What is a 51% attack? When one group controls a blockchain
By SendPay Business · · 2 min read
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A 51% attack is when one person or group controls more than half of a blockchain's mining power, or staked coins on a proof of stake network. With that control they can rewrite recent history and spend the same coins twice.
What an attacker can do
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What an attacker can't do
They can't create coins out of nothing beyond normal rewards, and they can't take coins from wallets whose private keys they don't hold. The damage is mostly to people who accepted their payments.
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Real examples
Smaller networks have been hit. Bitcoin Gold suffered a 51% attack in 2018, and Ethereum Classic was attacked more than once, including several times in August 2020. Exchanges that had credited deposits too quickly lost money.
Why big networks are safer
On a network as large as Bitcoin, gaining majority control would cost an enormous amount in equipment and energy. That is also why exchanges wait for more confirmations on smaller coins before crediting deposits.
Where SendPay fits
SendPay platforms let customers buy, sell and hold crypto, held with regulated partners, alongside GBP, EUR and USD accounts, powered by licensed partners. SendPay doesn't offer trading tools, DeFi or lending.
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Questions people ask
What is a hash function?
How SHA-256 turns data into a fingerprint, and why blockchains need it.
SHA-256 hash generator →What does mining cost and earn?
Work out mining income against power costs.
Bitcoin mining profit calculator →What is proof of work vs proof of stake?
Two ways blockchains agree on which transactions are valid.
PoW vs PoS →What is a blockchain node?
A computer that keeps a copy of the blockchain and checks new blocks.
Blockchain nodes →Read next
Spotted a mistake or something out of date? and we'll fix it. How our guides are made
This guide is general information, not legal or financial advice. SendPay Business is a technology company, not a bank, and does not take deposits; regulated services on the platforms are provided by licensed partners. Other companies named in this guide are reference points only and are not affiliated with SendPay Business.