The criminals responsible for a recently-discovered piece of ransomware called VirRansom have raided the dirty tricks locker and pulled out a technique experts assumed was extinct - old-fashioned virus-like replication.
Security firm Sophos doesn't say precisely when and where it discovered VirRansom, but as ransomware goes it is an unexpected experiment in old-world techniques.
Outwardly, it works rather like any other ransomware, encrypting a wide range of data files and throwing up a warning screen claiming piracy and demanding a payment of around 0.6 Bitcoins ($210).
Bizarrely, the encryption key is held on the victim's PC so retrieving the scrambled files is easy as long as the user can wield an anti-virus program that knows where to look for it.
The clever bit is that VirRansom appends or copies itself inside every file it can find locally or on the network, including executables and as data files without that being obvious to the end user. Even the icons and filenames appear to be the same.
Anyone who subsequently tries to run or open one of those infected files, possibly sent via email or via an itinerant USB stick, will immediately be infected with the same malware and so the process begins all over again.
It won't necessarily be obvious which file caused the problem either - after a delay when the malware runs itself executables appear to run normally. Data files opened within applications fail to open.
As well as encrypting files using what must be a symmetric key, the malware 'locks' the user's PC as it throws up its ransom demand.
If an uninfected user suspects a data file is infected but badly needs to gain access, they can but only at the expense of running the malware and infecting themselves - bam.
"You can liberate the data in the file, but only by opening the file directly, which runs the malware first: Catch 22," said Sophos researcher Paul Ducklin in his blog on VirRansom-A.
Frankly, if it weren't for the replication trick, VirRansom would just be another piece of not particularly innovative copycat ransomware. But that technique is precisely how all malware used to function back in the great age of the computer virus, an era that one might assume ended at least 15 years ago.
Apparently not. Despite its ransom appearance, VirRansom is a true virus because it is programmed to copy itself over and over as a way of spreading as far as it can.
It's a technique eschewed by malware these days for a variety of reasons, including that it draws attention to itself pretty quickly. It should also be easy to intercept and kill even if the malware in this case loads two processes that attempt to keep each other alive should one be intercepted.
Functionally speaking, then, behaving like a virus is obsolete even as millions of people still describe malware using the term 'virus'. It's a popular label that has stuck around even as worms and then Trojans took over the heavy lifting of malicious behaviour.
Viruses should be extinct but VirRansom proves it's a tactic that can still be useful. Why might the malware''s authors have revisited this technique despite its many limitations? The answer probably has something to do with how easily and silently VirRansom can spread, infecting every file it touches without the user realising.
That could take it far and wide and cause a massive nuisance not to mention part people from part of one Bitcoin. VirRansom isn't sophisticated so much as plain surprising.