Reentrancy Attack

A Reentrancy Attack is a critical smart contract vulnerability where an external call to an untrusted contract allows the malicious contract to recursively call back into the original function before the original transaction's state has been updated, enabling the attacker to drain funds or manipulate data repeatedly. This exploit is most famously associated with the 2016 DAO hack on the Ethereum network, which resulted in the loss of over $60 million worth of Ether and necessitated a hard fork of the blockchain.