Malware was sent to Dropbox users via phishing emails with slightly different subject lines. 10 of our internal users were targeted in the latest campaign. Malware is known as CryptoWall and refers to RSA-2048 encryption algorithm like CryptoDefense mentioned here. Once infected, users are instructed to pay 500 USD in Bitcoins to unlock their files. Attackers are using a base 36 number scheme: 348,637 348,610 potentially infected systems. Many poor unfortunate souls have lost tens of thousands of files.”]
Source: https://cofense.com/inside-look-dropbox-phishing-cryptowall-bitcoins/