Security within the blockchain space is something that shouldn't be taken likely. Upbit is a top volume exchange. Recently, breaches on these perceived secure exchanges are becoming a real cause for concern, even Binance wasn't exempt. Sadly, it can only get worse as more people enter into the space and the values of these crypts increase.
Hackers can not be prevented so this happens in small and large exchanges of course in any security there must be a weakness there we must realize and do not believe it fully, although Upbit exchange for it will still be vulnerable too.
If indeed this situation can certainly happen maybe there should be a blockchain system that can make the stolen assets back to where they should be, not just locked and then stored for a long time in the wallet. Because if this continues, crypto assets can collapse. In the future i hope Blockchain should be really flexible to certain situations