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ONTAP ARW “Possible Ransomware Activity Detected” Alerts on NAS Volume Due to High Entropy and Unsuitable Workload

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Applies to

  • ONTAP 9.17.1P5
  • Autonomous Ransomware Protection (ARP)
  • NAS volumes (SMB/NFS) with high file extension diversity

Issue

  • ONTAP ARW (Autonomous Ransomware Protection) repeatedly triggers “Possible Ransomware Activity Detected” alerts on a NAS volume.
[?] Sat Mar 28 15:25:48 +0530 [NASCLTR01-01:svc_queue_thread:callhome.arw.activity.seen:alert]: Callhome message for "POSSIBLE RANSOMWARE ACTIVITY DETECTED", Volume: "VOL1" in Vserver: "SVM_CIFS"
  • The detection is based on a spike in high-entropy data writes and a large number of observed file extensions.
  • No actual ransomware activity is present, but the system flags moderate probability attacks due to workload characteristics.
::> security anti-ransomware volume show -vserver SVM_CIFS -volume VOL1
Vserver Name: SVM_CIFS
Volume Name: VOL1
State: enabled
Dry Run Start Time: -
Attack Probability: moderate
Attack Timeline: 3/16/2026 10:42:56
Number of Attacks: 1
Attack Detected By: encryption_percentage_analysis
Block Device Detection Status: Active_unsuitable_workload
::> security anti-ransomware volume workload-behavior show -vserver SVM_CIFS -volume VOL1
Number of File Extensions Observed: 4002
Historical Statistics
HighEntropyDataWritePercentage: 99
::> security anti-ransomware volume attack-detection-parameters  show -vserver SVM_CIFS -volume VOL1
                                            Vserver Name : SVM_CIFS
                                             Volume Name : VOL1
          Block Device Auto Learned Encryption Threshold : 89

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