Surreptitious mining of cryptocurrency by hackers is spreading very, very fast.

The news: Over 4,000 websites, including those of the US federal judiciary and the UK National Health Service, have been weaponized by hackers to mine the cryptocurrency Monero on their behalf.

How it worked: Hackers injected malware into a widely used plug-in called Browsealoud that reads web pages our loud as a way of helping partially sighted people navigate the internet. The malware is based on the popular app Coinhive, a piece of software that uses processing power on someone’s device to mine cryptocurrency. The malicious version of the app does this without people's knowledge, coopting their computing power to enrich hackers.

Why it matters: Cryptojacking is rapidly becoming one of the world’s biggest cyberthreats. This news shows how easy it is for crooks to spread the trick, by infecting a single product used across multiples sites with a single hack.