51% Attack

What Is a 51% Attack?

A 51% attack is a threat to a blockchain network that uses proof-of-work consensus. It occurs when one person, group or organization gains control of more than half of the network's mining power, measured by its hash rate. With that majority, the controller can produce new blocks faster than the honest participants, which gives them the ability to interfere with the ledger's recent history.

How It Works

Proof-of-work networks treat the longest valid chain of blocks as the correct record. Honest miners build on the longest chain they can see. An attacker with majority hash power can secretly mine a private fork that grows faster than the public chain. When they release it, the network accepts it because it is longer, and any transactions removed from the public chain are reversed.

This allows several kinds of abuse:

• Double-spending: the attacker spends coins, then reverses that payment so the same coins can be spent again. double-spending is the main concern for merchants who accept payment before it is confirmed.
• Blocking transactions: the attacker can refuse to include certain transactions in blocks.
• Rewriting recent history: blocks near the tip of the chain can be replaced, though changing deeper history becomes increasingly difficult.

Limits and Realistic Impact

A 51% attack does not let the attacker create coins from nothing or spend coins from wallets they do not control, because transactions must still carry valid cryptographic signatures and follow the network's rules. Its main power is to disrupt recent activity. Smaller networks are generally more exposed, since acquiring enough mining power can be far cheaper than on large networks with very high hash rates.

Possible defenses include waiting for several confirmations before accepting a payment, and designs in which control stays spread across many independent participants, a property tied to the network's consensus mechanism. Large mining pools combine the hash power of many miners, so a single pool nearing majority control is a recognized concern. Networks using proof-of-stake face an analogous risk, where an attacker would need a majority of the staked tokens rather than computing power.

Why It Matters to Investors

For investors, a 51% attack shows how much a cryptocurrency's security depends on how widely its mining or validation is distributed. A successful attack can damage confidence in a network, lead to reversed transactions on exchanges, and affect the price of the affected asset. Understanding this risk is part of evaluating any proof-of-work network.