Reentrancy

Definitions

1. A class of vulnerability where a function can be called again before its first invocation completes, leading to unexpected state. In smart contracts, reentrancy attacks exploit external calls that trigger callbacks into the original contract. The 2016 DAO hack ($60M) was a reentrancy exploit.

   In plain English: A security bug where a function gets called again before it finishes the first time, like someone cutting in line at a bank and withdrawing money that was already being withdrawn.

   Example: "The withdraw function sent ETH before updating the balance — a textbook reentrancy bug that drained the contract."

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