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Securing the Blockchain: Preventing Smart Contract Exploits with Sample Code for Ethereum

As blockchain technology continues to evolve and become more widely adopted, there is an increasing interest in the security of blockchain networks. One of the primary concerns is the potential for malicious actors to hack the blockchain and compromise the integrity of the data stored within it. While the security of blockchain networks is generally considered to be robust, there are still potential vulnerabilities that can be exploited by hackers.

In this article, we will explore the concept of hacking the blockchain and provide a case study using the Ethereum blockchain as an example. We will also provide some sample code to demonstrate how these attacks can be carried out.

What is Blockchain Hacking?

Blockchain hacking is the act of exploiting vulnerabilities within a blockchain network to gain unauthorized access, modify data, or steal assets. Blockchain networks are generally considered to be highly secure due to their decentralized nature and the use of cryptography to secure transactions. However, as with any technology, there are potential vulnerabilities that can be exploited by skilled hackers.

There are several types of attacks that can be carried out against blockchain networks, including:

  1. 51% Attack – This type of attack involves a malicious actor gaining control of the majority of the computing power within a blockchain network. This allows the attacker to control the validation of transactions, which can be used to modify the blockchain’s history or double-spend coins.
  2. Sybil Attack – A Sybil attack is when a malicious actor creates multiple fake identities or nodes within a blockchain network to gain control of the network. This type of attack is often used to carry out other types of attacks, such as a 51% attack.
  3. Smart Contract Exploits – Smart contracts are self-executing contracts with the terms of the agreement between buyer and seller being directly written into lines of code. A smart contract exploit involves a hacker finding a vulnerability in the code that allows them to manipulate the contract and steal funds.
  4. Private Key Theft – Private keys are used to authenticate transactions on a blockchain network. If a hacker gains access to someone’s private key, they can steal their funds.

Ethereum Blockchain Hacking Case Study

As the second-largest blockchain network in the world, the Ethereum network is an attractive target for hackers. In recent years, there have been several high-profile attacks against the network, including the DAO attack in 2016 and the Parity wallet hack in 2017.

The DAO Attack

The DAO (Decentralized Autonomous Organization) was a smart contract on the Ethereum blockchain that raised $150 million in Ether through an initial coin offering (ICO). The DAO was designed to be a decentralized venture capital fund, with investors receiving tokens that represented their share of the fund.

In June 2016, a hacker exploited a vulnerability in the DAO’s smart contract code and drained approximately one-third of the fund’s Ether into a “child DAO.” The attack was made possible by a reentrancy vulnerability in the smart contract code, which allowed the hacker to repeatedly withdraw funds without updating the token balance.

The Ethereum community was faced with a difficult decision: either hard fork the blockchain to reverse the attack and return the stolen funds, or allow the hack to stand and potentially undermine investor confidence in the platform. Ultimately, the community decided to hard fork the blockchain, creating Ethereum Classic as a result.

The Parity Wallet Hack

In July 2017, a hacker exploited a vulnerability in the Parity multi-signature wallet software to steal approximately $30 million worth of Ether. The vulnerability was caused by a bug in the wallet’s smart contract code that allowed the hacker to gain control of the wallet’s library contract and freeze the funds.

The Parity wallet hack was particularly damaging because it affected a large number of users who trusted the wallet to store their Ether. The incident highlighted the importance of thorough code review and testing, as well as the need for adequate security measures to protect against smart contract exploits.

Sample Code: Smart Contract Exploit

To demonstrate the potential for smart contract exploits, we will provide a simple example of a vulnerability that could be exploited by a hacker. This example is not intended to be used for malicious purposes but is instead intended to highlight the importance of thoroughly testing smart contract code before deploying it on the blockchain.

Consider the following simple smart contract code:

pragma solidity ^0.8.0;

contract VulnerableContract {
    mapping(address => uint256) public balances;

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

    function withdraw(uint256 amount) public {
        require(balances[msg.sender] >= amount);
        msg.sender.call{value: amount}("");
        balances[msg.sender] -= amount;

This contract allows users to deposit and withdraw Ether. The deposit function simply adds the deposited Ether to the user’s balance, while the withdraw function allows users to withdraw their Ether balance.

However, there is a serious vulnerability in this code. The withdraw function uses the call function to transfer Ether to the user’s address. The call function is used to execute an external contract, and it can be vulnerable to a reentrancy attack.

A reentrancy attack is a type of smart contract exploit that involves calling the same function multiple times before the first call has completed. This can allow an attacker to repeatedly withdraw funds before the balance is updated, potentially draining the contract’s entire balance.

To exploit this vulnerability, a hacker would need to create a malicious contract that calls the withdraw function repeatedly before the balance is updated. This would allow the hacker to drain the contract’s balance and steal all of the deposited Ether.

To fix the vulnerability in the smart contract code, we can use the send function instead of the call function in the withdraw function. The send function ensures that the called contract cannot re-enter the vulnerable contract until the transaction has been completed.

Here’s the updated code:

pragma solidity ^0.8.0;

contract FixedContract {
    mapping(address => uint256) public balances;

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

    function withdraw(uint256 amount) public {
        require(balances[msg.sender] >= amount);
        balances[msg.sender] -= amount;
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed.");

In the withdraw function, we first check that the user has enough balance to withdraw the requested amount. We then subtract the withdrawn amount from the user’s balance. Finally, we use the send function to transfer the withdrawn amount to the user’s address.

The send function returns a boolean value indicating whether the transfer was successful or not. If the transfer fails, the require statement will revert the transaction, ensuring that the user’s balance is not incorrectly updated.

With these changes, the smart contract is now much more secure against reentrancy attacks. However, it is still important to thoroughly test the code and consider other potential attack vectors before deploying the contract on the blockchain.


Blockchain technology has the potential to revolutionize many industries, but it is not without its risks. Blockchain networks can be vulnerable to a variety of attacks, including 51% attacks, Sybil attacks, smart contract exploits, and private key theft.

To ensure the security of blockchain networks, it is essential to thoroughly test smart contract code before deploying it on the blockchain. Additionally, users should take appropriate security measures to protect their private keys and avoid falling victim to phishing scams or other forms of social engineering.

While blockchain hacking can be a serious threat, there are also many talented security professionals working to identify and patch vulnerabilities in blockchain networks. By remaining vigilant and taking appropriate security measures, we can help ensure the continued growth and success of the blockchain ecosystem.


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