The following smart contract follows the checks-effects-interaction pattern aiming at reducing the surface attack for malicious contracts that may attempt to hijack control flow after an external call:

  1- Checks : Validate all conditions before making any state change or external calls,
  2- Effects - Perform all necessary state changes,
  3- Interactions : interact with other contracts or transfer funds only after state changes are complete.
 

The goal of this pattern is to prevent re-entrancy attack (the famous dao-hack from 2016 kind of issue), where an external contract call can recursively call back into the vulnerable function before the initial execution complete, potentially leading to unexpected behavior and security issues:
 


// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.20;

import { ReentrancyGuard } from "../lib/openzeppelin-contracts/contracts/utils/ReentrancyGuard.sol"; 

contract SecureBanking is ReentrancyGuard { 
    // @dev mapping named' balances' that maps Ethereum addresses to unsigned integers, which represents the balance of each user.
    mapping(address => uint256) public balances ; 


    /**
    * @dev  Emitted when a user makes a deposit 
    * @param user The adress of the user making the deposit 
    * @param amount  The amount of Ether deposited by the user . 
    */
    event Deposit(address indexed user, uint amount); 


    /**
    * @dev emitted when a user withdrw funds . 
    * @param user The address of the user making the withdrawal 
    * @param amount The amount of Ether withdrawn by the user  
    */ 
    event Withdrawal(address indexed user, uint amount); 

    /**
     * @notice Allows a user to deposit Ether into the contract.
     * @dev updates the  balance of the sender by the amount of Ether sent with the transaction.
     * Emits a Deposit event indicating the depositor and the amount deposited.
     */
    function depositFunds() public payable  { 
      balances[msg.sender] +=  msg.value;  
      emit Deposit(msg.sender, msg.value);
    }

    /**
     * @notice Withdraws a specified amount of Ether from the contract.
     * @dev Reduces the sender's balance by the specified amount and transfers the amount to the sender.
     * Uses the `nonReentrant` modifier to prevent reentrancy attacks.
     * Emits a Withdrawal event indicating the sender and the amount withdrawn
     * @param amount The amount of Ether to withdraw.

     * Requirements:
     * - The sender must have a balance greater than or equal to the `amount`. 
     * - The contract must hold sufficient Ether to fullfill the withdrawal.
     */
      function withdrawFunds(uint256 amount) public nonReentrant { 
        //Checks - Ensure the sender  has sufficient balance to withdraw the specified amount; 
            require(balances[msg.sender] >= amount, "Insufficient balance");         

        // Effects - this line updates the state by reducing the balance of the user, substact the specified amount.
        balances[msg.sender] -= amount; 
        
        // Interactions  - Transfers the specified amount of Ether to the sender . 
        payable(msg.sender).transfer(amount); 
        emit Withdrawal(msg.sender, amount);
    }
}

 

The analysis of the provided smart contract snippet reveals it does indeed follow the
Checks-Effects-Interactions pattern and offers protection against re-entrancy attacks.

Short breakdown:

Checks: The withdrawFunds function starts with a check using require .
It ensures that the user ’s balance (balances[msg.sender]) is greater than or equal to the requested withdrawal amount (amount).

Effects: If the check passes, the function modifies the contract ’s state by subtracting the withdrawal amount from the user’s balance using balances[msg.sender] -= amount .

Interactions: The contract then interacts with the external environment by transferring the requested amount to the user using ‘payable(msg.sender).transfer(amount)’. This interaction occurs only after the balance update, effectively preventing manipulation during the transfer process.
 
Protection Against Re-entrancy:

The nonReentrant modifier provided by the imported ReentrancyGuard library plays a crucial role.This modifier ensures a function can only be called once per transaction, effectively blocking any recursive calls from a malicious external contract during the withdrawal process.

The withdrawFunds function follows the Checks-Effects-Interactions (CEI) pattern, a critical component of the Web3 security toolbox that every competent developer should understand. This pattern ensures that state changes (effects) occur before any external calls (interactions), minimizing the risk of reentrancy attacks. Combined with the nonReentrant modifier from the OpenZeppelin library, this approach creates a robust defense against common vulnerabilities, demonstrating best practices in smart contract development.