Reentrancy Attack occurs when
the attacker drains funds from the target by recursively calling
the target’s withdraw function. When the contract fails to
update its state, a victim’s balance, prior to sending funds,
the attacker can continuously call the withdraw function to
drain the contract’s funds. A famous real-world Reentrancy
attack is the The DAO Project attack, which caused a loss of 60 million US dollars.