Hackers can exploit a flaw in Bind and take down large parts of the internet. What is Bind? It is the software that translates domain names into IP addresses, so that servers can understand. And Bind is used a lot. So a bug that exists in every single version of Bind can give hackers the ability to take down large parts of the internet by using a DoS attack.
A patch has been released, but it requires manual installation and very few have installed it correctly.