Definition

What is Reentrancy?

Reentrancy is a vulnerability where an external call lets an attacker's contract call back into your function before its state updates finish, often draining funds.

The classic case is sending ETH before setting a balance to zero; the recipient's fallback re-enters withdraw and repeats. The fix is the checks-effects-interactions pattern (update state before making external calls) or a reentrancy guard such as OpenZeppelin's ReentrancyGuard.

Definition by Adwait Keshari, founder of Monsmith.