Erebus Ransomware

Hunt Hypothesis

This detection rule identifies the initial execution patterns and file encryption behaviors characteristic of the Erebus ransomware family within Azure Sentinel. Proactively hunting for these indicators is critical to validate early-stage infection signals before they escalate into high-severity incidents that could disrupt critical business operations.

YARA Rule

rule Erebus: ransom
{
	meta:
		description = "Erebus Ransomware"
		author = "Joan Soriano / @joanbtl"
		date = "2017-06-23"
		version = "1.0"
		MD5 = "27d857e12b9be5d43f935b8cc86eaabf"
		SHA256 = "0b7996bca486575be15e68dba7cbd802b1e5f90436ba23f802da66292c8a055f"
		ref1 = "http://blog.trendmicro.com/trendlabs-security-intelligence/erebus-resurfaces-as-linux-ransomware/"
	strings:
		$a = "/{5f58d6f0-bb9c-46e2-a4da-8ebc746f24a5}//log.log"
		$b = "EREBUS IS BEST."
	condition:
		all of them
}

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

Here are 4 specific false positive scenarios for the Erebus Ransomware detection rule in an enterprise environment, including suggested filters and exclusions:

• Enterprise Backup Agent File Locking and Encryption

  • Scenario: The backup agent (e.g., Veeam Backup & Replication, Commvault, or Rubrik) performs a scheduled incremental backup. During this process, the agent locks existing files and may re-encrypt metadata or create new encrypted snapshots, mimicking the rapid file modification and extension changes typical of ransomware activity.
  • Filter/Exclusion: Whitelist the specific service accounts used by the backup agents (e.g., svc-vbackup, commvault-agent) and exclude processes running from the vendor’s installation directory (e.g., C:\Program Files\Veeam\Backup & Replication\).

• Scheduled Antivirus Full System Scans

  • Scenario: A scheduled full system scan by an Endpoint Detection and Response (EDR) or AV solution (e.g., CrowdStrike Falcon, SentinelOne, or Microsoft Defender for Endpoint) triggers aggressive file scanning. This involves opening, reading, and potentially quarantining or re-signing thousands of files simultaneously, which can generate high-volume file I/O events that resemble ransomware encryption behavior.
  • Filter/Exclusion: Create a rule exclusion based on the parent process name (e.g., C:\Program Files\CrowdStrike\fsagent.exe) and correlate with the specific scheduled task name in Windows Task Scheduler (e.g., \Microsoft\CrowdStrike\Falcon Sensor).

• Database Maintenance and Index Rebuild Operations

  • Scenario: Database administrators run maintenance scripts using tools like SQL Server Management Studio (SSMS) or Oracle Enterprise Manager. These tasks often involve rebuilding indexes, updating statistics, or performing