FreeCryptor01build001GlOFF

Hunt Hypothesis

This detection identifies potential early-stage ransomware activity by flagging the presence of the specific FreeCryptor01 build executable, which may indicate an adversary’s initial reconnaissance or payload deployment phase. Proactively hunting for this signature in Azure Sentinel allows the SOC team to isolate and investigate these low-severity indicators before they escalate into full-scale encryption events that could compromise critical data assets.

YARA Rule

rule FreeCryptor01build001GlOFF
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 8B 04 24 40 90 83 C0 07 80 38 90 90 74 02 EB FF 68 26 [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

Here are 5 specific false positive scenarios for the FreeCryptor01build001GlOFF detection rule, including suggested filters and exclusions:

• Scenario: Automated Backup Encryption by Veeam

  • Context: The enterprise utilizes Veeam Backup & Replication to perform nightly incremental backups. During the encryption phase of the backup chain, the Veeam service process (vbrservice.exe) invokes a cryptographic engine that matches the signature of the FreeCryptor build 001.
  • Filter/Exclusion: Add an exclusion for the executable path C:\Program Files\Veeam\Backup and Replication Enterprise Edition\vbrservice.exe where the parent process is vbrsvc.exe. Alternatively, filter out alerts occurring between 02:00 and 04:00 UTC on weekdays.

• Scenario: Endpoint Protection Scanning by CrowdStrike Falcon

  • Context: The CrowdStrike Falcon sensor performs a real-time scan of the C:\Users\Public\Documents directory. When scanning encrypted archives (e.g., .7z or .gpg files), the scanner’s internal decryption module triggers the YARA rule, mistaking its own cryptographic activity for the target malware.
  • Filter/Exclusion: Exclude any detection where the parent process is C:\Program Files\CrowdStrike\Falcon Sensor\csfalcon.exe. Ensure the exclusion applies specifically when the file extension of the scanned object is .7z, .gpg, or .enc.

• Scenario: Scheduled Group Policy Script Execution

  • Context: A scheduled Group Policy Object (GPO) runs a PowerShell script (EncryptUserProfiles.ps1) every morning at 08:30 to encrypt user profile data on domain-joined workstations. The script calls