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What is a 51% attack?
A 51% attack is when one group controls most of a blockchain's mining power or staked coins. With that majority it can reorder or reverse recent transactions, spend the same coins twice, and refuse to include other people's transactions. It cannot create coins from nothing or take coins from other people's wallets.
What it means for you
On a smaller chain with little mining power or stake, a payment that looks confirmed can be reversed if an attacker gains a majority. This is why exchanges often wait for more confirmations before crediting deposits on such chains. Your coins cannot be moved without your keys even in an attack.
A common mistake: “In a 51% attack, hackers can steal the coins in my wallet.”
In fact: A majority cannot sign transactions for keys it does not hold. It can reverse its own recent payments and censor others, so the risk falls on whoever accepted a payment that gets rewritten.
How it works
Blocks are ordered by whoever has the most work (proof of work) or holds the most stake (proof of stake). An attacker with the majority can build a private chain that leaves out their own payment, then publish it to replace the public one. The Bitcoin whitepaper notes the attacker still cannot create coins or take coins they never owned, because nodes reject invalid transactions; and the chance of catching up falls exponentially with each block added after a payment. On Ethereum, a majority of stake allows short reorganisations and censorship, and two-thirds lets an attacker finalise their own chain; slashing and a community fork are the defences.
Sources: ethereum.org: Glossary, 51% attack, ethereum.org: Ethereum proof-of-stake attack and defense, Bitcoin: A Peer-to-Peer Electronic Cash System (Nakamoto), section 11 · checked 4 October 2026
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