FreeCryptor01build002GlOFF

Hunt Hypothesis

This rule identifies the presence of the FreeCryptor ransomware variant, a threat actor behavior that typically involves encrypting files to extort victims. Proactively hunting for this signature allows the SOC team to detect early-stage infections or dormant payloads in Azure Sentinel before the encryption process begins, minimizing potential data loss and operational disruption.

YARA Rule

rule FreeCryptor01build002GlOFF
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 8B 04 24 40 90 83 C0 07 80 38 90 90 74 02 EB FF 90 68 27 [2] 00 64 FF 35 00 00 00 00 64 89 25 00 00 00 00 FF E4 90 8B 04 24 64 A3 00 00 00 00 8B 64 24 08 90 83 C4 08 }

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

• Scenario: A developer or DevOps engineer manually compiles a custom encryption utility (e.g., using OpenSSL or a custom C++ wrapper) for a specific data migration task, where the binary retains the original YARA signature strings (e.g., FreeCryptor01build002GlOFF) in its debug symbols or version information.
  • Filter/Exclusion: Exclude files located in known development directories (e.g., C:\Users\<User>\Projects\, D:\Builds\) or binaries with a valid digital signature from the internal Code Signing Authority.
• Scenario: An IT administrator deploys a legacy backup or archival tool (e.g., a custom wrapper around 7-Zip or WinRAR) that was built from a source tree containing the FreeCryptor01build002GlOFF string in its resource section, and the tool is executed from a standard system path like C:\Program Files\BackupTools\.
  • Filter/Exclusion: Exclude processes running from C:\Program Files\ or C:\Program Files (x86)\ where the parent process is a known service host (e.g., svchost.exe) or a scheduled task service (Taskeng.exe).
• Scenario: A security team or QA engineer is testing the YARA rule itself by executing a known benign sample (e.g., a test executable) in a dedicated test VM or isolated folder, and the sample is copied to a shared network drive or local temp folder for analysis.
  • Filter/Exclusion: Exclude files in temporary directories (%TEMP%, C:\Windows\Temp) or specific test directories (e.g., C:\QA\YaraTests\) and exclude processes where the command line contains test-specific arguments (e.g., --test-mode).
• Scenario: A