OnlyPwner - Payday - Writeup

题目信息

• 题目名称: Payday
• 作者: bobface
• 难度: 中级
• 类型: Merkle 树安全

题目描述

Your competitor has just set up a node operator fee claiming contract for their users. It would be a shame if it stopped working properly...

Merkle 树结构：

• 包含 20 个收款人，每个可以提取指定的 ETH 余额
• 叶子存储 keccak256(recipient || amount || validUntil) 的哈希值

胜利条件

合约余额少于 1 ETH，且 20 个原始收款人均未领取（余额为 0）。

漏洞分析

1. 核心合约

contract Distributor {
    bytes32 public root;
    mapping(address => bool) public hasClaimed;

    function withdraw(
        bytes calldata params,
        bytes32[] calldata proof
    ) external {
        require(params.length == 64, "invalid params");

        bytes32 leaf = keccak256(params);  // ← 关键漏洞点
        require(MerkleProof.verifyProof(leaf, root, proof), "invalid proof");

        (address recipient, uint72 amount, uint184 validUntil) = decodeParams(params);

        require(!hasClaimed[recipient], "already claimed");
        require(validUntil >= block.timestamp, "expired");

        hasClaimed[recipient] = true;
        (bool success, ) = recipient.call{value: amount}("");
        require(success, "failed to send ether");
    }
}