When someone controls more than half of a blockchain’s hash rate, the total computing power used to validate transactions and secure the network. Also known as majority attack, it’s the one vulnerability that can break the trust in a decentralized system. If a single miner or group owns over 51% of that power, they can stop new transactions, reverse their own payments, and even prevent others from mining. This isn’t science fiction—it’s a real risk for smaller blockchains that don’t have enough miners to keep things balanced.
This kind of attack doesn’t let you steal coins directly. Instead, it enables double spending, the act of spending the same cryptocurrency twice by rewriting transaction history. For example, you could send 100 coins to an exchange, wait for confirmation, then use your control over the network to erase that transaction and keep your original coins. The exchange gets fooled, you walk away with cash—and the network loses credibility. This is why blockchain security, the system of incentives and technical barriers that prevent bad actors from taking over matters more than price or hype. Bitcoin, with its massive hash rate spread across thousands of miners worldwide, is practically immune. But smaller chains like Ethereum Classic and Verge have been hit before. In 2018, Ethereum Classic suffered two 51% attacks in one month, losing over $15 million. Those attacks didn’t break Bitcoin, but they exposed how fragile some networks really are.
The good news? Most attacks happen on chains with low mining rewards or weak decentralization. If a coin doesn’t attract enough miners, it becomes an easy target. That’s why you should always ask: Who’s securing this network? Is it just a few big players? Or is the hash rate spread across dozens of independent miners? The more distributed, the safer. And if you’re holding a smaller altcoin, check its mining stats before you invest. A 51% attack doesn’t always make headlines—but when it happens, your coins can vanish overnight. Below, you’ll find real cases, breakdowns of vulnerable networks, and how to avoid getting caught in the fallout.
In 2025, 51% attacks are no longer theoretical-Monero was hit, and smaller blockchains remain vulnerable. Bitcoin stays secure due to its massive hash rate, but alternatives face existential threats without major changes.
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