Introduction

The Blockchain ecosystem has continued to grow in prominence and is becoming widely adopted across several industries. This growth is attributable to its functional characteristics like decentralization, immutability, and cryptographic techniques that create a highly secured system.

The blockchain's decentralized nature removes the need for a central control entity to oversee its operation.

Additionally, data stored on the blockchain network is immutable which means it cannot be altered or manipulated.
It uses cryptographic techniques like digital signature, hash function, and public keys to maintain the security of the network.

However, despite the efforts to secure blockchain networks, malicious actors still attempt to launch cyber-attacks and disrupt the integrity of the system. One such attack is the Sybil attack.

What Is Sybil Attack? How does this attack weaken the security of the blockchain network and what security measures can be taken to counter it?

This article will provide answers to these questions, read on to learn more.

What is a Sybil attack?

A Sybil attack is a malicious act in which attackers create multiple fake nodes often known as Sybil nodes to attack a system with the intent of gaining the majority of influence in the network and making it work for their selfish interest.

The name ‘Sybil' was inspired by a book in 1973, named ‘Sybil' where a character in the book suffered from a dissociative identity disorder. A disorder that is characterized by having multiple personalities.

In the blockchain network, a sybil attack involves a single entity creating multiple fake nodes to hijack the authority of the blockchain network, corrupt its reputation, and exploit the system to carry out illegal activities.

Attackers can use these multiple fake nodes to gain a large control of the system. This control gives them the ability to alter the decision-making in the system, manipulate transactions, and undermine the consensus mechanism.

Consensus mechanism is the method used to reach an agreement on the state of the blockchain. This ensures that the majority of the nodes in the network are holding the same copy of the transactions to validate that the transaction is legitimate.

Attackers can manipulate this consensus mechanism by amassing enough sybil nodes to outvote the authentic nodes. This enables them to approve or reject transactions, block authentic nodes, and create confusion among them.

How can a single attacker control multiple fake nodes in a decentralized network?

Although a decentralized network cannot be controlled by a central entity, attackers can control these multiple sybil nodes because they're a single entity disguised as multiple independent networks.

Once a sybil attack is launched into a decentralized network, the attackers can single-handedly control the multiple sybil nodes to perform fraudulent activities.

Types of blockchain sybil attack

Sybil nodes can attack the blockchain network in two major ways.

1.Direct Sybil Attack: This is a form of attack where sybil nodes are disguised as real nodes to interact with the original nodes in the system.
The original nodes being unable to identify those fake nodes interact with them. Through this interaction, attackers can gain influence and exploit the original nodes to carry out their fraudulent activities.
For instance, attackers can manipulate the original nodes to approve malicious transactions.

An illustration that explains how direct sybil attack works
Source: cyfrin updraft

2.Indirect Sybil Attack: This attack involves an indirect method of using an intermediary or proxy node to interact with the original nodes.
The sybil nodes first exploit the intermediary nodes and then use the exploited intermediary node to launch an attack on the original nodes.

An illustration that explains how indirect sybil attack works.
source: cyfrin updraft.

This indirect attack is difficult to detect because there's no direct interaction between the sybil nodes and original nodes.

Consequence of the sybil attack on the blockchain network

Successful sybil attacks in a blockchain network can cause catastrophic effects on the blockchain system such as:

1. Restriction of authorized users in the network: In a sybil attack, attackers have the majority of control over the network which can restrict the original nodes from participating in the voting system or validating transactions.

2. Undermining consensus mechanism: Attackers can deteriorate the integrity of the consensus mechanism by using the sybil nodes to take the majority of the votes in the network.

3.Security breaches: Attackers can use the sybil nodes to extract sensitive information like personal information and financial transactions from the original nodes which is against the blockchain principle of prioritizing security and anonymity.

Sybil Resistance Mechanism

A Sybil resistance mechanism is a technique used to prevent hackers from creating multiple fake nodes that can affect a peer-to-peer network or blockchain system.

This system is implemented to ensure that attackers cannot easily gain the majority of influence in the network by enforcing users to first prove their honesty before they can join the network.

Types of sybil resistance mechanisms

The types of sybil resistance are:

1. Proof of work (POW): This mechanism enforces miners(users) to solve computationally intensive tasks to prove their honesty.
This task involves miners expending their computational resources to find a valid hash.
A hash is a unique fingerprint for data.

Miners go through a continuous trial and error process of changing the nonce value in the block header. When this nonce value is combined with the block transaction and hashed with a hash function like SHA256 used in Bitcoin, it must meet the targeted hash value set for the task they are to perform in the network.

It is only after they get the valid hash and other participants in the network verify it before they are added to the block.

This process means that malicious actors trying to launch multiple fake nodes must go through the difficult process of finding answers to the required complex problems for each node that they want to add to the network.

This approach seems unachievable and a waste of computational resources because they need to amass enough nodes (more than 50%) before they can control the network.

Therefore, the proof of work mechanism prevents a sybil attack by enforcing miners to prove their honesty by solving computationally intensive tasks before they can add a node to the network.

2. Proof of Stake(POS): This is another mechanism used to prevent Sybil attacks where users(validators) are required to stake their cryptocurrencies as collateral before they can participate in the network.

Unlike POW where miners compete against each other to be the first to get a valid hash before they can propose a block, validators are pseudo-randomly picked, i.e., Validators are selected randomly and those who stake high have a higher chance of being selected.

Once they're selected, other validators in the network validate it to ensure It's legitimate.

Validators that successfully propose a valid block earn a reward and if a validator tries to compromise the network by acting dishonestly, a substantial amount of their staked assets is slashed and they'll be removed from the network.

This process makes it difficult for malicious actors to launch a sybil attack because they have to stake an asset upfront for each node they want to run.
Sponsoring more than 50% of these nodes to gain majority control in the network is way too expensive and they are at risk of losing everything once the attack fails.

Final Notes

Sybil's attack poses a serious threat to the blockchain, however, the Sybil resistance mechanism such as proof of work and proof of stake can counter this attack by making it extremely difficult for malicious actors to create multiple sybil nodes.