A 51% attack occurs when a single entity or coordinated group gains control of the majority of a blockchain's mining hash rate, on proof-of-work networks, or staked tokens, on proof-of-stake networks. With majority control, an attacker can potentially reverse recent transactions to double-spend coins or censor transactions from being confirmed.
A blockchain's resistance to this kind of attack scales with how decentralized and well-secured its network is — large networks like Bitcoin, with enormous distributed hash rate, are considered extremely costly to attack this way, while smaller-cap chains with thinner hash rate or stake distribution have historically proven more vulnerable and have, in some documented cases, actually experienced such attacks.
Network security and decentralization are worth researching for any smaller-cap chain, since they're not uniform across the industry. This is general information, not financial advice, and not a recommendation about any specific network's safety.
Key takeaways
- A 51% attack lets an entity controlling the majority of a network's hash rate or staked tokens rewrite recent transactions and enable double-spending, though it cannot create new coins or steal from unrelated wallets.
- Bitcoin's enormous distributed hash rate makes a sustained 51% attack economically prohibitive, while smaller-cap proof-of-work and proof-of-stake chains with thinner security have suffered documented attacks in the past.
- Network decentralization and security vary widely across chains, so treating 51% attack risk as uniform across crypto is a mistake; checking hash rate or stake distribution matters before assuming any chain is safe.
51% Attack — frequently asked questions
Can a 51% attack steal my crypto directly from my wallet?
No. A 51% attack lets an attacker rewrite recent transaction history on the chain it controls, mainly enabling double-spending, but it cannot access private keys or drain unrelated wallets directly.
Has Bitcoin ever suffered a 51% attack?
No confirmed 51% attack has occurred on Bitcoin; its hash rate is distributed across enormous global mining capacity, making an attack extremely costly. Smaller proof-of-work chains with far less hash rate have been attacked.
Are proof-of-stake chains immune to 51% attacks?
No. On proof-of-stake networks the same risk applies to staked tokens instead of hash rate; an entity controlling a majority of staked tokens could similarly attempt to reverse transactions or censor the chain.
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