51% Attack

A majority attacker can mine a private chain faster than the honest network, then release it to replace recent blocks — reversing their own transactions (double-spending) or censoring others. They cannot steal arbitrary funds or change protocol rules; the attack's scope is narrower than its reputation suggests.

Feasibility is purely economic: matching Bitcoin's hashrate would cost billions in hardware and gigawatts of power, while smaller chains (Ethereum Classic in 2019–2020, Bitcoin Gold) have been successfully attacked with rented hashrate. This is why exchanges demand more confirmations on smaller PoW coins.