The Vibe: When someone controls over half a blockchain’s power and starts rewriting recent history, like double-spending coins or censoring transactions.
The Details: A 51% attack occurs when one party (or coordinated group) gains control of more than 50% of a blockchain network’s consensus power. In Proof-of-Work (PoW) networks like Bitcoin, this means dominating over half the total mining hashrate (computational power). In some Proof-of-Stake (PoS) systems, it involves controlling a majority of staked tokens/coins.
This lets them: reverse recent transactions (enabling double-spending, where the same coins are spent twice), censor or block other people’s transactions, or prevent new blocks from being confirmed normally. They can’t steal coins from wallets they don’t own, create new coins out of thin air, or change old history forever (beyond recent blocks they control). It’s a huge threat to smaller or less secure networks, but extremely expensive and unlikely on big ones like Bitcoin due to massive costs.
Pro Tip: Stick to high-hashrate networks like Bitcoin for large holdings—51% attacks are practically impossible there in 2026. On smaller chains, wait for more confirmations before trusting a transaction.