Four months ago Microsoft scored a cyber coup. Its digital sleuths identified a “botnet”, or fake server, that had installed malware on computers worldwide, and then it worked with the Federal Bureau of Investigation and others to shut it down. To their alarm they discovered that no fewer than 12m — yes 12m — PCs were infected, according to Tom Burt, the company’s deputy general counsel.

If you are tempted to shout “hooray”, that is understandable. After all, botnets pose a particularly pernicious threat since they are fiendishly hard to find. And cyber attacks in general are increasing explosively, costing global businesses $400bn a year, according to data from Microsoft.

There is a catch, though. Microsoft and the FBI now hope to bring the cyber hackers who created that botnet to court. But since this botnet was not entirely run from US soil — and those 12m infected computers sit everywhere around the world, from China and India to Chile and the US — the saga could be about to plunge into a legal grey zone.