When someone controls more than half of a blockchain’s computing power, they can pull off a 51% attack, a scenario where a single entity or group dominates the network’s hash power to reverse transactions or block others. Also known as a majority attack, this isn’t science fiction—it’s a real threat that’s happened before on smaller chains. The goal? Double-spend coins, stop new transactions, or disrupt trust in the network. But here’s the catch: pulling this off on Bitcoin or Ethereum is nearly impossible. Why? Because the cost and energy needed are astronomical.
Think of it like a voting system where each miner gets one vote based on their computing power. If one person owns 51% of the votes, they can force any decision—even if everyone else disagrees. That’s why hash power, the total computational strength used to mine and secure a blockchain is the backbone of security. Networks with low hash power, like some altcoins, are easy targets. In 2018, Verge suffered a 51% attack that stole over $1 million. Ethereum Classic had one too in 2019. These weren’t hacks of wallets—they were attacks on the rules themselves.
So how do big networks stay safe? consensus mechanism, the system that lets nodes agree on the state of the blockchain plays a huge role. Bitcoin uses Proof of Work, which makes attacks expensive. Ethereum switched to Proof of Stake, where attackers would need to own 51% of all ETH—a financial suicide move. Even if you had the money, selling that much ETH would crash its price, making your attack worthless. That’s why no one’s ever pulled off a 51% attack on Bitcoin. The cost is higher than the reward.
You’ll find posts here that dig into how exchanges protect keys with HSMs, how blockchain forensics tools trace stolen coins after an attack, and why some tokens are too small to be safe. You’ll also see how EIP-1559 changed Ethereum’s fee structure to make it harder to manipulate, and why projects like Moca Network and Wise Monkey don’t even have mining—so a 51% attack doesn’t apply to them. The real lesson? Not all blockchains are built the same. Some are designed to be tamper-proof; others are just lucky. Know the difference before you invest.
Historical double-spending incidents reveal how small cryptocurrencies were hacked through 51% attacks, while Bitcoin remained secure. Learn which coins were targeted, how attacks worked, and how the industry responded.