Rovnix downloader with sinkhole checks

Hunt Hypothesis

This detection identifies the initial execution of a Rovnix downloader that attempts to communicate with known malicious infrastructure, specifically validating connections against sinkholed domains. Proactively hunting for this behavior in Azure Sentinel is critical because it allows SOC teams to identify early-stage infection vectors and validate endpoint posture before the malware progresses to more complex payload delivery stages.

YARA Rule

rule rovnix_downloader : downloader 
{
	meta:
		author="Intel Security"
		description="Rovnix downloader with sinkhole checks"
		reference = "https://blogs.mcafee.com/mcafee-labs/rovnix-downloader-sinkhole-time-checks/"
	strings:
			$sink1= "control"
			$sink2 = "sink"
			$sink3 = "hole"
			$sink4= "dynadot"
			$sink5= "block"
			$sink6= "malw"
			$sink7= "anti"
			$sink8= "googl"
			$sink9= "hack"
			$sink10= "trojan"
			$sink11= "abuse"
			$sink12= "virus"
			$sink13= "black"
			$sink14= "spam"
			$boot= "BOOTKIT_DLL.dll"
			$mz = { 4D 5A }
	condition:
		$mz in (0..2) and all of ($sink*) and $boot
}

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 16 string patterns in its detection logic.

References

• https://blogs.mcafee.com/mcafee-labs/rovnix-downloader-sinkhole-time-checks/
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the Rovnix downloader with sinkhole checks rule in an enterprise environment, including suggested filters and exclusions:

• Enterprise Software Update Agents (e.g., SCCM/Intune)

  • Scenario: Scheduled maintenance windows where tools like Microsoft Endpoint Configuration Manager (SCCM) or Intune agents download large definition updates or patch bundles. These downloads often initiate connections to external update servers that may be flagged by sinkhole checks if the destination IP is dynamically assigned or part of a known CDN range not yet whitelisted in the sinkhole database.
  • Filter/Exclusion: Create an exclusion for processes running under C:\Program Files\Microsoft Intune Management Extension and C:\Windows\CCM\ccmexec.exe. Additionally, filter out traffic destined for Microsoft Update CDN IP ranges (e.g., 13.64.0.0/12) during the defined maintenance window hours (e.g., 02:00–05:00 UTC).

• Antivirus and EDR Definition Refreshes

  • Scenario: Automated background jobs for security suites like CrowdStrike Falcon, SentinelOne, or Symantec Endpoint Protection. These agents frequently download fresh threat intelligence signatures from vendor update servers. The Rovnix rule may trigger if the downloader component uses a generic user-agent string that mimics standard browser behavior, causing it to be misidentified as a suspicious standalone executable rather than a trusted security agent.
  • Filter/Exclusion: Exclude processes with known digital signatures from major EDR vendors (e.g., CrowdStrike, SentinelOne). Implement a filter to ignore events where the User-Agent header contains specific vendor identifiers (e.g., Falcon-Update-Agent) and the destination port is strictly 443.

• **IT Administration Script Execution