# Spot the Vulnerability

{% hint style="info" %}
[**Book an audit with Zokyo**](https://www.zokyo.io/)
{% endhint %}

{% hint style="info" %}
Welcome to our "Spot The Vulnerability" challenges! In this section, we dive deep into the intricate world of smart contract development and its associated vulnerabilities. The aim of these exercises is to cultivate a deep and nuanced understanding of potential pitfalls and vulnerabilities in Ethereum smart contract code. These exercises are not purely academic - they are based on real-world bug bounty findings and vulnerabilities discovered in actual smart contracts.
{% endhint %}

The vulnerability in the depositTokens() function is that it fails to check the return value of the approve() function call. As per the ERC20 standard, approve() should return a boolean indicating the success or failure of the approval operation. If the token contract in question is one that returns false instead of reverting when an approval fails, the approve() call could silently fail. This silent failure could lead to unexpected behavior later when the pool tries to transfer these tokens, potentially causing token balances to be incorrectly accounted for or transactions to fail unexpectedly.

This vulnerability can be mitigated by utilizing the safeIncreaseAllowance() function from OpenZeppelin's SafeERC20 library, which checks the return value and reverts the transaction if the approval operation fails. Also, accounting for potential fees on transfer tokens, the approved amount should be calculated based on the actual balance received.

Here is how the corrected contract code might look:

```
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";

contract DeFiPlatform {
    using SafeERC20 for ERC20;
    address public pool;

    constructor(address _pool) {
        pool = _pool;
    }

    function depositTokens(address token, uint256 amount) public {
        // User must approve contract before deposit
        require(
            ERC20(token).allowance(msg.sender, address(this)) >= amount,
            "Token allowance too small"
        );

        uint256 balanceBefore = ERC20(token).balanceOf(address(this));

        ERC20(token).transferFrom(msg.sender, address(this), amount);

        uint256 balanceAfter = ERC20(token).balanceOf(address(this));
        
        ERC20(token).safeApprove(pool, 0);
        
        // safe approval operation, checking the return value
        ERC20(token).safeIncreaseAllowance(pool, balanceBefore + (balanceAfter - balanceBefore));
    }
}
```

The use of safeIncreaseAllowance() in this way provides a much stronger guarantee that the subsequent logic of the contract will execute correctly, avoiding any potential mishaps arising from incorrect assumption of successful approvals. It is always a best practice to handle the return value of the approve() function and account for potential fees on transfer to maintain the integrity and reliability of smart contracts."
