The term Sybil Attack originates from the book "Sybil," which tells the case study of a woman with dissociative identity disorder, reflecting the behavior of attackers creating multiple false identities. A Sybil Attack refers to malicious attackers creating multiple false identities or nodes in a blockchain network to gain undue influence and control. Attackers can use these numerous false identities to manipulate the network, disrupt its functionality, or engage in other malicious activities.
Sybil Attacks have existed since the birth of the internet, mainly because real identities cannot be directly mapped to online identities. The most common example of a Sybil Attack in daily life is vote manipulation. For instance, in a competition where votes determine prizes, you might seek out individuals who specialize in vote manipulation to cast votes for you, or you might create multiple fake accounts to vote for yourself. Although these votes may come from different devices and IPs, they are fundamentally fake identities created by you, making this the most common example of a Sybil Attack.
The primary goal of a Sybil Attack is not necessarily to directly damage the network but to expand one's influence thereupon, thereby causing further disruption. This can include spreading false information, denying services to legitimate nodes, or even influencing the consensus mechanism to validate only certain transactions. Just like in our previous example, the act of vote manipulation doesn't necessarily damage the voting system but rather leverages influence (more votes) to gain benefits (prizes).
Sybil attacks can hinder regular users from using and accessing the network normally. Attackers create a sufficient number of fake identities to deceive honest nodes in voting, causing the blockchain network to stop transmitting or receiving blocks, thus preventing other users from participating in the network. For example, if a decision in a cryptocurrency project is made through node voting on the network, attackers can create thousands of fake accounts to influence the decision-making process.
Typically, the target of a Sybil attack is the entire network, with an aim to tamper with the credibility system of the network protocol. A successful Sybil attack can provide attackers with over half (i.e., ≥51%) of the total computing power, granting them access and control. When attackers control more than 51% of the network's computing power, they can reverse transactions or change the order of transactions, leading to the "double-spending" problem.
Double-spending refers to the same funds being spent multiple times. On networks like Bitcoin SV (BSV), Ethereum Classic (ETC), etc., there have been instances of double-spending issues due to attackers controlling over 51% of computing power.
Airdrop hunting has become a new form of Sybil attack. Airdrop hunters create numerous accounts and interact purposefully with smart contracts and protocols to obtain a large share of project tokens distributed through airdrops. You may see some users on the network achieving financial freedom after certain project airdrop releases. Essentially, they use the Sybil attack method to create a large number of accounts and participate in early-stage exchanges of projects to ultimately profit from airdrop distributions.
This form of Sybil attack disrupts the original intention of projects to distribute tokens equally, leading project teams to take anti-Sybil actions before airdrop distributions. These actions may include IP detection, account association analysis, mutual reporting, and other measures to prevent tokens from concentrating in the hands of a few airdrop hunters, thus avoiding situations where tokens are immediately sold off after listing, causing price drops.
Many blockchains use different consensus mechanisms to resist Sybil attacks, such as POW (proof of work) or POS (proof of stake), which increase the computational cost of creating blocks to prevent Sybil attacks in the case of POW, or asset risk, in the case of POS. Consensus mechanisms only increase the cost of a successful Sybil attack, making the attack impractical, but they do not eliminate Sybil attacks entirely.
For example, on the Bitcoin network, if an attacker wants to control over half of the network's computing power, they would need to purchase a large number of advanced mining equipment. Additionally, the costs of electricity, space, and ongoing maintenance are unimaginable. The Proof of Work (POW) consensus mechanism ensures the security of the Bitcoin network and increases the cost of attacks for attackers.
Sybil attacks occur because real-world identities cannot be directly matched to online identities. Using third-party identity verification verifies individual identities. If personal identities and their corresponding online identities are uniquely determined and cannot be forged, theoretically, Sybil attacks would not occur. In the blockchain industry, projects in the decentralized identity (DID) sector, such as on-chain identities and on-chain reputations, are attempts to solve the uniqueness of real-world and online identities.
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