A 51% attack is a potential threat to blockchain networks that rely on consensus mechanisms like Proof of Work (PoW) or Proof of Stake (PoS). 

This attack typically happens when a single entity or group gains control of more than half the network’s mining (PoW) or staking (PoS) power, allowing them to manipulate the blockchain in dangerous ways.

Here’s what the attackers can do with that power:

• Double-spending: The attacker can spend the same cryptocurrency twice by reversing already confirmed transactions.
• Rewriting blocks: They can alter the order of transactions, exclude certain transactions, or even censor them entirely.
• Blocking new transactions: By preventing new transactions from being validated, they can grind the network to a halt.

A successful 51% attack undermines trust in the blockchain, disrupting the very decentralization that makes these networks secure. But before you panic—pulling off such an attack is insanely difficult and expensive, especially on major blockchains like Bitcoin or Ethereum.

How much would a 51% attack cost?

The price tag for a 51% attack depends on the blockchain’s size, security, and consensus model. 

• In Proof of Work (PoW) networks, the attacker needs to outmatch the combined computing power of honest miners, which requires massive amounts of hardware and electricity. For a major network like Bitcoin, this would cost billions of dollars in mining equipment alone—not to mention ongoing electricity costs. On the other hand, smaller PoW blockchains (like Bitcoin Gold or Ethereum Classic) are more vulnerable, since their total mining power is much lower. In some cases, an attacker might only need hundreds of thousands or a few million dollars to launch an attack.
• In Proof of Stake (PoS) networks, it’s not about computing power—it’s about money. To attack Ethereum (which switched to PoS in 2022), an attacker would need to stake over 10 million ETH—worth tens of billions of dollars. And that’s just to gain control, without even factoring in the risk of losing it all.

Can a 51% attack happen on proof of stake?

Technically, yes. But in reality? Not likely. Here’s why:

• Massive financial barrier: PoS attacks require buying up over 50% of all staked tokens. 
• Slashing penalties: Many PoS networks, including Ethereum, punish malicious behavior by confiscating an attacker’s staked funds. Try to cheat? You could lose everything.
• Network decentralization: A PoS network typically has thousands of validators spread worldwide. Gaining majority control isn’t just expensive—it’s logistically nightmarish.
• Market impact: Buying up that much of a token would skyrocket its price, making the attack even more expensive. And if the attack succeeds? The token’s value would probably crash, leaving the attacker with a pile of worthless coins.

While smaller PoS networks with low staking participation could be more at risk, the sheer cost and built-in protections make a 51% attack on major networks like Ethereum extremely unlikely—if not outright impossible.