Possible Division by Zero

Overview

• Severity: Low
• Confidence: Medium
• Affected Versions: All

What is the Possible Division by Zero vulnerability?

In Solidity, as in other programming languages, divide-by-zero errors can cause program execution to halt. Whether using checked or unchecked arithmetic, a divide-by-zero calculation will cause a Panic exception in the Ethereum Virtual Machine (EVM). Functions that use variable divisors without checking these values may be subject to such exceptions.

Further reading: Solidity Documentation: Checked or Unchecked Arithmetic

Technical example of vulnerable code

  // SPDX-License-Identifier: Unlicense
  pragma solidity ^0.8.0;

  contract SimplifiedLendingProtocol {
      mapping(address => uint256) public balances;
      uint256 public totalDeposited;
      uint256 public totalInterestPaid;

      uint256 public interestRatePerPeriod = 5e16; // 5% annual interest rate
      uint256 public constant SECONDS_PER_YEAR = 365 days;

      event InterestPaid(address user, uint256 interestAmount);

      function deposit() external payable {
          require(msg.value > 0, "Deposit amount must be greater than zero");
          balances[msg.sender] += msg.value;
          totalDeposited += msg.value;
      }

      function computeAnnualizedInterestRate(uint256 amountDeposited, uint256 scaleFactor) private view returns (uint256) {
          // Adjust the interest rate based on the scale factor
          uint256 adjustedInterestRate = (interestRatePerPeriod / scaleFactor);
          uint256 interest = (amountDeposited * adjustedInterestRate) / 1e18;
          return interest;
      }

      // Function to pay interest to a depositor with a scale factor for depositor tiers
      function payInterest(address depositor, uint256 scaleFactor) external {
          uint256 userBalance = balances[depositor];

          uint256 interest = computeAnnualizedInterestRate(userBalance, scaleFactor);
          payable(depositor).transfer(interest);
          totalInterestPaid += interest;

          emit InterestPaid(depositor, interest);
      }

      // Additional functionality omitted for brevity
  }

In the example above, contract SimplifiedLendingProtocol represents a platform that allows depositors to deposit Ether and earn interest (with borrowing functionality omitted). The function payInterest() to make payments to depositors includes a variable, scaleFactor, which can be used to scale the interest rate offered to certain tiers of borrowers. However, when this variable is passed to computeAnnualizedInterestRate(), it is used as a divisor without any check; thus, inadvertent calls to the outer function passing zero could lead to execution errors.

Technical example of how to fix the vulnerability

  // SPDX-License-Identifier: Unlicense
  pragma solidity ^0.8.0;

  contract SimplifiedLendingProtocolUpdated {
      mapping(address => uint256) public balances;
      uint256 public totalDeposited;
      uint256 public totalInterestPaid;

      uint256 public interestRatePerPeriod = 5e16; // 5% annual interest rate
      uint256 public constant SECONDS_PER_YEAR = 365 days;

      event InterestPaid(address user, uint256 interestAmount);

      function deposit() external payable {
          require(msg.value > 0, "Deposit amount must be greater than zero");
          balances[msg.sender] += msg.value;
          totalDeposited += msg.value;
      }

      function computeAnnualizedInterestRate(uint256 amountDeposited, uint256 scaleFactor) private view returns (uint256) {
          require(scaleFactor > 0, "Scale factor must be greater than zero");
          // Adjust the interest rate based on the scale factor
          uint256 adjustedInterestRate = (interestRatePerPeriod / scaleFactor);
          uint256 interest = (amountDeposited * adjustedInterestRate) / 1e18;
          return interest;
      }

      // Function to pay interest to a depositor with a scale factor for depositor tiers
      function payInterest(address depositor, uint256 scaleFactor) external {
          uint256 userBalance = balances[depositor];

          uint256 interest = computeAnnualizedInterestRate(userBalance, scaleFactor);
          payable(depositor).transfer(interest);
          totalInterestPaid += interest;

          emit InterestPaid(depositor, interest);
      }

      // Additional functionality omitted for brevity
  }

In the revised example, contract SimplifiedLendingProtocolUpdated now performs a check in the computeAnnualizedInterestRate() function to ensure that the scaleFactor variable is greater than zero before performing division, avoiding the possibility of a divide-by-zero error.