Blockchain is certainly one of the most exciting new innovations in the financial world. However, this new technology comes with some potential hazards. If you want to avoid major issues, you need to be aware of problems like Sybil attacks. What is a Sybil attack in blockchain? This guide will help you understand what it is, and the types of Sybil attacks that exist today.
What Is a Sybil Attack in Blockchain?
A Sybil attack is a type of malicious assault that targets peer-to-peer (P2P) networks. It involves a single entity operating multiple identities at the same time to get undue influence over the network's actions.
This type of attack existed long before blockchain networks. It can be used in just about any sort of P2P network, so it’s a potential threat to many applications. This computer network service attack has gone by many other names, including “pseudospoofing” and “sock puppets.”
The term "Sybil attack" was first coined in 2002 by Brian Zill, at that time a researcher at Microsoft. He named the attack after Sybil Dorsett (neé Shirley Ardell Mason), a renowned psychological patient known for having dissociative identity disorder. Reminiscent of Dorsett's case, a Sybil attack involves multiple identities all being controlled by a single node in a network. Though the identities appear to be completely distinct nodes, the reality is that they all correspond to the same local entity.
This type of attack was initially used to gain excessive control over P2P systems. However, with the advent of blockchain, Sybil attacks became a lot more prevalent. In modern times, a large-scale Sybil attack can be one of the most efficient ways of taking over a system such as bitTorrent’s Mainline DHT. A Sybil attack can be used to manipulate outcomes in a network, or it can disrupt a network's functioning entirely.
Types of Sybil Attacks
To better understand what a Sybil attack is, it’s helpful to take a closer look at how these attacks work. Sybil attacks are usually classified into two main categories.
Direct Attacks
A direct attack is the most straightforward. It starts with one or more nodes that spoof other nodes within the network. These nodes, which are called Sybil nodes, mimic the identity of an authentic node in the network.
In a direct attack, other real nodes end up communicating directly with the Sybil nodes. The authentic nodes don’t realize the Sybil node is a fake, so they communicate with it directly and accept influence from the Sybil nodes.
Indirect Attacks
An indirect attack involves both normal nodes and Sybil nodes. However, the normal nodes and the fake nodes don’t directly interact with each other; instead, a Sybil node maliciously influences a middle node in the network. This node then becomes a malicious node that communicates with other nodes on behalf of the Sybil node. An indirect attack allows the Sybil node to affect the network without being easily identifiable.
Problems Caused by a Sybil Attack
Why should you be concerned about a Sybil attack? A few false identities might seem relatively harmless, but as more develop, they can greatly harm a network. Here are some common examples of how a Sybil attack causes problems.
1. Sybil Attacks Can Block Users From a Network
The main concern with a Sybil attack is that it can create enough fake identities to completely overwhelm all the honest network nodes. Once the Sybil nodes have control of the network, they can completely alter the way the network runs. The Sybil nodes can refuse to either transmit or receive blocks in a network. This ends up essentially blocking other users from being able to access the network.
These attacks can temporarily cause a drop in crypto values, so they can be very problematic. In fact, ETHereum's founder has stated that he plans to entirely change the crypto's algorithm simply due to the problems caused by recent Sybil attacks.
2. Sybil Attacks Can Disrupt the Network with a 51% Attack
In modern times, a Sybil attack is often used to enact a 51% attack. This type of online assault involves making sure that 51% or more of the network is made up of false nodes. These nodes can then outvote all the honest nodes within the network. This sort of Sybil attack tends to cause quite a few issues with voting on operational decisions in a network.
The attack can be used to stop transactions from being confirmed, order new transactions, or even reverse transactions to cause double-spending. For example, in August 2021, Bitcoin SV underwent a 51% attack that let malicious miners double-spend coins. This caused BSV values to plunge by 4%.
3. A Sybil Attack Can Compromise Privacy
Nodes are responsible for handling the flow of information within a network, so any compromised node is a privacy risk. A Sybil node can be used to retrieve information about other nodes within the network. At the very least, a Sybil attack can do things like obtaining users' IP addresses to create more fake nodes.
When used on a P2P model like the Tor network, this privacy breach can be even more malicious. An attacker can use Sybil nodes to spy on data transfer and to monitor network traffic. A widespread attack from 2017–2020 used 900 servers to discover the data of hundreds of Tor users. This in turn defeated the entire purpose of Tor which was to guarantee anonymity.
Are All Blockchains Susceptible to a Sybil Attack?
As you can see, a Sybil attack can cause all sorts of problems. Just about any P2P system can be harmed by a Sybil attack. Since a blockchain is a type of decentralized P2P network, a Sybil attack can affect it. However, this doesn't necessarily mean that all blockchains are at risk of a Sybil attack. There are plenty of ways to protect against these attacks, so only certain types of blockchain need to worry about a Sybil attack.
In some cases, a Sybil attack can be devastating. For example, the cryptocurrency Verge suffered a massive Sybil attack in 2021. The attacker managed to perform the deepest reorganization ever of a blockchain. Over 200 days' worth of transactions were deleted. In response, Bittrex had to entirely pause the Verge wallet.
There have also been instances of a Sybil attack being used to compromise blockchain privacy. Some types of crypto, like Monero, are focused on providing a private, secure mETHod for completing transactions. In 2020, an unidentified assailant used a Sybil attack to disrupt the network and link transactions to IP addresses. Though Monero was mostly able to halt the attack, some private user information was still leaked.
While there have been many examples of a Sybil attack on smaller blockchains, it’s worth noting that big blockchains are less susceptible. For example, no attacker has managed to successfully complete a 51% attack on Bitcoin. Since the blockchain is so large, the costs of adding so many fake nodes would outweigh most benefits of an attack. Furthermore, since new blocks are added at such a rapid rate in Bitcoin, an attacker has only a very small window to make alterations.
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.
Closing Thoughts
Now that you know what is a Sybil attack in blockchain, it is ultimately one of the leading threats in modern blockchain technology. These online assaults can damage crypto values, steal funds and affect user privacy. Most prevention strategies simply rely on making the cost of an attack too pricey, but these mETHods don't guarantee security. To truly avoid the negative results of a Sybil assault, blockchain networks need to design their systems carefully and implement extra forms of user verification security.
