Smart Contract / Presence of Unused Variables

Description

Presence of unused variables, CWE-561, is a type of vulnerability that occurs when a software component contains variables that are not used anywhere within the codebase. This is commonly seen in Smart Contract development, where Solidity is the most commonly used language. This type of vulnerability can lead to a variety of issues, such as a lack of security, a lack of code maintainability, and increased difficulty in debugging and testing. Unused variables can also increase the attack surface, making it easier for attackers to find and exploit vulnerabilities. (Reference: CWE directory, OWASP Testing Guide)

Risk

Presence of unused variables can lead to many security risks. Since unused variables are not actively used within the code, they can be easily exploited by attackers, who can use the variables to bypass security and access sensitive information or corrupt the system. Additionally, the presence of unused variables can lead to confusion and difficulty in maintaining, testing, and debugging the code.

Solution

The best way to mitigate the risk of presence of unused variables is to regularly audit the code and remove any unused variables. This should be done whenever a codebase is updated or changed. Additionally, developers should ensure that all variables are properly used and needed.

Example

The following code, taken from the CVE directory, shows an example of an unused variable:

contract Example {
  uint256 public a;
  uint256 public b;
  uint256 c;
  
  function Example() public {
    a = 0;
    b = 0;
  }
  
  function doSomething() public {
    a = 1;
    b = 1;
  }
}

In the above code, the c variable is unused, meaning that it is not used anywhere within the codebase. As such, it can be removed to help improve the codebase's security, code maintainability, and debugging and testing processes.