version identifiers.
  • Filter/Exclusion: Exclude processes named axcrypt.exe, 7z.exe, or 7za.exe when the working directory is within a designated share folder (e.g., \\fileserver\shares\external) or when the command line includes flags like -p (password) or -mhe=on (header encryption).
• Scenario: A scheduled maintenance job runs a custom PowerShell script or batch file to encrypt log files or temporary data before deletion. The script might invoke a small native helper binary (e.g., log_encryptor.dll or enc_helper.exe) that is not on the standard allowlist but is part of the internal toolchain.
  • Filter/Exclusion: Exclude processes where the parent is powershell.exe or cmd.exe and the command line contains keywords like encrypt, cipher, or secure, specifically if the process path resides in a known internal tools directory