51% Attack

What is a 51% attack in crypto?

A 51% attack refers to a situation where a single entity or group gains control of more than 50% of the mining hash rate or validating power of a blockchain network. 

This dominant control allows the attacker to potentially manipulate the blockchain in several critical ways: 

• Double-Spending: The attacker could reverse transactions that they made while in control, allowing them to spend the same cryptocurrency more than once, which undermines the fundamental trust of the network.
• Censorship of Transactions: The attacker can refuse to validate or confirm new transactions, effectively stalling the network for other users.
• Blocking Other Miners or Validators: By gaining majority control, the attacker could prevent others from mining or validating blocks, giving themselves disproportionate rewards. 

Who is vulnerable to a 51% attack?

This type of attack is particularly dangerous in Proof of Work (PoW) systems where mining power translates directly to control over block creation. While theoretically possible, it's extremely expensive and resource-intensive to carry out on large networks like Bitcoin or Ethereum (when it was PoW), making them unlikely targets. However, smaller blockchains with lower hash rates are much more vulnerable to this kind of attack. 

Why is the 51% attack important?  

The 51% attack is often used as a benchmark for network security. A blockchain’s resistance to this threat indicates the robustness and decentralization of its ecosystem. It also influences user trust, investor interest, and overall market legitimacy. Preventing such attacks is one reason why maintaining a decentralized and sufficiently distributed network is critical to any blockchain's long-term health.