sigma ransomware

Hunt Hypothesis

This detection identifies early-stage ransomware activity by monitoring for specific file encryption patterns and process behaviors defined in the Sigma Ransomware YARA rule. Proactively hunting for these indicators within Azure Sentinel is critical to validate the rule’s sensitivity against false positives and ensure rapid response before encryption spreads across tenant workloads.

YARA Rule

rule sigma_ransomware {

  meta:
    author = "J from THL <[email protected]>"
    date = "20180509"
    reference1 = "https://www.virustotal.com/#/file/705ad78bf5503e6022f08da4c347afb47d4e740cfe6c39c08550c740c3be96ba"
    reference2 = "https://www.virustotal.com/#/file/bb3533440c27a115878ae541aba3bda02d441f3ea1864b868862255aabb0c8ff"
    version = 1
    maltype = "Ransomware"
    filetype = "memory"

  strings:
    $a = ".php?"
    $b = "uid="
    $c = "&uname="
    $d = "&os="
    $e = "&pcname="
    $f = "&total="
    $g = "&country="
    $h = "&network="
    $i = "&subid="

  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 9 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the sigma_ransomware detection rule in an enterprise environment, including suggested filters or exclusions:

• Enterprise Antivirus Real-Time Scanning

  • Scenario: The YARA rule detects high-frequency file creation and modification patterns typical of ransomware encryption. However, this is triggered by legitimate background scans from tools like CrowdStrike Falcon, SentinelOne, or Microsoft Defender for Endpoint. These agents often rename infected files (e.g., appending .quarantine or moving to a specific folder) simultaneously across thousands of endpoints during peak hours.
  • Filter/Exclusion: Exclude processes owned by known security vendors from the alert logic. Add a filter to ignore events where process.name matches C:\Program Files\Microsoft Defender\MsMpEng.exe, CrowdStrike Falcon Sensor, or SentinelOne Agent.

• Scheduled Data Backup and Archiving Jobs

  • Scenario: The rule triggers on bulk file operations (create, rename, delete) occurring during off-hours. This is caused by scheduled backup agents like Veeam Backup & Replication, Commvault, or Druva. These tools frequently compress and archive large datasets, creating temporary files and modifying metadata in a manner that mimics the “encryption phase” of ransomware attacks.
  • Filter/Exclusion: Create an exclusion based on scheduled execution windows (e.g., 02:00–05:00 local time) combined with specific service accounts. Filter out events where user.name is a dedicated backup account (e.g., svc_backup_prod) and the process path contains \Veeam\BackupService\ or \Commvault\Agent\.

• Software Deployment and Patch Management

  • Scenario: The detection logic flags mass file modifications during system updates