A Sybil attack is a kind of security threat on an online system where one person tries to take over the network by creating multiple accounts, nodes or computers.

This can be as simple as one person creating multiple social media accounts.

But in the world of cryptocurrencies, a more relevant example is where somebody runs multiple nodes on a blockchain network. 

The word “Sybil” in the name comes from a case study about a woman named Sybil Dorsett, who was treated for Dissociative Identity Disorder – also called Multiple Personality Disorder.


What problems can Sybil attacks cause?

  • Attackers may be able to out-vote the honest nodes on the network if they create enough fake identities (or Sybil identities). They can then refuse to receive or transmit blocks, effectively blocking other users from a network.

  • In really large-scale Sybil attacks, where the attackers manage to control the majority of the network computing power or hash rate, they can carry out a 51% attack. In such cases, they may change the ordering of transactions, and prevent transactions from being confirmed. They may even reverse transactions that they made while in control, which can lead to double spending. 

Over the years, computer scientists have dedicated a lot of time and research to figure out how to detect and prevent Sybil attacks, with varying degrees of effectiveness. For now, there’s no guaranteed defense.


So how do blockchains mitigate Sybil attacks?

Many blockchains use different “consensus algorithms” to help defend against Sybil attacks, such as Proof of Work, Proof of Stake, and Delegated Proof of Stake.

These consensus algorithms don’t actually prevent Sybil attacks, they just make it very impractical for an attacker to successfully carry out a Sybil attack.

For example, Bitcoin’s blockchain applies a specific set of rules to the generation of new blocks.

One of the rules is that the ability to create a block must be proportional to the total processing power of the Proof of Work mechanism. That means that you have to actually own the computer power required to create a new block, which makes it very difficult and costly for an attacker to do. 

Since mining Bitcoin is so intensive, miners have a very strong incentive to keep mining honestly, instead of attempting a Sybil attack.


Besides Sybil attacks, there are a few other common types of attacks. Stay tuned to Binance Academy for more!