51% Attack

Gaining majority control of a blockchain's computing power to rewrite its recent history.

A 51% attack occurs when a single party controls more than half of a proof-of-work blockchain's mining power, letting it reverse its own recent transactions — spending coins, then erasing the spend. Smaller blockchains have been hit repeatedly: Ethereum Classic suffered multiple 51% attacks in 2019 and 2020, with attackers double-spending millions. For large networks like Bitcoin the attack is theoretically possible but economically brutal — acquiring that much mining power would cost billions and destroy the value of the very coins being stolen.

The exchange froze deposits from the smaller chain after a 51% attack let attackers double-spend their coins.