OnlyPwner - 13th Airdrop - Writeup

题目信息

• 题目: 13th Airdrop
• 作者: bobface
• 目标: 使 Airdrop 合约余额 <= 30 ETH
• 初始状态: Airdrop 有 60 ETH（6 个用户各 10 ETH）

漏洞分析

合约结构

contract Airdrop is IAirdrop {
    mapping(address => uint256) public balances;
    uint256 private savedContractBalance;
    bool private entered = false;
    
    function claim(address recipient) external notContract(recipient) {
        savedContractBalance = address(this).balance;
        
        require(!entered, "ReentrancyGuard: reentrant call");
        entered = true;
        (bool success, ) = msg.sender.call{value: balances[msg.sender]}("");
        require(success, "Transfer failed.");
        entered = false;
        
        updateUserBalance();
    }
    
    function updateUserBalance() internal {
        assembly {
            function checkFunds(originalUserBalance, originalContractBalance)
                -> newUserBalance
            {
                let expectedBalance := sub(originalContractBalance, originalUserBalance)
                if iszero(gt(balance(address()), expectedBalance)) {
                    let diff := sub(expectedBalance, balance(address()))
                    newUserBalance := sub(newUserBalance, diff)
                    leave
                }
                return(0, 0)  // 关键漏洞点！
            }
            
            mstore(0x0, caller())
            mstore(0x20, 0)
            let slot := keccak256(0x0, 0x40)
            let originalContractBalance := sload(1)
            let originalUserBalance := sload(slot)
            
            sstore(slot, 0)  // 先设为 0
            
            let newUserBalance := checkFunds(originalUserBalance, originalContractBalance)
            
            sstore(slot, newUserBalance)  // 如果 return(0,0) 执行，这行不执行
        }
    }
}

漏洞点

在 updateUserBalance() 的 assembly 代码中：