Attack Vectors in Solidity #1: Inappropriate Access Control
Tags: solidity, web3, blockchain, webdev
Published: 2022-12-26 09:25:38

### **Introduction:**

In Solidity, access control refers to the ability to restrict certain functions or variables to only be accessible or modifiable by certain parties. This is typically achieved through the use of modifiers, which are special functions placed before the function or variable they are modifying. Proper access control is essential for ensuring the security and integrity of smart contracts by limiting the ability of unauthorized parties to access or modify sensitive data or functions.

However, incorrect usage or placement of access controls within functions can pose a very high security risk and be used as an attack vector by hackers. Functions that are important or contain sensitive information should have address checks (such as "owner" or "controller") to ensure that only authorized parties can access or use them. This is especially important when using modifiers. Failing to add these checks allows hackers to gain access to critical logic.

### **How Do These Vulnerabilities Occur?**

A contract's functionality is typically accessed through its public and external functions. Access control helps keep a contract's private values and logic safe from attackers. However, attackers can still sometimes access them in sneaky ways. There are several types of vulnerabilities that can occur in Solidity contracts, including:

* Using the deprecated tx.origin to validate callers.
    
* Handling large authorization logic with lengthy require statements.
    
* Making reckless use of delegatecall in proxy libraries or proxy contracts.
    

It is important to be aware of these vulnerabilities and take steps to mitigate them in order to ensure the security and integrity of your contract.

### **Example of Inappropriate Access Control**

For example, consider the following function that allows anyone to change the owner of a contract:

```solidity
function changeOwner(address newOwner) public {
owner = newOwner;
}
```

This function is inappropriate because it means that anyone can take ownership of the contract, potentially causing security issues. **A solution to this issue is to add a requirement that the caller must be the current owner of the contract in order to change the owner:**

```solidity
function changeOwner(address newOwner) private {
require(msg.sender == owner, "Only the current owner can change ownership.");
owner = newOwner;
}
```

This ensures that only the current owner can change ownership, preventing unauthorized changes.

### **Conclusion**

In conclusion, access control is an essential aspect of Solidity programming that helps to ensure the security and integrity of smart contracts. By using modifiers and address checks, developers can restrict access to certain functions and variables to only authorized parties. However, it is important to be aware of vulnerabilities such as the use of deprecated tx.origin, lengthy require statements, and reckless delegatecall usage, and to take steps to mitigate these vulnerabilities in order to protect against unauthorized access to or modification of sensitive data or functions. Proper access control is crucial for the security and reliability of smart contracts, and developers should take care to implement it correctly in their code.

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