Specific sample match for WannaCryptor

Hunt Hypothesis

This detection rule identifies the presence of specific WannaCryptor ransomware samples within the Azure Sentinel environment to confirm active infection or historical artifacts. The SOC team should proactively hunt for these matches because early identification allows for rapid containment and prevents lateral movement before encryption spreads across critical workloads.

YARA Rule

rule Wanna_Sample_4da1f312a214c07143abeeafb695d904: Wanna_Sample_4da1f312a214c07143abeeafb695d904
{
        meta:
                description = "Specific sample match for WannaCryptor"
                MD5 = "4da1f312a214c07143abeeafb695d904"
                SHA1 = "b629f072c9241fd2451f1cbca2290197e72a8f5e"
                SHA256 = "aee20f9188a5c3954623583c6b0e6623ec90d5cd3fdec4e1001646e27664002c"
                INFO = "Looks for offsets of r.wry and s.wry instances"
 
        strings:
                $rwnry = { 72 2e 77 72 79 }
                $swnry = { 73 2e 77 72 79 }
 
        condition:
                $rwnry at 88195 and $swnry at 88656 and $rwnry at 4495639
}

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 5 specific false positive scenarios for the WannaCryptor detection rule in an enterprise environment, along with suggested filters or exclusions:

• Antivirus Engine Updates and Scans

  • Scenario: Enterprise Endpoint Protection tools (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint, or SentinelOne) frequently download and execute specific sample files to update their local threat intelligence databases. These updates often include known ransomware signatures, including WannaCryptor variants, which trigger the “specific sample match” logic even though no infection is occurring.
  • Filter/Exclusion: Create an exclusion rule for process names associated with major EDR agents (e.g., C:\Program Files\CrowdStrike\csagent.exe, MsMpEng.exe) and restrict alerts to non-standard execution paths or exclude specific file hashes known as “clean” update signatures.

• Scheduled Patch Management Deployments

  • Scenario: Automated patch management solutions like Microsoft SCCM (Configuration Manager) or Ivanti Endpoint Manager deploy security patches during maintenance windows. These deployments often involve the extraction and execution of temporary installer files that contain embedded WannaCryptor sample data for testing compatibility, triggering a match on file creation or process execution events.
  • Filter/Exclusion: Exclude alerts originating from scheduled task triggers (e.g., TaskScheduler running under the SYSTEM account) during defined maintenance windows (e.g., 02:00–04:00 UTC) and filter by the specific parent process of the patching engine.

• IT Admin Manual Remediation Tasks

  • Scenario: Security analysts or system administrators manually run remediation scripts using tools like PowerShell, Ansible, or CyberArk Privileged Access Management. When an admin executes a script to analyze a known WannaCryptor