The websites of US telly giant CBS’s Showtime contained JavaScript that secretly commandeered viewers’ web browsers over the weekend to mine cryptocurrency.

The flagship Showtime.com and its instant-access ShowtimeAnytime.com sibling silently pulled in code that caused browsers to blow spare processor time calculating new Monero coins — a privacy-focused alternative to the ever-popular Bitcoin. The hidden software typically consumed as much as 60 per cent of CPU capacity on computers visiting the sites...