5-force.md

Force

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 Help page above, section "Beyond the console"

pragma solidity ^0.4.18;

contract Force {/*

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

*/}

Since this contract has no payable fallback or any other method and the challenge requires you to pay it somehow, it must be possible to send funds to a contract by some other means.

Reading the docs, we find this quote at Security Considerations:

Neither contracts nor “external accounts” are currently able to prevent that someone sends them Ether. Contracts can react on and reject a regular transfer, but there are ways to move Ether without creating a message call. One way is to simply “mine to” the contract address and the second way is using selfdestruct(x).

Since we haven't read about mining so far, selfdestruct seems like a better alternative. Let's read about it:

The only possibility that code is removed from the blockchain is when a contract at that address performs the selfdestruct operation. The remaining Ether stored at that address is sent to a designated target and then the storage and code is removed from the state.

This means we have to create a contract on the blockchain, send some funds to it, and then make it call selfdestruct with the instance address.

Let's do it:

pragma solidity ^0.4.18;

contract ForceWriteup {

  function ForceWriteup(address _address) payable public {
    selfdestruct(_address);
  }
}

1. Paste the code in Remix;
2. Go to the Run tab;
3. Make sure you have Injected Web3 as Environment and Metamask is on the Ropsten testnet;
4. Set Value to whatever nonzero amount you want;
5. Set Address to your Instance Address;
6. Click Create.

Then check whether it went through:

(await getBalance(instance)).toNumber()

And finally submit the instance back to the level.