51% attack
Also · Majority attack
A 51% attack is control of most of the hashrate by a hostile party, who could reorganise recent blocks to double spend or censor transactions, but not steal coins or print new ones.
With more than half of the network’s hashrate, an attacker can mine a private chain faster than everyone else, then publish it to replace recent blocks. Transactions in the replaced blocks are undone, which allows the attacker to spend coins twice. The attacker can also refuse to include chosen transactions for as long as the majority holds.
What the attack cannot do is as important. Hashrate does not allow spending other people’s coins, since their spending conditions still apply, nor creating more subsidy than the rules permit. Nodes would reject such blocks. The cost scales with the hashrate to match, the hardware and energy needed, and how far back the attacker wants to reach.
In French · Attaque des 51 %