Attacks that were once thought to exist only theoretically are more and more common today. Monacoin, bitcoin gold, zencash, verge, and litecoin cash, at least five digital cryptocurrencies suffered 51% of attacks last month, and the attackers hijacked the blockchain network of these digital currencies briefly after accumulating sufficient computing power. Arranging transactions and stealing millions of dollars.
51% attack is a term which signifies that an attacker has accumulated more computing power in his hands than all the other members of the network combined: a sort of controlling interest of generating power.
Another possible way of application is an attack on already commercially successful currency for profit. However this case implies that the coin is commercially mined by network members who use their computing power for generation of cherished coins. In the case of a successful currency this means that the total network power is enormous as there’s a massive number of people who are engaged in mining process. Thus a 51% attack becomes hard to conduct. via_Wiki
51% of attacks were considered extremely rare. Some experts believe that miners in large blockchain networks will never be victims. They believe that 51% of attacks are too expensive to be profitable. But reality denies these points. Joseph Bonneau, a computer science researcher at New York University, published a research report last year that estimated the cost of leasing instead of purchasing all equipment to perform a 51% attack.
One of his conclusions is that such attacks will gradually increase, and the blockchain community believes that 51% of the attacks are very distant threats. He warned about this, but he himself did not expect that the attacks would come so quickly.