The blockchain is a robust and efficient technology, but it is not without flaws. Security flaws persist, and one of the most serious issues with blockchain security is known as a Sybil Attack. This guide will teach you everything you need to know about Sybil Attacks, including what they are, how they work, examples of them, and how to avoid them. So, without further ado, let us get started and learn more about Sybil Attacks!

What Is Sybil Attacks?

A Sybil attack is a type of attack in which an adversary creates multiple identities in order to gain control of a network. Flora Rheta Schreiber, a psychologist, coined the term in 1973 to describe a woman with dissociative identity disorder (DID).

In a blockchain context, the term refers to someone who uses multiple identities or accounts — also known as sockpuppets — to confuse and manipulate others. Individuals or organizations with sufficient resources can create a large number of bogus accounts to make it appear that there are more people involved in supporting a particular cause than actually exist. This can be used to sway public opinion and make fraudulent transactions appear legitimate.

Sybil Attacks: How Do They Work?

An attacker must first create multiple identities in order to carry out a Sybil attack. These new identities are autonomous and have their own transactions on the blockchain network. As a result, it may appear that many people are using your service when, in fact, only one person is responsible for all of their activities.

As you might expect, such an attack has numerous potential applications:

  • Using bots to vote in elections for someone else. With the rise of Decentralized Autonomous Organizations (DAOs), this has become a common issue.
  • Attackers may also be able to out-vote honest nodes on a network if they create a large enough number of fake identities (or Sybil identities). They can then refuse to receive or transmit blocks, effectively excluding other network users.
  • Attackers may even be able to execute a 51% attack in large-scale Sybil attacks by seizing control of the network’s processing power or hash rate. Under such circumstances, they might be able to rearrange the transactions and stop them from being approved. They have the ability to undo transactions they completed while in charge, which could result in double-spending.

Sybil Attack Examples

Here are a couple of real-world examples of Sybil attacks:

  • For several months in 2014, a notable Sybil attack was launched against the Tor anonymity network. Tor anonymity is achieved by routing traffic through three distinct nodes. The first is aware of the user’s IP address, while the third is aware of the traffic’s destination. The middle acts as a trusted intermediary, so nodes one and three are unaware of each other. Running a large number of servers has the potential to breach those guarantees of anonymity.
  • Another notable example is the 51% theft of Bitcoin gold in May 2018, which resulted in the theft of $18 million in Bitcoin gold. That was the network’s second such attack, with the first occurring in late 2017, resulting in over $72,000 worth of Bitcoin gold tokens being double-spent.

How to Prevent Sybil Attacks

One way to avoid Sybil attacks is to use proof-of-stake consensus algorithms instead of proof-of-work. The proof of stake consensus mechanism requires master node or service node operators to stake (lock up a significant amount of cryptocurrency for a period of time). Sybil Attacks are deterred by this stake. A node that is detected by the network as engaging in fraudulent or malicious activity risks losing a portion of its stake as well as the right to participate in the future. Because the stake is higher than the potential reward, the cost of attacking exceeds the reward once more.

Furthermore, in order to gain control of the entire network, the attacker would need to obtain 51% of the cryptocurrency’s circulating supply. The cost of doing so would be so high that no attacker could possibly afford it.

Large proof-of-work networks, such as Bitcoin, are also immune to 51% attacks. The amount of mining hardware required to control 51% of the network’s hash power (enabling a Sybil Attack) would be prohibitively expensive, making it unprofitable for the attacker. Simply put, the cost of an attack would outweigh the potential rewards.


Sybil attacks remain a significant challenge for blockchain technology. However, by understanding the different types of attacks and implementing effective prevention strategies, it is possible to mitigate their impact and ensure the long-term security and sustainability of blockchain networks.