Raphael Centeno and Noel Llimos said in a blog post that the malware, which they had named Viro, was not connected to any known families of ransomware.
Viro was first noticed in the wild on 17 September, they said, adding that once it gained a presence on a Windows computer, it checked the registry keys to determine the machine GUID and product key to determine if the system should be encrypted.
No indication was given of how the infection occurred in the first place, though presumably it was through the traditional route of phishing through suitable worded spam emails.
Curiously, though Viro had been noticed to be infecting US users, the warning notice that was displayed to users (seen above) after files were encrypted was in French, Centeno and Llimos said.
Viro was also able to carry out keylogging and would periodically connect to its C&C server to send keystrokes it had logged from the infected Windows machine.
In some cases, it would download additional malware from the C&C server and execute it using PowerShell.
Centeno and Llimos said the Viro's capabilities could be measured by the fact that it could use Microsoft Outlook on an infected machine to send spam emails to addresses on the user's contact list. These contained a copy of itself or malware downloaded from the C&C server.
The C&C had been taken down recently, hence the ability to encrypt files had been temporarily removed.
Screenshot: courtesy Trend Micro