Detects Petya Ransomware

Hunt Hypothesis

This rule identifies adversary behavior characterized by the specific encryption patterns and system file modifications indicative of a Petya ransomware infection within Azure Sentinel. Proactively hunting for this signature is essential to validate detection efficacy against known variants and ensure rapid containment before the malware can propagate across critical workloads, despite its current low severity classification.

YARA Rule

rule Petya_Ransomware {
	meta:
		description = "Detects Petya Ransomware"
		author = "Florian Roth"
		reference = "http://www.heise.de/newsticker/meldung/Erpressungs-Trojaner-Petya-riegelt-den-gesamten-Rechner-ab-3150917.html"
		date = "2016-03-24"
		hash = "26b4699a7b9eeb16e76305d843d4ab05e94d43f3201436927e13b3ebafa90739"
	strings:
		$a1 = "<description>WinRAR SFX module</description>" fullword ascii

		$s1 = "BX-Proxy-Manual-Auth" fullword wide
		$s2 = "<!--The ID below indicates application support for Windows 10 -->" fullword ascii
		$s3 = "X-HTTP-Attempts" fullword wide
		$s4 = "@CommandLineMode" fullword wide
		$s5 = "X-Retry-After" fullword wide
	condition:
		uint16(0) == 0x5a4d and filesize < 500KB and $a1 and 3 of ($s*)
}

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 6 string patterns in its detection logic.

References

False Positive Guidance

Here are 4 specific false positive scenarios for the Detects Petya Ransomware rule in an enterprise environment, along with suggested filters and exclusions:

Windows Update Agent Performing Scheduled Maintenance
- Scenario: The Windows Update service (wuauserv) or a third-party patch management tool like Microsoft Endpoint Configuration Manager (MECM/SCCM) executes a scheduled task to apply critical security updates. This process often involves writing large files to the system drive and modifying boot configuration data (BCD), which mimics Petya’s behavior of encrypting the Master File Table (MFT) and rebooting the machine.
- Suggested Filter: Exclude events where the ProcessName is wuapp.exe, ccmexec.exe, or msiexec.exe AND the ParentProcessName is svchost.exe running under the context of the “Windows Update” service, specifically during defined maintenance windows (e.g., 02:00–04:00 local time).
Enterprise Antivirus Full System Scan
- Scenario: A scheduled full disk scan by enterprise EDR solutions such as CrowdStrike Falcon, SentinelOne, or Symantec Endpoint Protection triggers. During a deep scan, these agents read and write to the MFT and system metadata to verify file integrity, generating I/O patterns that closely resemble the initial encryption phase of Petya ransomware.
- Suggested Filter: Create an exclusion for ProcessName matching known EDR service executables (e.g., CsService.exe, S1Agent.exe) and filter out alerts where the UserAccount is a local system account (e.g., NT AUTHORITY\SYSTEM) rather than an interactive user session.
**Group Policy Object (GPO) Enforcement and Registry Modification