How Do You Prevent Reentrancy Attacks in Solidity?

Problem Faced:
A reentrancy attack occurs when an attacker re-enters a smart contract function before the previous execution completes, leading to unexpected behavior like multiple withdrawals.
**
Solution:**
Use the Checks-Effects-Interactions pattern and Solidity’s reentrancy guard modifier.

• Check: Validate the caller and amount.
• Effects: Update the contract’s state.
• Interactions: Transfer ETH last.

Fix Using ReentrancyGuard:
solidity

pragma solidity ^0.8.0;
import "@openzeppelin/contracts/security/ReentrancyGuard.sol";

contract SecureContract is ReentrancyGuard {
    mapping(address => uint256) balances;

    function withdraw() external nonReentrant {
        uint256 amount = balances[msg.sender];
        require(amount > 0, "Insufficient balance");

        balances[msg.sender] = 0; // State update before transfer
        payable(msg.sender).transfer(amount);
    }
}

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