It's interesting to me that most of the traffic originated it Brazil and Vietnam. I assume that this attack was carried out with a botnet. What makes computers in those two countries particularly vulnerable to infection? Maybe a popular software package was infected with malware, but I can't think of any software that would be specifically more popular in those two locations vs the rest of the world.

I would expect that the number of attacks from a country would be roughly tied to population, but that doesn't match with the data.