> ## Documentation Index
> Fetch the complete documentation index at: https://docs.theblockchainlibrary.com/llms.txt
> Use this file to discover all available pages before exploring further.

# 51% Attack

> An attack where one entity or coalition controls enough consensus power to reorganize blocks, censor transactions, or attempt double spending.

# 51% Attack

An attack where one entity or coalition controls enough consensus power to reorganize blocks, censor transactions, or attempt double spending.

## Definition

An attack where one entity or coalition controls enough consensus power to reorganize blocks, censor transactions, or attempt double spending.

## Why it matters

Security concepts help users and developers recognize common blockchain attack surfaces and defenses.

## How it works

The attacker acquires enough computational power or staked tokens to control the majority of block production. They then ignore the honest network nodes, building a secret fork that follows their own rules. Eventually, they publish this chain to the network, which, due to the protocol's longest-chain rule, is accepted by nodes as the legitimate version.

## Real-world example

The Ethereum Classic (ETC) network has suffered several 51% attacks where attackers reorganized blocks to perform double-spend operations on exchanges.

## Advantages

* Identifies weak consensus mechanisms
* Encourages wider distribution of mining power
* Validates robustness of network security

## Limitations

* Extremely expensive to execute on large networks
* Damages the reputation of the attacked blockchain
* Difficult to maintain over a long duration

## Common misconceptions

* People think 51% attacks can allow hackers to steal everyone's private keys. In reality, they cannot access individual wallets but can only reorder or block transactions.
* Some believe these attacks are common on major chains like Bitcoin, but the cost required makes such an attack virtually impossible.

## Related knowledge

* [Phishing](/generated/v2/glossary/phishing) — term
* [Smart Contract Audit](/generated/v2/glossary/smart-contract-audit) — term
* [Sybil Attack](/generated/v2/glossary/sybil-attack) — term

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**Canonical knowledge ID:** `glossary:51-attack`