Why do PoW and PoS still fall victim to the curse of the 51% Attack?

Question

Some say that blockchain is the ultimate firewall, but that's not the case. Whether you are mining with PoW or staking with PoS, as long as it involves a consensus mechanism, theoretically, it cannot escape the “original sin” of a 51% Attack—once someone controls more than 50% of the computing power or stake across the network, they can do as they please.

The Root of the Problem: The Paradox of Decentralization

Blockchain does not have a central bank dad, it relies on distributed nodes to maintain the ledger by themselves. But the problem arises - who has the right to keep accounts? Who has the final say? This is where the consensus mechanism comes in to make a decision.

The gameplay of PoW is very straightforward: competing for bookkeeping rights based on computing power. Nodes burn electricity madly to calculate SHA256 hash values, and whoever first finds a nonce value that meets the difficulty requirements gets the bookkeeping rights for this block. Sounds fair, right? Actually, it is not.

Fatal Flaw of PoW: Longest Chain Principle

There is an iron rule in Blockchain called the “Longest Valid Chain Principle” - the entire network recognizes the longest chain as the main chain. This was originally meant to ensure consistency, but it also opened the door to 51% attacks.

Imagine this scenario:

Step One: The attacker controls 51% of the network's Computing Power and intentionally broadcasts two different transactions to two regions of the network.

Step Two: The miners on both sides package these two transactions into blocks, forming two forked chains - Branch A and Branch B. According to the rules, the entire network will temporarily recognize the longer of the two chains.

Step Three: This is the moment of dark magic. The attacker uses their own 51% Computing Power to crazily mine on branch B, and soon the length of chain B will surpass that of chain A, causing the entire network to automatically switch to B as the main chain. All transactions on chain A will be voided and rolled back.

Result: The attacker has returned the coins spent on Chain A back to himself, but he has already obtained real goods or money from an exchange or merchant. The same coins were spent twice—this is known as double spending.

Real-World Bloody Cases

Bitcoin Gold (BTG) Incident: In 2018, attackers controlled 51% of the Computing Power and successfully reversed 22 Blocks. They first deposited BTG to the exchange to sell for cash, then reorganized the chain to invalidate the transactions, resulting in the theft of funds worth $18.6 million. This is not a trivial matter — this is an outright robbery.

A bunch of small coins have also been affected:

Verge (XVG): 35 million coins were mined within a few hours, resulting in a direct evaporation of $1.75 million.
Monacoin: Miners control 57% Computing Power, directly execute Block Attack.
Litecoin Cash ( LCC ), Zen, Ethereum Classic ( ETC ): are all on the list of 51% Attack

Why is it so hard to defend?

This is a vicious cycle. If you want decentralization, you have to delegate power to miners/validators. But as long as there is power, there is the potential for abuse. Even if you switch to PoS (Proof of Stake), the problem still exists—controlling 51% of the coin power can still lead to reckless behavior.

The only real defense line is: maintaining sufficient computing power/stake decentralization. Once a certain mining pool or whale holds more than 50%, the system enters a danger zone. This is also why major cryptocurrencies like Bitcoin and Ethereum are relatively safe— the ecosystem is large enough, and the computing power is sufficiently decentralized. But those small coins, new coins, and neglected coins? They are like lambs to the slaughter, and the cost of a 51% attack might be less than a few hundred thousand dollars.

Conclusion: The promise of decentralization in Blockchain can only ever be relative.