Regla para detectar RANSOM.CRYPTXXX

Hunt Hypothesis

This hunt hypothesis targets adversary behavior characterized by the execution of CRYPTXXX ransomware processes, which typically involve rapid file encryption and system resource consumption indicative of an active data exfiltration or extortion campaign. Proactively hunting for these indicators in Azure Sentinel is critical to identify early-stage infections before they escalate into high-severity incidents that could disrupt business continuity across cloud workloads.

YARA Rule

rule Ransom_CryptXXX_Dropper
{
    /*
      Regla para detectar el dropper de Ransom.CryptXXX con MD5 d01fd2bb8c6296d51be297978af8b3a1
    */
    meta:
        description = "Regla para detectar RANSOM.CRYPTXXX"
        author      = "CCN-CERT"
        version     = "1.0"
        ref = "https://www.ccn-cert.cni.es/seguridad-al-dia/comunicados-ccn-cert/4002-publicado-el-informe-del-codigo-danino-ransom-cryptxxx.html"
    strings:
        $a = { 50 65 31 57 58 43 46 76 59 62 48 6F 35 }
        $b = { 43 00 3A 00 5C 00 42 00 49 00 45 00 52 00 5C 00 51 00 6D 00 6B 00 4E 00 52 00 4C 00 46 00 00 }
    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 RANSOM.CRYPTXXX detection rule in an enterprise environment, along with recommended filters or exclusions:

• Enterprise Backup and Snapshot Operations

  • Scenario: Scheduled backup agents (e.g., Veeam Backup & Replication, Commvault, or Microsoft Azure Backup) perform massive file read/write operations during nightly maintenance windows. These processes often rename, encrypt, or compress thousands of files simultaneously to create snapshots, mimicking the rapid file modification patterns typical of ransomware encryption.
  • Suggested Filter: Exclude known backup service accounts (e.g., svc-veeam-backup, AZURE-BACKUP-SVC) and restrict the rule trigger to non-business hours if backups are scheduled outside standard work times, or whitelist specific executable paths like C:\Program Files\Veeam\Backup.

• Endpoint Data Loss Prevention (DLP) Scanning

  • Scenario: Real-time scanning by DLP solutions such as Symantec DLP, Microsoft Purview Information Protection, or Forcepoint triggers when users save sensitive documents. These agents may temporarily lock, encrypt, and rename files to apply classification labels or rights management policies (e.g., appending .encrypted extensions), which generates high-volume file event logs indistinguishable from a ransomware attack.
  • Suggested Filter: Create an exclusion for the specific DLP service executables (e.g., SymantecDlpAgent.exe, MIPService.exe) and filter out events where the “File Extension Change” involves known corporate extensions like .enc or .irm.

• IT Administration Bulk Migration Tasks

  • Scenario: System administrators performing bulk data migrations using tools like Robocopy, rsync, or PowerShell scripts (e.g., Move-Item, `Copy