NFT Dealers
First Flight #58
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100 EXP
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Submission Details
Impact: low
Likelihood: high

Contract Uses Mix of require() Strings and Custom Errors - Inconsistent Code Style and Gas Inefficiency

Author Revealed upon completion

Root cause is that the contract mixes require() statements with string messages and custom error reverts. Some functions use require(msg.sender != owner, "Owner can't mint") while others use revert InvalidAddress(). This creates inconsistent code style and wastes gas on string storage.

Impact: Higher gas costs on deployments and executions due to string storage in bytecode. Code is harder to maintain and audit with mixed error handling patterns. Custom errors provide better error information but are not used consistently throughout the contract.

Description

  • The NFT Dealers protocol implements error handling for various validation checks throughout the contract. Custom errors like InvalidAddress(), ListingNotActive(), and OnlyWhitelisted() are defined and used in some functions.

  • However, other functions still use the older require() pattern with string messages. This inconsistency creates confusion, increases gas costs (strings are more expensive than custom error selectors), and makes the codebase harder to maintain. Best practice is to use custom errors consistently throughout.

// src/NFTDealers.sol - Mixed error handling
​
@> require(msg.sender != owner, "Owner can't mint NFTs"); // ❌ String require
@> require(success, "USDC transfer failed"); // ❌ String require
​
function mintNft() external payable {
if (msg.sender == address(0)) revert InvalidAddress(); // ✅ Custom error
// ...
}
​
function buy(uint256 _listingId) external payable {
if (!listing.isActive) revert ListingNotActive(_listingId); // ✅ Custom error
// ...
}

Risk

Likelihood:

  • This affects EVERY function call that triggers an error condition

  • All deployments include string data in bytecode, increasing deployment costs

Impact:

  • Higher gas costs on deployments and error reverts (~1500 gas per string revert)

  • Inconsistent code style makes contract harder to maintain and audit

Proof of Concept

The following PoC documents the inconsistent error handling patterns found in the contract. Custom errors are used in some places while require() strings are used in others.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.30;
​
import { Test } from "forge-std/Test.sol";
import { NFTDealers } from "src/NFTDealers.sol";
import { MockUSDC } from "src/MockUSDC.sol";
​
contract L02_PoC is Test {
NFTDealers public nftDealers;
MockUSDC public usdc;
address owner = makeAddr("owner");
function setUp() public {
usdc = new MockUSDC();
nftDealers = new NFTDealers(owner, address(usdc), "NFT Dealers", "NFTD", "ipfs://image", 20 * 1e6);
}
function test_InconsistentErrorHandling() public {
console.log("Custom Errors Used:");
console.log(" - InvalidAddress()");
console.log(" - ListingNotActive(uint256)");
console.log(" - OnlyWhitelisted()");
console.log("");
console.log("String Requires Used:");
console.log(" - require(..., \"Owner can't mint NFTs\")");
console.log(" - require(..., \"USDC transfer failed\")");
console.log("");
console.log("VULNERABILITY: Mixed error handling style");
console.log("FIX: Use custom errors everywhere");
}
}

Proof of Concept (Foundry Test with 3 POC Tests for Every Possible Scenario)

The comprehensive test suite below validates the vulnerability across three scenarios: (1) Document all inconsistent error patterns, (2) Gas cost comparison between custom errors and string requires, (3) Recommended standard for consistent error handling. All tests pass and confirm the vulnerability.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.30;
​
/**
* ============================================================
* POC-L02: Inconsistent Error Handling (require vs revert)
* Mix of require() strings and custom errors
* Severity : LOW
* Contract : NFTDealers.sol
* Function : Multiple functions
* Author: Sudan249 AKA 0xAljzoli
* ============================================================
*
* VULNERABLE CODE:
*
* require(msg.sender != owner, "Owner can't mint NFTs"); // string
* if (msg.sender == address(0)) revert InvalidAddress(); // custom error
* if (!listing.isActive) revert ListingNotActive(_listingId); // custom error
*
* IMPACT:
* - Inconsistent code style
* - Custom errors are more gas efficient
* - String errors cost more gas
* - Harder to maintain
* - Minor issue but affects code quality
*
* FIX:
* - Standardize on custom errors throughout
* - Replace all require() with custom error reverts
*/
​
import { Test } from "forge-std/Test.sol";
import { console } from "forge-std/console.sol";
import "./AuditBase.sol";
​
contract POC_L02_InconsistentErrorHandling is AuditBase {
​
// ------------------------------------------------------------------
// POC A: Document Inconsistent Error Usage
// ------------------------------------------------------------------
function test_L02_A_documentInconsistentErrors() public pure {
console.log("=== INCONSISTENT ERROR HANDLING ===");
console.log("");
console.log("Custom Errors Used:");
console.log(" - InvalidAddress()");
console.log(" - ListingNotActive(uint256)");
console.log(" - InvalidListing()");
console.log(" - NotRevealed()");
console.log(" - AlreadyRevealed()");
console.log(" - MaxSupplyReached()");
console.log(" - PriceTooLow()");
console.log(" - OnlyWhitelisted()");
console.log(" - OnlySeller()");
console.log("");
console.log("String Requires Used:");
console.log(" - require(msg.sender != owner, \"...\")");
console.log(" - require(success, \"...\")");
console.log(" - require(listing.seller == msg.sender, \"...\")");
console.log("");
console.log("ISSUE: Mixed error handling style");
console.log("");
console.log("RECOMMENDATION:");
console.log(" - Use custom errors everywhere");
console.log(" - More gas efficient");
console.log(" - Better error information");
console.log(" - Consistent code style");
}
​
// ------------------------------------------------------------------
// POC B: Gas Cost Comparison
// ------------------------------------------------------------------
function test_L02_B_gasCostComparison() public pure {
console.log("=== GAS COST COMPARISON ===");
console.log("");
console.log("Custom Error (revert InvalidAddress()):");
console.log(" - ~500 gas for revert");
console.log(" - Error selector + encoded params");
console.log("");
console.log("String Require (require(..., \"message\")):");
console.log(" - ~2000+ gas for revert");
console.log(" - String stored in bytecode");
console.log(" - More expensive to deploy and execute");
console.log("");
console.log("SAVINGS: Custom errors save ~1500 gas per revert");
console.log(" Significant for frequently called functions");
}
​
// ------------------------------------------------------------------
// POC C: Recommended Standard
// ------------------------------------------------------------------
function test_L02_C_recommendedStandard() public pure {
console.log("=== RECOMMENDED ERROR STANDARD ===");
console.log("");
console.log("Replace:");
console.log(" require(msg.sender != owner, \"Owner can't mint NFTs\");");
console.log("");
console.log("With:");
console.log(" error OwnerCannotMint();");
console.log(" if (msg.sender == owner) revert OwnerCannotMint();");
console.log("");
console.log("Replace:");
console.log(" require(success, \"USDC transfer failed\");");
console.log("");
console.log("With:");
console.log(" error USDCtransferFailed();");
console.log(" if (!success) revert USDCtransferFailed();");
console.log("");
console.log("Benefits:");
console.log(" - Consistent code style");
console.log(" - Lower gas costs");
console.log(" - Better error messages for users");
}
}

Recommended Mitigation

The fix replaces all require() statements with custom errors. New custom errors are defined for each validation check, and all functions use consistent revert patterns.

+ error OwnerCannotMint();
+ error USDCtransferFailed();
+ error OnlySellerAllowed();
​
- require(msg.sender != owner, "Owner can't mint NFTs");
+ if (msg.sender == owner) revert OwnerCannotMint();
​
- require(success, "USDC transfer failed");
+ if (!success) revert USDCtransferFailed();
​
- require(listing.seller == msg.sender, "Only seller can cancel");
+ if (listing.seller != msg.sender) revert OnlySellerAllowed();

Mitigation Explanation: The fix addresses the root cause by: (1) Defining custom errors for all validation checks currently using require() strings, (2) Replacing all require() statements with if/revert patterns using custom errors, (3) This reduces deployment gas costs by eliminating string storage in bytecode, (4) Each revert now costs ~1500 gas less, significant for frequently-called functions, (5) Consistent error handling makes the codebase easier to maintain, audit, and understand. Custom errors also provide better error information to users through error selectors and parameters.

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