51% Attack

In simple terms

A 51% attack is what happens when one group controls more than half a network's power. They cannot steal from wallets, and they can reverse their own recent transactions, which lets them spend the same coins twice.

Definition

An attack in which one party controls enough of a network's mining or staking power to rewrite recent history.

In depth

With a majority of hash rate or stake, an attacker can produce a competing chain faster than the honest network and, on release, cause a reorganisation that orphans recent blocks. This enables double spending and transaction censorship. It does not permit forging signatures, minting coins outside the rules, or altering long-settled history, because every node still validates every rule independently. Feasibility scales inversely with a network's security budget, which is why small chains sharing a hashing algorithm with a large one have been attacked repeatedly using rented capacity.

How does 51% Attack work?

The attacker mines a private chain while making a payment on the public one, waits for the recipient to accept it, then publishes the longer private chain. Nodes follow the chain with the most accumulated work, so the public branch is discarded and the payment along with it, while the goods or funds received elsewhere are kept. Requiring more confirmations raises the cost, because the attacker must sustain the majority for longer.

An example

A small chain has an illustrative one percent of the hash rate of the large network sharing its algorithm. Renting enough capacity to overpower it for an hour is affordable, so exchanges listing such chains typically demand far higher confirmation counts, sometimes hundreds of blocks, before crediting a deposit.

Figures are illustrative only.

What beginners get wrong

• Believing an attacker could empty arbitrary wallets. Spending still requires the private key, and majority power does not provide it.
• Assuming every blockchain is equally protected. Security follows the size of the security budget, and small chains are genuinely vulnerable.
• Dismissing it as theoretical. Several smaller networks have been attacked successfully, more than once.

Related terms

• Mining
• Hash Rate
• Double Spend
• Confirmation
• Consensus Mechanism
• Bitcoin Security

Part of

How do crypto scams work, and how do you avoid them? — the subject page for security and scams, with all 17 of its definitions in one place.