A cloud storage server receives a request from a client device to store a file. The cloud storage server stores the file in a storage device of the cloud storage server. The cloud storage server determines features of the server-stored file and detects ransomware activity based on the features of the server-stored file. The cloud storage server sends a notification of the ransomware activity to the client device. The client device confirms the presence or an absence of the ransomware activity in the server-stored file. The cloud storage server updates an operation of the detection of the ransomware activity based on the received ransomware confirmation.