Blockchain's Once-Feared 51% Attack Is Now Becoming Regular

At least five cryptocurrencies have recently been hit with an attack that used to be more theoretical than actual, all in the last month.

More surprising may be that so-called 51% attacks are a well-known and dangerous cryptocurrency attack vector.

To make money using this attack vector, hackers need a few pieces to be in place.

Through three successful attacks of zencash, the attacker was able to run off with about more than 21,000 zen worth well over $500,000 at the time of writing.

The attack on verge was a bit different since the attacker exploited insecure rules to confuse the network into giving him or her money.

"Hackers are now realizing it can be used to attack networks," he said.

As a data point for this, someone even erected a website Crypto51 showing how expensive it is to 51% attack various blockchains using a mining marketplace.

Attacking bytecoin, for example, might cost as little as $719 to attack using rented computing power.

Though a 51% attack is perhaps the most famous cryptocurrency attack, it's not necessarily the worst in his mind.

Either way, one way for users or exchanges to make sure they aren't defrauded is to only accept money that's older, or has been buried by more blocks of transactions, called "Confirmations." The more confirmations there have been, the harder the funds are to steal in a 51% attack.