A blog post on the company's website said the new variant had been recoded to fix earlier flaws that allowed the use of free decryption tools to escape without paying the ransom demanded.

As with all ransomware, it spreads through spam and possibly other means, encrypts files on Windows systems, and then informs the user through a pop-up message where to send the ransom and how much is being demanded. [Screenshots used with permission from SentinelOne]

SentinelOne researchers said CryptXXX appeared to be in active development and hence the likelihood that the people behind it would continue a cat and mouse game to defeat any defences against it was likely.

"Some factors which may contribute to this are the increasing reliance on computers to store and process valuable information and the increasing popularity of Bitcoin which is semi-anonymous, works globally, and is difficult to regulate because it's completely decentralised," they wrote.

A packed sample of the ransomware has the filename F0F3.tmp.dll. SentinelOne said the properties of this malicious DLL showed it was using what seemed to be "the details of a legitimate DLL named _BigBang.dll from a product called CyberLink PowerDVD Cinema. After hunting down a legitimate copy of _BigBang.dll, though of a slightly older version, it's clear that the details have been copied exactly".

The detailed technical analysis of the new variant is a recommended read.

The researchers wrote that ransom notes were created in every folder where a file was encrypted, one in plain text and the other in HTML. And when the ransomware completed file encryption, all shadow volume copies were deleted making it impossible to restore those files from backups.

When an infected computer is rebooted, a screen appears on login, indicating that the damage has been done and directing the user to the website they should go to in order to pay. Interestingly, the researchers said, at the CryptXXX creators' website - which is on the dark web and hence only accessible through Tor - the user was given the opportunity to decrypt any one file.

"This is a good idea from a psychological standpoint since the malware authors know that people are more likely to pay for something if they know that it will work. This tool allows their victims to confirm decryption is possible, but doesn't allow them to decrypt any sizeable file since there's a limit of 512 KB," the SentinelOne researchers wrote.

The company has a paper here on what one can do about ransomware. It is holding a webinar on the fundamentals of ransomware at 3am AEST on 29 June (10am Pacific time in the US on 28 June) with its chief of security strategy, Jeremiah Grossman.