The recent discussions about 51% attacks on blockchain security are actually quite complex. On the surface, it sounds scary, but once you understand the mechanics and impact, you can see that it’s very different between large networks and small chains.

The basic mechanism of a 51% attack is simple. If a single entity controls more than half of the network’s total hash rate, in theory, they can rewrite the chain’s history. Proof-of-Work-based blockchains rely on majority consensus to determine the truth, so if the majority is controlled, an attacker can secretly create a “shadow chain” and then present a longer version than the original chain.

The attack process has three stages. First, the attacker mines a hidden chain while sending their assets on the public chain. Once an exchange confirms the deposit, the attacker exchanges those funds for other assets and withdraws. Immediately afterward, they release the longer hidden chain to the network. The network automatically considers the longer chain as the true one, causing the original transactions to disappear. As a result, the attacker successfully performs a double-spend of the same coins.

However, there are limits to 51% attacks. An attacker cannot create coins from nothing, nor can they steal someone else’s private keys. Essentially, it’s only a temporary rewriting of transaction history, and the fundamental cryptographic rules of the network remain unbroken.

Has this attack actually happened? Yes, it has. Ethereum Classic experienced about $1.1 million worth of double spending in early 2019. In August 2020, it was attacked three times in one month. Bitcoin SV also suffered a major 51% attack in August 2021. Smaller altcoins like Vertcoin and Verge have been targeted multiple times. What do they have in common? All are small networks with low hash rates.

Why isn’t Bitcoin targeted? It’s an issue of economics and logistics. To launch a 51% attack on Bitcoin, you would need more mining power than all honest miners combined, which is enormous. It’s impossible to produce that many ASIC miners in such a short time within the global supply chain. Plus, powering millions of miners would require a scale comparable to a medium-sized country’s energy consumption. Secretly doing this is practically impossible.

Most importantly, if such an attack succeeded, trust in Bitcoin would collapse instantly. The value of the network would plummet, and the attacker’s double-spend rewards and investments would vanish together—economic suicide. Investing billions in Bitcoin makes destroying the network far less profitable than honestly earning mining rewards.

To protect your portfolio, a simple strategy is to focus on large, proven networks. Networks like Bitcoin, which are economically impossible to acquire or take over, have almost no risk of a 51% attack. If you hold smaller altcoins, managing your private keys yourself (self-custody) is crucial. Even if the network is attacked, your cryptographic keys are not compromised, so your funds remain safe.

While 51% attacks are a real risk, there’s no need to panic. Understanding how they work and managing risks properly allows you to participate safely in the world of decentralized finance.