Reentrancy Attacks


A reentrancy attack happens when a function is externally invoked during its execution, allowing it to be run multiple times in a single transaction. This typically occurs when a contract calls another contract before it resolves its state.


A successful reentrancy attack can lead to fund drains, unauthorized function calls, or state changes that disrupt the normal operations of the contract.

Steps to Fix

  1. Make sure you follow the Checks-Effects-Interactions (CEI) pattern: check conditions, then make changes, then interact with other contracts.
  2. Use a reentrancy guard or mutual exclusion lock (mutex) to block recursive calls from external contracts during a function’s execution.
  3. Regularly update to the latest version of Solidity, which includes inherent protection against reentrancy attacks.


The infamous DAO hack was a reentrancy attack. An attacker exploited a reentrancy vulnerability to drain around 3.6 million Ether from the contract.