# Reentrancy

> A smart contract bug that lets an attacker call back into a contract before its first call finishes, draining funds.

Canonical URL: https://fudfomo.co/glossary/reentrancy
Source: What The Block! Dictionary v1.0 (last updated 2026-04-25), browsable at https://wtb.fudfomo.co.

## Definition

Reentrancy is a class of bug where a smart contract makes an external call before fully updating its own balances. The attacker uses that gap to call back into the contract repeatedly, withdrawing funds each time before the balance has been corrected.

The 2016 DAO hack is the most famous reentrancy attack. Modern Solidity practice avoids it by following the checks-effects-interactions pattern and using reentrancy guards.

## Related terms

- [Smart Contract](https://fudfomo.co/glossary/smart-contract): A program that runs on a blockchain and does what it says, automatically.
- [Solidity](https://fudfomo.co/glossary/solidity): The most common programming language for writing smart contracts on Ethereum.
- [DAO](https://fudfomo.co/glossary/dao): Decentralised autonomous organisation. A group whose decisions and treasury are managed on a blockchain.

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