Reentrancy

A class of smart-contract bug in which an external call from a contract to another address allows the called address to re-enter the original contract before the first call's state changes have completed. The canonical exploit is the 2016 DAO hack ($60 million at the time), where the attacker's contract repeatedly withdrew from the DAO before its balance was decremented. The defensive pattern is checks-effects-interactions (validate inputs, update state, then call external addresses) and OpenZeppelin's ReentrancyGuard modifier. Despite being well-understood since 2016, reentrancy variants — including read-only reentrancy and cross-function reentrancy — continue to cause major exploits (Cream Finance, Fei, Curve).