BBC hacks into thousands of PCs

I was reading this story
from the article:
"The BBC has deliberately hacked into 22,000 PCs to prove the power of botnets, and the damage that can be done with a network of compromised computers.

Click – BBC News’ technology programme – with the help of anti-virus company Prevx, took over thousands of computers in order to demonstrate a growing problem in the modern world.

Botnets are networks of computers that have been compromised by cyber-criminals and can be used to launch distributed denial of service (DDoS) attacks on servers or, most commonly, to send out the deluge of spam that lands in the world’s inboxes daily."

i wonder what kind of exploit or stuff they used to compromised the user systems? is it a publicly available exploit or a 0-day? i am not sure.....
you can read the full article here.