A 51% attack is a situation in which more than half of the computing power (hash rate) of a cryptocurrency network running on the Proof-of-Work algorithm comes under the control of a single miner or a coordinated group of miners. Since in such networks the longest chain of blocks is considered correct, the holder of the majority of the power gains the ability to outpace the rest of the network and impose its own version of transaction history.

Control over the majority of the power theoretically gives the attacker a number of possibilities for manipulating the network. This does not mean that an attacker can arbitrarily steal other people's coins or create them out of nothing, but the list of potentially dangerous actions is still serious.

What a 51% attack potentially makes possible

• Creating transactions that conflict with already existing ones.
• Delaying or blocking the confirmation of other people's transactions.
• Carrying out double-spending of the same coins.
• Preventing other miners from finding and adding blocks.

In practice, such an attack requires enormous resources, so for large and well-distributed networks it is economically impractical. The greatest risk is faced by small projects with a low total hash rate, where concentrating the majority of the power is significantly easier.