8.
A 51 percent attack, or majority attack, is a case when a user or a group of users control the majority of mining power.
The attackers get enough power to control most events in the network.
They can monopolize generating new blocks and receive rewards since they’re able to prevent other miners from completing blocks.
They can reverse transactions.
Let’s assume Alice sent Bob some money using Blockchain. Alice is involved in the 51 percent attack case, Bob is not. This transaction is placed in the block. But the attackers don’t let the money be transferred. There is a fork happening in the chain.
Further, miners join one of the branches. And as they have the majority of the computational power, their chain contains more blocks.
In the network, a branch that lasts longer remains, and shorter one is rejected. So the transaction between Alice and Bob does not take place. Bob doesn’t receive the money.
Following these steps, the attackers can reverse transactions.
51 percent attack is not a profitable option. It requires an enormous amount of mining power. And once it gets public exposure, the network is considered compromised, which leads to the outflow of users. This will inevitably move the cryptocurrency price down. All consequently, the funds lose their value.