A Sybil attack is an online security threat involving an attacker trying to take over a network by creating and operating multiple nodes or accounts. In the decentralized environment of a blockchain network, a Sybil attack would require the attacker to own or control various nodes within the network.
A Sybil attack could become a significant threat to a blockchain’s distributed or decentralized operations as the attacker could foster dishonest behavior among nodes. Additionally, gaining control over multiple nodes could potentially empower the attacker to manipulate the blockchain's computing power or hash rate.
A severe Sybil attack could also see the attacker gain majority control over the blockchain's nodes, turning the episode into a 51% attack on the network. This could raise the risks of launching double spending attacks, manipulating the order of transactions, and preventing transactions from recording on the distributed ledger.