On May 24, it was reported that a pair of mining groups were able to successfully execute what is called a "51% attack" on the blockchain of Bitcoin Cash.

A 51% attack is one in which a group of cryptocurrency miners controls more than half of the mining power of a digital currency that uses a proof-of-work algorithm, such as Bitcoin Cash. When this occurs, this group can prevent others from both mining cryptocurrency and reversing transactions on the blockchain, as they in effect become the verifier of everything that happens on the blockchain.

While such attacks are usually considered malicious in nature, in this particular case the attack was relatively benign. A pair of mining pools called BTC.top and BTC.com were able to reverse two blocks of transactions and replace them with their own. They did this to prevent someone else from taking currency that was freely available as a result of a programming update to Bitcoin Cash's blockchain. The update happened two years ago when the currency experienced what is called a hard fork.

According to previously collected data, the two mining pools recently only had ownership of 43% on mining power on the currency's blockchain.

At one time, attacks of this kind were considered largely theoretical and impractical. This is because such an attack generally requires a huge amount of computing resources, and because such an attack would cause users to avoid making any further transactions on the blockchain after it occurred.

Though this is not the first time that someone has executed a successful 51% attack. In January of this year, an attack reportedly occurred on the blockchain of Ethereum Classic. In this instance, the perpetrator of the attack was able to reverse 4 transactions after it took control of the blockchain. This resulted in a loss of more than 54,000 ETC — the currency used on the Ethereum Classic blockchain — on a cryptocurrency exchange called Gate.io. In response, the exchange insisted that they would compensate those who suffered losses. They also reported the attacker's digital address to other exchanges.