What is the Hafnium Microsoft hack and why has the UK linked it to China?

Britain has joined the US and other allies in formally accusing China of being behind the cyber-attack

In March, tens of thousands of organisations around the world discovered their private internal discussions had been cracked open and lain bare by a group of Chinese hackers.

Four previously undiscovered weaknesses in Microsoft's Exchange software, known as zero days" because of the amount of time the company had had to fix the flaws before they were exploited, lay behind the mass hack. The vulnerabilities, which affected software released from 2012 onwards, allowed the group to take permanent control of the corporate servers, siphoning emails, calendars, and anything else they desired.