Access Control Vulnerabilities in Solidity Smart Contracts
Published: 2023-09-05 08:15:31

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## What is Solidity

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/pj0ft068ndbnzj8zdtz0.png)

The Solidity programming language enables the development of Ethereum smart contracts. A smart contract defines the rules and conditions by which transactions and other interactions can take place on a blockchain. With Solidity, developers can write these contracts in a high-level language similar to JavaScript, which makes it easy to use and understand.

On the Ethereum blockchain, Solidity enables developers to build and deploy smart contracts. Using these contracts, you can automate various processes and interactions, such as the transfer of assets or the execution of an agreement. The Ethereum blockchain is used by Solidity to execute these contracts, which ensures that they are transparent and cannot be altered or deleted because they are decentralized and secure.

## Access Control Vulnerabilities

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/smlwhq1vojj8znkup8lo.png)

Software and systems that contain access control vulnerabilities allow unauthorized users to access or modify data or functions through security flaws. A security control system determines who and what may access specific resources or perform certain actions by establishing rules and mechanisms. When these rules and mechanisms are not properly implemented, unauthorized users may bypass them and gain access to sensitive information or functionalities as a result of an access control vulnerability.

It is possible for attackers to steal sensitive data, manipulate information, or disrupt the functionality of a system or application due to access control vulnerabilities. In order to prevent these vulnerabilities from occurring, developers must carefully design and implement access control mechanisms in their software and systems.

## Access Control Vulnerabilities in Solidity

An access control vulnerability in a Solidity smart contract is a type of security flaw that lets unauthorized users access or modify the contract’s data or functions. The Ethereum blockchain uses Solidity for smart contracts. If the contract’s code does not properly restrict access to its data or functions according to the user’s permission level, access control vulnerabilities can occur.

## Solidity Access Control Vulnerability Example

An example would be a contract allowing users to deposit and withdraw ether (the native currency of the Ethereum blockchain). It might have a public function called “withdraw” that lets users withdraw ether. The attacker could withdraw ether from the contract without the user’s permission if the contract does not check the user’s permission level before executing the function. Since the contract does not control access to its data and functions properly, this is an access control vulnerability.

Here is an example of a Solidity smart contract with access control vulnerability:

```
pragma solidity ^0.5.0;

contract Bank {
    // The contract's balance of ether
    uint256 public balance;

    // A mapping that stores the balances of each user
    mapping (address => uint256) public userBalances;

    // Allows users to deposit ether into their account
    function deposit(uint256 amount) public {
        // Update the user's balance
        userBalances[msg.sender] += amount;

        // Update the contract's balance
        balance += amount;
    }

    // Allows users to withdraw ether from their account
    function withdraw(uint256 amount) public {
        // Check if the user has sufficient balance
        require(userBalances[msg.sender] >= amount, "Insufficient balance");

        // Update the user's balance
        userBalances[msg.sender] -= amount;

        // Update the contract's balance
        balance -= amount;
    }
}
```

In this example, the Bank contract allows users to deposit and withdraw ether from their account. The contract uses a mapping called userBalances to store the balance of each user. However, this contract is vulnerable to an access control attack because the withdraw function does not properly check the user's permission level before executing. An attacker could call the withdraw function and withdraw ether from the contract without the user's permission.

## Fixing the Vulnerability

To fix this vulnerability, the contract should check the user’s permission level before executing the withdraw function. For example, the contract should define a variable that stores the address of the owner and then check this variable against msg.sender in the withdraw function:

```
pragma solidity ^0.5.0;

contract Bank {
    // The contract's balance of ether
    uint256 public balance;

    // The owner of the contract
    address public owner;

    // A mapping that stores the balances of each user
    mapping (address => uint256) public userBalances;

    constructor() public {
        // Set the contract owner
        owner = msg.sender;
    }

    // Allows users to deposit ether into their account
    function deposit(uint256 amount) public {
        // Update the user's balance
        userBalances[msg.sender] += amount;

        // Update the contract's balance
        balance += amount;
    }

    // Allows users to withdraw ether from their account
    function withdraw(uint256 amount) public {
        // Check if the user is the owner of the contract
        require(msg.sender == owner, "Only the owner can withdraw");

        // Check if the user has sufficient balance
        require(userBalances[msg.sender] >= amount, "Insufficient balance");

        // Update the user's balance
        userBalances[msg.sender] -= amount;

        // Update the contract's balance
        balance -= amount;
    }
}

```

In this updated version of the contract, the owner variable stores the address of the contract owner, and the withdraw function checks this variable against msg.sender before allowing the withdrawal to proceed. This prevents attackers from calling the withdraw function and withdrawing ether from the contract without the user's permission.

## Summary
A smart contract’s security can be seriously compromised by access control vulnerabilities. An attacker could steal funds, manipulate data, or disrupt the contract. To ensure that their contracts are not vulnerable to access control attacks, contract developers should carefully design and test them.

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