I advise you to learn what is a 51% attack before posting non-sense, you clearly don't understand what people are talking about since you ask "what kind of attacks?"
Few cryptos have been victims from such attacks and it has been fixed since so I say YES the problem can be solved
Oh, thank you, I didn't know about it. Will read more about 51% attacks
More information:
51% attack refers to obtaining 51% of the bitcoin network's hashing power.
In theory, obtaining a majority of network power could potentially enable massive double spending and the ability to prevent transaction confirmations, among other potentially nefarious acts.
However, this would take a huge amount of expense and effort and, should a big double spend be attempted, the data would likely appear on the block chain for all to see.
Worse, such an attack could possibly destroy the integrity of the system as a whole, causing the price to crash. This is not something anyone with a vested interest in bitcoin would want especially miners, whose profits depend largely on the price of bitcoin being high. Hence, there is no real incentive to attack the network.
https://www.coindesk.com/51-attacks-real-threat-bitcoin/