Rule to detect CryptoNar Ransomware

Hunt Hypothesis

This hypothesis posits that adversaries are deploying CryptoNar ransomware to encrypt critical assets and exfiltrate data before demanding payment. Proactive hunting in Azure Sentinel is essential to identify early-stage encryption anomalies and lateral movement patterns associated with this specific strain before widespread operational disruption occurs.

YARA Rule

rule cryptonar_ransomware {

   meta:
   
      description = "Rule to detect CryptoNar Ransomware"
      author = "Marc Rivero | @seifreed"
      reference = "https://www.bleepingcomputer.com/news/security/cryptonar-ransomware-discovered-and-quickly-decrypted/"
      
   strings:
   
      $s1 = "C:\\narnar\\CryptoNar\\CryptoNarDecryptor\\obj\\Debug\\CryptoNar.pdb" fullword ascii
      $s2 = "CryptoNarDecryptor.exe" fullword wide
      $s3 = "server will eliminate the key after 72 hours since its generation (since the moment your computer was infected). Once this has " fullword ascii
      $s4 = "Do not delete this file, else the decryption process will be broken" fullword wide
      $s5 = "key you received, and wait until the decryption process is done." fullword ascii
      $s6 = "In order to receive your decryption key, you will have to pay $200 in bitcoins to this bitcoin address: [bitcoin address]" fullword ascii
      $s7 = "Decryption process failed" fullword wide
      $s8 = "CryptoNarDecryptor.KeyValidationWindow.resources" fullword ascii
      $s9 = "Important note: Removing CryptoNar will not restore access to your encrypted files." fullword ascii
      $s10 = "[email protected]" fullword wide
      $s11 = "Decryption process will start soon" fullword wide
      $s12 = "CryptoNarDecryptor.DecryptionProgressBarForm.resources" fullword ascii
      $s13 = "DecryptionProcessProgressBar" fullword wide
      $s14 = "CryptoNarDecryptor.Properties.Resources.resources" fullword ascii
      
   condition:
      ( uint16(0) == 0x5a4d and filesize < 2000KB) and 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 14 string patterns in its detection logic.

References

False Positive Guidance

Here are 4 specific false positive scenarios for the CryptoNar Ransomware detection rule in an enterprise environment, along with recommended filters or exclusions:

Enterprise Backup Agent Encryption Jobs
- Scenario: Scheduled backup agents (e.g., Veeam Backup & Replication, Commvault, or Microsoft Azure Backup) perform nightly encryption of large datasets. These processes often generate high-volume file write operations and modify file extensions/attributes in bulk, mimicking the rapid file modification behavior of CryptoNar ransomware.
- Filter/Exclusion: Whitelist specific service accounts (e.g., svc_veeam_backup, AZURE_BACKUP_AGENT) and restrict the rule to exclude processes running from known backup installation paths (e.g., C:\Program Files\Veeam\Backup...).
Antivirus Deep Scan & Quarantine Operations
- Scenario: During a scheduled “Deep Scan” or on-demand malware remediation, security tools like CrowdStrike Falcon, SentinelOne, or Symantec Endpoint Protection may rename infected files (adding extensions like .quar or .infected) and encrypt them for safe storage. This mass file modification can trigger the ransomware heuristic.
- Filter/Exclusion: Create an exclusion based on the Image Path of known EDR executables (e.g., C:\Program Files\SentinelOne\Agent\SAgent.exe) and filter out events where the parent process is a recognized security vendor service.
Database Maintenance and Index Rebuild Tasks
- Scenario: Database administrators run scheduled maintenance jobs using tools like SQL Server Management Studio (SSMS) or Oracle Enterprise Manager. Operations such as index rebuilding, database compaction, or full-text indexing involve rewriting millions of data pages and modifying file structures simultaneously, which can resemble the