Regla para detectar Ransom.CryptXXX original

Hunt Hypothesis

This hunt hypothesis targets adversary behavior consistent with the initial reconnaissance and file encryption phases of a Ransom.CryptXXX attack, specifically looking for anomalous process execution patterns that precede widespread data locking. Proactive hunting in Azure Sentinel is critical to identify these low-severity early indicators before they escalate into full-scale ransomware incidents, allowing the SOC team to contain threats during their most vulnerable window.

YARA Rule

rule Ransom_CryptXXX_Real
{
    /*
      Regla para detectar el codigo Ransom.CryptXXX fuera del dropper con MD5 ae06248ab3c02e1c2ca9d53b9a155199
    */
    meta:
        description = "Regla para detectar Ransom.CryptXXX original"
        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 = { 52 59 47 40 4A 41 59 5D 52 00 00 00 FF FF FF FF }
		$b = { 06 00 00 00 52 59 47 40 40 5A 00 00 FF FF FF FF }
		$c = { 0A 00 00 00 52 5C 4B 4D 57 4D 42 4B 5C 52 00 00 }
		$d = { FF FF FF FF 0A 00 00 00 52 5D 57 5D 5A 4B 43 70 }
		$e = { 3F 52 00 00 FF FF FF FF 06 00 00 00 52 4C 41 41 }
		$f = { 5A 52 00 00 FF FF FF FF 0A 00 00 00 52 5C 4B 4D }
		$g = { 41 58 4B 5C 57 52 00 00 FF FF FF FF 0E 00 00 00 }
		$h = { 52 2A 5C 4B 4D 57 4D 42 4B 20 4C 47 40 52 00 00 }
		$i = { FF FF FF FF 0A 00 00 00 52 5E 4B 5C 48 42 41 49 }
		$j = { 5D 52 00 00 FF FF FF FF 05 00 00 00 52 4B 48 47 }
		$k = { 52 00 00 00 FF FF FF FF 0C 00 00 00 52 4D 41 40 }
		$l = { 48 47 49 20 43 5D 47 52 00 00 00 00 FF FF FF FF }
		$m = { 0A 00 00 00 52 5E 5C 41 49 5C 4F 70 3F 52 00 00 }
		$n = { FF FF FF FF 0A 00 00 00 52 5E 5C 41 49 5C 4F 70 }
		$o = { 3C 52 00 00 FF FF FF FF 08 00 00 00 52 49 41 41 }
		$p = { 49 42 4B 52 00 00 00 00 FF FF FF FF 06 00 00 00 }
		$q = { 52 5A 4B 43 5E 52 00 00 FF FF FF FF 08 00 00 00 }
		$v = { 52 48 3A 4C 4D 70 3F 52 00 00 00 00 FF FF FF FF }
		$w = { 0A 00 00 00 52 4F 42 42 5B 5D 4B 70 3F 52 00 00 }
		$x = { FF FF FF FF 0A 00 00 00 52 5E 5C 41 49 5C 4F 70 }
		$y = { 3F 52 00 00 FF FF FF FF 0A 00 00 00 52 5E 5C 41 }
		$z = { 49 5C 4F 70 3C 52 00 00 FF FF FF FF 09 00 00 00 }
		$aa = { 52 4F 5E 5E 4A 4F 5A 4F 52 00 00 00 FF FF FF FF }
		$ab = { 0A 00 00 00 52 5E 5C 41 49 5C 4F 70 3D 52 00 00 }
		$ac = { FF FF FF FF 08 00 00 00 52 5E 5B 4C 42 47 4D 52 }
		
    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 25 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, including suggested filters and exclusions:

• Enterprise Backup Agent Encryption Jobs

  • Scenario: Scheduled backup agents (e.g., Veeam Backup & Replication, Commvault, or Rubrik) perform nightly encryption of large data sets. These processes often generate high volumes of file creation, modification, and attribute changes across critical directories, mimicking the rapid file locking behavior of ransomware.
  • Filter/Exclusion: Exclude process paths containing Veeam, CommServe, or Rubrik from the rule scope. Additionally, filter out events occurring during defined maintenance windows (e.g., 02:00–06:00 UTC) where backup jobs are known to run.

• Antivirus Real-Time Scanning and Quarantine

  • Scenario: Endpoint Protection suites like Microsoft Defender for Endpoint, CrowdStrike Falcon, or Symantec Enterprise Security frequently scan user directories in real-time. When a large number of files are downloaded (e.g., via OneDrive sync), the AV engine may lock, scan, and re-encrypt files simultaneously, triggering high file I/O alerts that resemble ransomware encryption patterns.
  • Filter/Exclusion: Create an exclusion for processes with known benign hashes or names such as MsMpEng.exe, csagent.exe, or Symantec Endpoint Protection. Specifically, exclude events where the parent process is identified as a trusted AV service rather than a user application.

• Software Deployment and Patching Operations

  • Scenario: Automated deployment tools like Microsoft SCCM (ConfigMgr), Ivanti, or Ansible Tower push updates to thousands of endpoints simultaneously. During these operations, the installer extracts compressed