Mirai Widens Distribution with New Trojan that Scans More Ports

I read today an article entitled Mirai Widens Distribution with New Trojan that Scans More Ports and it was very interesting how this bot has just morphed in to something that can just cause havoc. Sometimes, releasing source code can be a good thing, but in this case, it is for all bad. This is something probably good for the security community at the time it was done, but now, who knows what else can happen. If we were to defend from this, how would we be doing this successfully?