51% Attack
A 51% attack happens when one actor or group controls most of a blockchain's mining power (or stake) and uses it to rewrite recent blocks, enabling double-spends and blocking transactions. It is not only theoretical: smaller proof-of-work chains have suffered real ones.
How the attack works
In proof of work, nodes follow the valid chain with the most accumulated work. An attacker with more hashrate than everyone else combined can secretly mine a private version of the chain. Meanwhile they spend coins on the public chain, for example by depositing them at an exchange and withdrawing something else.
Once the private chain is longer, they publish it. Nodes switch to it, the blocks containing the original payment disappear in a reorganization (reorg), and the attacker still holds the coins they already spent. This is called a double-spend.
What an attacker can and cannot do
With majority control, an attacker can reverse their own recent transactions, refuse to include other people's transactions, and orphan blocks found by other miners.
They cannot steal coins from addresses whose private keys they do not hold, create coins beyond the protocol's rules, or change old history without redoing all the work since then. Every node still checks each block against the rules.
Real cases
Ethereum Classic suffered several 51% attacks, in January 2019 and again in August 2020, with deep reorgs used for double-spends. Bitcoin Gold was hit in 2018. These chains had relatively little hashrate, and attackers could rent enough computing power to overwhelm them.
Why size matters
On a large network like Bitcoin, the hardware and electricity needed would be enormous, and a visible attack would likely damage the value of the coins the attacker is mining. Small chains that share an algorithm with a bigger one are the most exposed, because rented hashrate can be pointed at them.
In proof of stake the equivalent threat requires controlling a large share of staked coins, and protocols can punish provable misbehavior with slashing.
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Frequently asked questions
Has Bitcoin ever had a 51% attack?
No successful 51% attack on Bitcoin has been documented. Smaller proof-of-work chains such as Ethereum Classic and Bitcoin Gold have been attacked.
How do exchanges protect themselves?
They wait for more confirmations before crediting deposits, especially on smaller chains, so a reorg would have to be very deep to reverse a payment.
Does an attacker need exactly 51%?
Majority control makes success near certain over time. With less, double-spends are still possible but become a gamble that grows less likely with each confirmation.
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