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What Is a Sybil Attack?

5 mins
Updated by Ryan Glenn

Technology has revolutionized the way people interact in all areas of life. However, these upgrades don’t come without risk. While blockchain specifically may have provided a more secure, transparent means for transactions in general, the risk of an attack remains. A Sybil attack is one of the leading threats in blockchain technology. It is an online security attack where one computer controls multiple fake identities. However, Sybil attack prevention is possible

In this guide, we go over the different types of Sybil attacks along with the problems they cause on an online network. We also go over the ways in which you can prevent them. This way, you can use online networks with more confidence and trust. 

What is a Sybil attack?

what is a sybil attack

A Sybil attack is an online security threat in which one computer — referred to as a node — operates multiple fake identities on a peer-to-peer (P2P) network. Similar to when someone creates multiple social media accounts, one single user can run multiple nodes (IP addresses or user accounts) simultaneously on the network. 

Did you know?

The attack name was inspired by a character in Flora Rheta Schreiber’s 1973 book “Sybil” named Sybil Dorsett.

In Schreiber’s book, Sybil had Dissociative Identity Disorder — or Multiple Personality Disorder. One person exhibiting several distinct personalities characterizes this disorder. Similar to attacks on a peer-to-peer network. However, computer scientist Brian Zill coined the term Sybil and John R. Douceur discussed it in his paper

How does a Sybil attack take place?

Sybil attacks take place when a single entity (node) impersonates real users on a network they wish to infiltrate by creating multiple accounts. Each new identity acts independently from the other and performs its own transactions. It appears as if each node is operated by its own individual entity when in reality, it’s one person controlling them all.

Sybil attacks are not limited to just blockchains. However, because blockchain networks are governed by majority influence, a large-scale Sybil attack risk is high since it gives the attacker more centralized authority on a decentralized platform. 

This can happen on networks such as Bitcoin because many decisions depend on a majority vote. With this much control over the system, an attacker can rewrite the distributed public ledger. Likewise, a Sybil attack on social media influences the majority opinion, since one person manipulates many of the users. 

Types of Sybil attacks

sybil attack types

To better understand how Sybil attacks work, it would help to know the different types of Sybil attacks that exist. Both malicious and authentic nodes run on the same network, but how they communicate is classified into two categories: direct and indirect. 


In a direct attack, Sybil nodes directly influence honest nodes on the network . The malicious nodes communicate with authentic nodes while simultaneously mimicking an honest node. The real nodes don’t realize they are lead astray. As a result, they readily accept influence from the fake ones and allow direct communication. 


In contrast to a direct attack, indirect attacks involve another set of nodes that act as middlemen. These unsuspecting nodes are actually under the influence of the Sybil nodes, so they remain compromised. Therefore, the communication between the Sybil nodes and the honest nodes is indirect; they don’t interact with each other. These types of attacks allow Sybil nodes to run a network more undetected than in a direct attack. 

Problems caused by Sybil attacks

The goal of a Sybil attack is to gain influence over an entire network to control decisions. A successful attack can create problems, including:

  • Setting up a 51% Attack: A 51% attack on a blockchain network is characterized by control of over half of the network’s computer power — referred to as a hash rate. This can potentially cause a network disruption since transactions can be modified by majority power. In a 51% attack, an attacker can create fraudulent transaction blocks while invalidating others. In addition, with this much control, they can also enable double-spending by reversing transactions. 
  • Blocking users from a network: When enough false nodes are set up on a network, the attacker can essentially vote out honest nodes. This also enables the Sybil node to refuse either transmitting or receiving blocks. As a result, honest nodes are unable to access the system. 

How to prevent a Sybil attack

Sybil attack problems

For years, computer scientists have put in a lot of effort to find ways to prevent which Sybil attacks. As of now, no one can fully guarantee defense against an attack. However, there are measures to maximize security and prevention.  


The consensus algorithm secures blockchains. A consensus algorithm is the process that determines agreement regarding the transactions on a blockchain. In the case of proof-of-work, miners (nodes) on the network use up computing power to solve complex mathematical problems to verify transactions it is known as mining. 

Therefore, to reach a collaborative consensus on the blockchain, enough miners need to agree on the authenticity of the data. This process makes it almost nearly impossible for one entity to gain control of more than half of the network when there are so many miners needed to verify. In addition, it would be very costly for one person to own the majority of the equipment since a single computer represents each node. 

ID verification

Depending on the network, there are multiple ways in which ID verification can be done, either directly or indirectly. Through direct validation, a central authority verifies the new identities. When validation is done indirectly, established members who have already been verified can then verify the new identities. These procedures often require new members to identify themselves via credit cards, IP addresses, or two-factor authentication. 

Another way to ensure ID verification is to require a fee for every identity that is created which will make it extremely costly for one entity to acquire so many nodes.   

Reputation system

A reputation system gives power in varying degrees to the members of the network. These power levels are based on the reputation level of the member. Generally, those who have been in the system for a longer period are given more power to perform tasks or more interactions.

These members have built trust over time and have proven to be honest nodes, so they can exert more power over Sybil nodes and override them. This type of power in a system often discourages attacks from happening since attackers would have to wait a long time to reach higher reputation levels.  

Are all blockchains vulnerable to Sybil attacks? 

Theoretically, all blockchains are vulnerable to Sybil attacks. However, the size of the network does often make a difference. The more miners needed to validate transactions, the better, since it’s extremely difficult for one entity to control 51% of the miners. Due to Bitcoin’s large network, for example, it has proven to be more resistant against both Sybil and 51% attacks. No one has ever been able to successfully perform a 51% attack on Bitcoin.

Sybil attacks are something to be concerned about, but for the most part, teams need only take a few precautions to prevent it from happening. As always, security decisions in the design process are a priority in the development process.

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Frequently asked questions

What is a Sybil attack?

Is Sybil attack possible on Bitcoin?

How do you prevent Sybil attacks?

How does blockchain mitigate Sybil attacks?

What is Sybil attack in IoT?

What is a 51% attack?


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