Fallback Attack

Smart Contract Security Practice | Lv1 Fallback Attack

!!! DON'T TRY ON MAINNET !!!

SUMMARY

Look carefully at the contract's code below. You find a security risk on the contract and expose it.

You will beat this level if

• you claim ownership of the contract
• you reduce its balance to 0

Things that might help

• How to send ether when interacting with an ABI
• How to send ether outside of the ABI
• Difference between send - call - transfer
• Converting to and from wei/ether units (see help() command)
• Fallback methods

SMART CONTRACT CODE

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.5 <0.9.0;

contract Fallback {
  mapping(address => uint256) public contributions;
  address payable public owner;

  constructor() {
    owner = payable(msg.sender);
    contributions[msg.sender] = 1000 * (1 ether);
  }

  modifier onlyOwner {
    require(msg.sender == owner, "caller is not the owner");
    _;
  }

  event NewOwner(address _newOwner);

  function contribute() public payable {
    require(msg.value < 0.001 ether);
    contributions[msg.sender] += msg.value;
    if (contributions[msg.sender] > contributions[owner]) {
      owner = payable(msg.sender);
      emit NewOwner(msg.sender);
    }
  }

  function getContribution() public view returns (uint256) {
    return contributions[msg.sender];
  }

  function withdraw() public onlyOwner {
    owner.transfer(address(this).balance);
  }

  fallback() external payable {
    require(msg.value > 0 && contributions[msg.sender] > 0);
    owner = payable(msg.sender);
    emit NewOwner(msg.sender);
  }
}

HOW TO EXPOSE THE SECURITY RISK?

Fallback method changes the owner of the contract, so how can we make it happen?

You could find the code line that uses call function instead of transfer or send in order to send ether to the target contract in Hacker.sol. In many documents, call is dangerous, and needs care about using it. And I do recommend not to use it for your real smart contract.

One of the important differences between call and transfer or send, is that when using call, you can set the amount of gas that will be available in the transaction generated by call call.

The call in Hacker contract will call fallback function in Fallback contract.

Look at that fallback function, and try to predict how many gas it would need? Here are some Ethereum specifications that help you to calculate it.

To occupy a 256 Bit slot of Storage costs 20,000 gas. Changing a value of an already occupied slot costs 5,000 gas.

In the fallback function, it replaces the owner with the new one, and the owner is a state variable that stores on Storage. Then you can easily get the amount of gas required for the fallback function as roughly more than 25,000 gas.

Why to use call?

But trasfer and send functions are limited with 2300 gas stipend and not adjustable. So, if you attack with transfer or send, you will get "Out of Gas" exception, and in many cases, Remix, truffle and etc, they don't give the exact error description.

With call you can adjust the amount gas used in the called contract, and the sufficient amount of gas will allow the target contract to replace the owner, ultimately you will get success on the attack.

DEPLOY & TEST

Installation

npm install
npx hardhat node

Deployment

npx hardhat run --network [NETWORK-NAME] scripts/deploy.js

Test

npx hardhat test

You can test fallback on the local hardhat node as well.