51% Attack

A 51% attack happens when a single person or group gains control of more than half of the computing power (or staking power, depending on the blockchain) that validates transactions on a blockchain network. In most blockchain systems, transactions are verified by a distributed group of participants called miners or validators. The security assumption is that no single entity controls the majority — if they did, they could bend the rules. When someone crosses that 51% threshold, they have enough power to manipulate the blockchain’s history in ways that should be impossible under normal conditions.

What can an attacker actually do with 51% control? Most notably, they can perform “double spending” — spending the same cryptocurrency twice. They can also prevent other transactions from being confirmed, effectively censoring the network. What they generally cannot do is steal funds directly from other wallets or create new coins out of thin air, because those actions would violate the cryptographic rules that the rest of the network enforces. The attack is about controlling which history gets written, not about breaking the mathematics.

The risk of a 51% attack is much higher on smaller blockchains where the total amount of computing or staking power is low, because it is cheaper to acquire a majority. Bitcoin and Ethereum are considered highly resistant to this type of attack because the investment required to gain 51% of their network power would be astronomical. Several smaller cryptocurrencies have been successfully attacked this way.

Example: Imagine a town where all decisions are made by majority vote among 100 residents. The town runs fairly as long as no single person controls how most people vote. But if a billionaire buys influence over 51 of those residents, they can now pass any law they want, override fair decisions, and rewrite the rules in their favor — even if the other 49 residents protest. A 51% attack is the blockchain equivalent of that hostile takeover.