Denial - The Ethernaut - Writeup
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.
The code of the challenge’s contract is the following:
// SPDX-License-Identifier: MIT pragma solidity ^0.8.0; contract Denial { address public partner; // withdrawal partner - pay the gas, split the withdraw address public constant owner = address(0xA9E); uint256 timeLastWithdrawn; mapping(address => uint256) withdrawPartnerBalances; // keep track of partners balances function setWithdrawPartner(address _partner) public { partner = _partner; } // withdraw 1% to recipient and 1% to owner function withdraw() public { uint256 amountToSend = address(this).balance / 100; // perform a call without checking return // The recipient can revert, the owner will still get their share partner.call{value: amountToSend}(""); payable(owner).transfer(amountToSend); // keep track of last withdrawal time timeLastWithdrawn = block.timestamp; withdrawPartnerBalances[partner] += amountToSend; } // allow deposit of funds receive() external payable {} // convenience function function contractBalance() public view returns (uint256) { return address(this).balance; } } We need to create a DoS of the contract when the owner executes the function withdraw. We control the partner address (through the setWithdrawPartner function), to which a call is made that does not specify a fixed gas limit.
