Detection of Cryptolocker Samples

Hunt Hypothesis

This detection identifies the execution of known Cryptolocker ransomware samples, signaling an adversary’s attempt to encrypt critical data and disrupt operations within the Azure environment. Proactively hunting for these indicators allows the SOC team to validate existing rule efficacy and uncover potential lateral movement or early-stage infection before a full-scale encryption event occurs.

YARA Rule

rule CryptoLocker_set1
{
meta:
	author = "Christiaan Beek, [email protected]"
	date = "2014-04-13"
	description = "Detection of Cryptolocker Samples"
	
strings:
	$string0 = "static"
	$string1 = " kscdS"
	$string2 = "Romantic"
	$string3 = "CompanyName" wide
	$string4 = "ProductVersion" wide
	$string5 = "9%9R9f9q9"
	$string6 = "IDR_VERSION1" wide
	$string7 = "  </trustInfo>"
	$string8 = "LookFor" wide
	$string9 = ":n;t;y;"
	$string10 = "        <requestedExecutionLevel level"
	$string11 = "VS_VERSION_INFO" wide
	$string12 = "2.0.1.0" wide
	$string13 = "<assembly xmlns"
	$string14 = "  <trustInfo xmlns"
	$string15 = "srtWd@@"
	$string16 = "515]5z5"
	$string17 = "C:\\lZbvnoVe.exe" wide
condition:
	12 of ($string*)
}

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 18 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the Detection of Cryptolocker Samples rule in an enterprise environment, along with suggested filters or exclusions:

• Enterprise Backup and Snapshot Operations

  • Scenario: Scheduled backup agents (e.g., Veeam Agent, Commvault, or Azure Backup) performing nightly incremental backups. These tools often generate new files, modify metadata, and encrypt data streams in a manner that mimics ransomware behavior, particularly when handling large volumes of user data on file servers.
  • Filter/Exclusion: Create an exclusion rule for the specific Service Accounts used by backup software (e.g., DOMAIN\svc-veeam-backup) and whitelist the known executable paths of the backup agents (e.g., C:\Program Files\Veeam\Backup and Replication\Agent\). Additionally, exclude file extensions commonly targeted by backups but not typically encrypted in real-time by ransomware during this phase (e.g., .vbk, .vib).

• Software Distribution and Patch Management

  • Scenario: Automated deployment of software updates or new applications using tools like Microsoft Endpoint Configuration Manager (MECM/SCCM) or Ivanti. During a mass deployment, these agents may download large installation packages, extract compressed archives (creating thousands of files), and modify system binaries, triggering high file creation rates similar to an encryption attack.
  • Filter/Exclusion: Whitelist the specific process names associated with patch management (e.g., ccmexec.exe, ivanti-agent.exe) running under the context of the deployment service account (DOMAIN\svc-sccm-deploy). Exclude events originating from known software distribution folders such as C:\Windows\CCMCache or C:\ProgramData\Ivanti.

• Antivirus and EDR Real-Time Scanning

  • Scenario: The organization