A Sybil attack is used to target peer-to-peer networks (P2P): the attacker creates multiple malicious nodes controlled by a single entity which in turn tries to gain the majority share in the network and outvote the healthy nodes.
Similarly, in blockchain, the main malicious node further creates multiple malicious nodes creating a pool of nodes large enough to manipulate the voting process of a block being added to the ledger.
Note: This can lead to a
if carried out on a large scale. 51% attack When a person gains access to a majority share of the computing power of the blockchain or controls a majority of the nodes responsible for voting a new block in to the ledger, they can stop the block formation, validate malicious transactions and cause double spend.
The verification process used at the time of the addition of new nodes in the network is the key reason why such attacks are successful. It gives the attackers an easy way to create nodes that may be malicious in nature.
Public blockchains like Bitcoin do not use any verification process and are the ones most easily affected by this because they allow anyone and everyone to join the network. This makes it relatively easier for the attacker to take control of the nodes. However, this problem is not as prominent in private blockchains because the new nodes are added after verification.
Blockchain, specifically public blockchains, try to mitigate this problem by using different consensus mechanisms such as Proof of Work (PoW) and Proof of Stake (PoS). These mechanisms help ensure that achieving the majority becomes very difficult, and in the end, the reward does not justify the cost. These mechanisms ensure that to get the majority, the attacker either needs to control the majority of the total hash rate, which is very expensive in the case of PoW, or in the case of PoS, stake their own cryptocurrency, which ensures that the ledger can not be tampered with unless the attacker stakes a significant amount of money which will make the whole process extremely expensive.
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