Detection of CryptoLocker Variants

Hunt Hypothesis

This detection rule identifies adversary behavior consistent with CryptoLocker variants by monitoring for specific file encryption patterns and ransom note generation indicative of active ransomware campaigns. Proactively hunting for these indicators in Azure Sentinel is critical to validate early-stage infection signals before they escalate into widespread data unavailability, even when initial alert severity remains low.

YARA Rule

rule CryptoLocker_rule2
{
meta:
	author = "Christiaan Beek, [email protected]"
	date = "2014-04-14"
	description = "Detection of CryptoLocker Variants"
strings:
	$string0 = "2.0.1.7" wide
	$string1 = "    <security>"
	$string2 = "Romantic"
	$string3 = "ProductVersion" wide
	$string4 = "9%9R9f9q9"
	$string5 = "IDR_VERSION1" wide
	$string6 = "button"
	$string7 = "    </security>"
	$string8 = "VFileInfo" wide
	$string9 = "LookFor" wide
	$string10 = "      </requestedPrivileges>"
	$string11 = " uiAccess"
	$string12 = "  <trustInfo xmlns"
	$string13 = "last.inf"
	$string14 = " manifestVersion"
	$string15 = "FFFF04E3" wide
	$string16 = "3,31363H3P3m3u3z3"
condition:
	12 of ($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 17 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the Detection of CryptoLocker Variants rule in an enterprise environment, along with recommended filters or exclusions:

• Enterprise Antivirus Scheduled Scans and Updates

  • Scenario: The organization’s endpoint protection suite (e.g., CrowdStrike Falcon, SentinelOne, or Microsoft Defender for Endpoint) performs a scheduled full system scan or pushes a signature update. These processes often spawn high-volume file creation, modification, and encryption activities that mimic the behavior of ransomware encrypting user data.
  • Filter/Exclusion: Create an exclusion based on the specific process names (e.g., C:\Program Files\CrowdStrike\csfalcon.exe, MsMpEng.exe) and restrict the rule to trigger only when these processes are operating outside of their defined maintenance windows or if the file modification rate exceeds a baseline threshold for that specific service account.

• Backup Agent Data Synchronization

  • Scenario: Enterprise backup agents (such as Veeam Backup & Replication, Commvault, or Rubrik) initiate a scheduled incremental or full backup job. During this process, the agent reads and writes large volumes of data to local staging folders before transferring them to the repository, often triggering file encryption and hashing events that look like ransomware activity.
  • Filter/Exclusion: Exclude traffic originating from known backup service accounts (e.g., DOMAIN\VeeamService, DOMAIN\CommvaultAgent) and specific process paths associated with backup software. Additionally, filter out alerts where the destination path is a dedicated backup staging directory rather than user home directories or critical application data stores.

• Software Deployment via Configuration Management Tools

  • Scenario: Automated deployment tools like Microsoft SCCM (Configuration Manager), Ansible, or Chef execute scheduled software updates or patch installations