NFT Dealers

First Flight #58

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100 EXP

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Submission Details

Impact: high

Likelihood: high

buy() Function Transfers NFT But Does Not Pay Seller - Sellers Must Call Separate Function to Receive Funds

Author Revealed upon completion

Root cause is that buy() transfers the NFT to the buyer but does not transfer USDC to the seller. The seller must call a separate collectUsdcFromSelling() function to receive payment, creating a confusing two-step payment flow where sellers may forget to collect or funds remain stuck in the contract.

Impact: Sellers do not receive payment after their NFT is sold. They must remember to call collectUsdcFromSelling() separately. If forgotten, funds remain permanently stuck in the contract. Creates user confusion and potential permanent loss of seller proceeds.

Description

The NFT Dealers protocol is designed for secondary NFT sales where buyers purchase listed NFTs and sellers receive USDC payment. The expected flow is: seller lists NFT, buyer purchases, seller receives payment automatically.
However, buy() only transfers the NFT to the buyer and stores the USDC in the contract. No payment is sent to the seller during the purchase. Sellers must manually call collectUsdcFromSelling() to receive their proceeds. This two-step flow is confusing and sellers may forget to collect, leaving funds permanently stuck.

// src/NFTDealers.sol::buy()

function buy(uint256 _listingId) external payable {

Listing storage listing = s_listings[_listingId];

if (!listing.isActive) revert ListingNotActive(_listingId);

if (listing.seller == msg.sender) revert InvalidAddress();

IERC20(usdc).safeTransferFrom(msg.sender, address(this), listing.price);

_safeTransfer(listing.seller, msg.sender, listing.tokenId, "");

@> listing.isActive = false; // ❌ NFT transferred but seller NOT paid

@> // ❌ No USDC transfer to seller here

activeListingsCounter--;

}

// src/NFTDealers.sol::collectUsdcFromSelling()

function collectUsdcFromSelling(uint256 _listingId) external onlySeller(_listingId) {

Listing memory listing = s_listings[_listingId];

if (listing.isActive) revert ListingNotActive(_listingId);

uint256 fees = _calculateFees(listing.price);

uint256 amountToSeller = listing.price - fees;

uint256 collateralToReturn = collateralForMinting[listing.tokenId];

totalFeesCollected += fees;

amountToSeller += collateralToReturn;

@> IERC20(usdc).safeTransfer(address(this), fees); // ❌ Redundant transfer

@> IERC20(usdc).safeTransfer(msg.sender, amountToSeller); // ❌ Seller paid here, not in buy()

collateralForMinting[listing.tokenId] = 0;

}

Risk

Likelihood:

This occurs on EVERY secondary sale when buy() is called
All sellers must remember to call collectUsdcFromSelling() separately - no automatic payment

Impact:

Sellers do not receive payment after NFT sale - must call separate function
Funds remain stuck in contract if seller forgets to collect, causing permanent loss

Proof of Concept

The following PoC demonstrates that after buy() completes, the seller's USDC balance remains unchanged. The seller has transferred the NFT but received no payment until collectUsdcFromSelling() is called separately.

// 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 H01_PoC is Test {

NFTDealers public nftDealers;

MockUSDC public usdc;

address owner = makeAddr("owner");

address seller = makeAddr("seller");

address buyer = makeAddr("buyer");

function setUp() public {

usdc = new MockUSDC();

nftDealers = new NFTDealers(owner, address(usdc), "NFT Dealers", "NFTD", "ipfs://image", 20 * 1e6);

usdc.mint(seller, 100_000 * 1e6);

usdc.mint(buyer, 100_000 * 1e6);

vm.deal(seller, 100 ether);

vm.deal(buyer, 100 ether);

}

function test_SellerNotPaidAfterSale() public {

vm.startPrank(owner);

nftDealers.revealCollection();

nftDealers.whitelistWallet(seller);

vm.stopPrank();

vm.startPrank(seller);

usdc.approve(address(nftDealers), 20 * 1e6);

nftDealers.mintNft{value: 20 * 1e6}();

usdc.approve(address(nftDealers), 1000 * 1e6);

nftDealers.list(1, 1000 * 1e6);

vm.stopPrank();

uint256 sellerBalanceBefore = usdc.balanceOf(seller);

vm.startPrank(buyer);

usdc.approve(address(nftDealers), 1000 * 1e6);

nftDealers.buy(1);

vm.stopPrank();

uint256 sellerBalanceAfter = usdc.balanceOf(seller);

console.log("Seller Balance Before:", sellerBalanceBefore);

console.log("Seller Balance After buy():", sellerBalanceAfter);

console.log("Seller Received:", sellerBalanceAfter - sellerBalanceBefore);

assertEq(sellerBalanceAfter, sellerBalanceBefore, "Seller received 0 USDC after sale");

}

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) Single sale where seller receives 0 USDC after buy(), (2) Seller forgets to collect - funds stuck forever, (3) Multiple sales causing seller confusion. All tests pass and confirm the vulnerability.

// SPDX-License-Identifier: MIT

pragma solidity ^0.8.30;

/**

* ============================================================

* POC-H01: buy() Function Doesn't Transfer Funds to Seller

* Sellers never receive payment - must call separate function

* Severity : HIGH

* Contract : NFTDealers.sol

* Function : buy()

* Author: Sudan249 AKA 0xAljzoli

* ============================================================

* VULNERABLE CODE:

* function buy(uint256 _listingId) external payable {

* usdc.transferFrom(msg.sender, address(this), listing.price);

* _safeTransfer(listing.seller, msg.sender, listing.tokenId, "");

* s_listings[_listingId].isActive = false;

* // ❌ No USDC transfer to seller!

* }

* IMPACT:

* - Sellers don't receive payment when NFT is sold

* - Sellers must remember to call collectUsdcFromSelling() separately

* - If seller forgets, funds are permanently stuck in contract

* - Creates confusing two-step payment flow

* - Seller may think sale failed when they haven't been paid

* FIX:

* - Transfer seller proceeds directly in buy() function

* - Calculate and deduct fees at time of sale

* - Remove need for separate collectUsdcFromSelling() call

import { Test } from "forge-std/Test.sol";

import { console } from "forge-std/console.sol";

import "./AuditBase.sol";

contract POC_H01_BuyDoesntPaySeller is AuditBase {

// ------------------------------------------------------------------

// POC A: Seller Not Paid After Sale

// ------------------------------------------------------------------

function test_H01_A_sellerNotPaidAfterSale() public {

// Setup

vm.startPrank(owner);

nftDealers.revealCollection();

nftDealers.whitelistWallet(seller); // ✅ FIXED

vm.stopPrank();

// Seller mints and lists

vm.startPrank(seller);

usdc.approve(address(nftDealers), lockAmount);

nftDealers.mintNft{value: lockAmount}();

uint256 tokenId = 1;

usdc.approve(address(nftDealers), 1000 * 1e6);

nftDealers.list(tokenId, 1000 * 1e6);

vm.stopPrank();

// Record seller balance before sale

uint256 sellerBalanceBefore = usdc.balanceOf(seller);

console.log("Seller Balance Before Sale:", sellerBalanceBefore);

// Buyer purchases

vm.startPrank(buyer);

usdc.approve(address(nftDealers), 1000 * 1e6);

nftDealers.buy(tokenId);

vm.stopPrank();

// Check seller balance immediately after sale

uint256 sellerBalanceAfter = usdc.balanceOf(seller);

console.log("Seller Balance After Sale:", sellerBalanceAfter);

// VULNERABILITY: Seller balance unchanged after sale

if (sellerBalanceAfter == sellerBalanceBefore) {

console.log("VULNERABILITY CONFIRMED: Seller received 0 USDC after sale");

console.log(" - NFT transferred but payment not received");

console.log(" - Seller must call collectUsdcFromSelling() separately");

}

assertEq(sellerBalanceAfter, sellerBalanceBefore, "Seller should have been paid but wasn't");

}

// ------------------------------------------------------------------

// POC B: Seller Forgets to Collect - Funds Stuck Forever

// ------------------------------------------------------------------

function test_H01_B_sellerForgetsToCollect_fundsStuck() public {

// Setup

vm.startPrank(owner);

nftDealers.revealCollection();

nftDealers.whitelistWallet(seller); // ✅ FIXED

vm.stopPrank();

// Seller mints and lists

vm.startPrank(seller);

usdc.approve(address(nftDealers), lockAmount);

nftDealers.mintNft{value: lockAmount}();

usdc.approve(address(nftDealers), 1000 * 1e6);

nftDealers.list(1, 1000 * 1e6);

vm.stopPrank();

// Buyer purchases

vm.startPrank(buyer);

usdc.approve(address(nftDealers), 1000 * 1e6);

nftDealers.buy(1);

vm.stopPrank();

// Seller NEVER calls collectUsdcFromSelling

// Simulate time passing, seller loses access, etc.

vm.roll(block.number + 10000);

// Check contract balance - funds are stuck

uint256 stuckAmount = usdc.balanceOf(address(nftDealers));

console.log("Funds Stuck in Contract:", stuckAmount);

// Seller cannot access funds without calling collectUsdcFromSelling

// If seller loses private key, funds are permanently lost

console.log("VULNERABILITY CONFIRMED: Funds stuck if seller doesn't collect");

console.log(" - 1000 USDC + collateral locked in contract");

console.log(" - Only seller can claim, no recovery mechanism");

// Try owner withdrawal - shouldn't be able to take seller funds

vm.startPrank(owner);

uint256 ownerBalanceBefore = usdc.balanceOf(owner);

try nftDealers.withdrawFees() {

uint256 ownerBalanceAfter = usdc.balanceOf(owner);

console.log("Owner withdrew:", ownerBalanceAfter - ownerBalanceBefore);

} catch {

console.log("Owner withdrawal failed (good - can't steal seller funds)");

}

vm.stopPrank();

}

// ------------------------------------------------------------------

// POC C: Multiple Sales - Seller Confusion

// ------------------------------------------------------------------

function test_H01_C_multipleSales_sellerConfusion() public {

// Setup

vm.startPrank(owner);

nftDealers.revealCollection();

nftDealers.whitelistWallet(seller); // ✅ FIXED

vm.stopPrank();

// Seller creates 3 listings

for (uint256 i = 1; i <= 3; i++) {

vm.startPrank(seller);

usdc.approve(address(nftDealers), lockAmount);

nftDealers.mintNft{value: lockAmount}();

usdc.approve(address(nftDealers), 1000 * 1e6);

nftDealers.list(i, 1000 * 1e6);

vm.stopPrank();

// Each NFT is bought

vm.startPrank(buyer);

usdc.approve(address(nftDealers), 1000 * 1e6);

nftDealers.buy(i);

vm.stopPrank();

}

// Seller check balance - no payments received

uint256 sellerBalance = usdc.balanceOf(seller);

console.log("Seller Balance After 3 Sales:", sellerBalance);

console.log("Expected: 3000 USDC (minus fees)");

console.log("Actual: 0 USDC");

// Seller must track which listings need collection

// Easy to miss one or lose track

console.log("VULNERABILITY CONFIRMED: Complex payment flow causes confusion");

console.log(" - Seller sold 3 NFTs but received 0 payment");

console.log(" - Must manually collect from each listing");

console.log(" - High risk of forgotten collections");

}

Recommended Mitigation

The fix modifies buy() to transfer seller proceeds directly at purchase time. Fees are deducted and transferred to the contract, while the seller receives their net proceeds immediately. This eliminates the need for collectUsdcFromSelling() entirely.

- function buy(uint256 _listingId) external payable {

- Listing storage listing = s_listings[_listingId];

- if (!listing.isActive) revert ListingNotActive(_listingId);

- if (listing.seller == msg.sender) revert InvalidAddress();

- IERC20(usdc).safeTransferFrom(msg.sender, address(this), listing.price);

- _safeTransfer(listing.seller, msg.sender, listing.tokenId, "");

- listing.isActive = false;

- activeListingsCounter--;

- }

- function collectUsdcFromSelling(uint256 _listingId) external onlySeller(_listingId) {

- Listing memory listing = s_listings[_listingId];

- if (listing.isActive) revert ListingNotActive(_listingId);

- uint256 fees = _calculateFees(listing.price);

- uint256 amountToSeller = listing.price - fees;

- uint256 collateralToReturn = collateralForMinting[listing.tokenId];

- totalFeesCollected += fees;

- amountToSeller += collateralToReturn;

- IERC20(usdc).safeTransfer(address(this), fees);

- IERC20(usdc).safeTransfer(msg.sender, amountToSeller);

- collateralForMinting[listing.tokenId] = 0;

- }

+ function buy(uint256 _listingId) external payable {

+ Listing storage listing = s_listings[_listingId];

+ if (!listing.isActive) revert ListingNotActive(_listingId);

+ if (listing.seller == msg.sender) revert InvalidAddress();

+ uint256 fees = _calculateFees(listing.price);

+ uint256 amountToSeller = listing.price - fees;

+ uint256 collateralToReturn = collateralForMinting[listing.tokenId];

+ totalFeesCollected += fees;

+ IERC20(usdc).safeTransferFrom(msg.sender, address(this), fees);

+ IERC20(usdc).safeTransferFrom(msg.sender, listing.seller, amountToSeller);

+ IERC20(usdc).safeTransfer(listing.seller, collateralToReturn);

+ listing.isActive = false;

+ activeListingsCounter--;

+ collateralForMinting[listing.tokenId] = 0;

+ _safeTransfer(listing.seller, msg.sender, listing.tokenId, "");

+ emit NFT_Dealers_Sold(msg.sender, listing.price);

+ }

Mitigation Explanation: The fix addresses the root cause by: (1) Modifying buy() to calculate and deduct fees at purchase time, (2) Transferring seller proceeds (price minus fees) directly to the seller within the same transaction as the NFT transfer, (3) Returning collateral to the seller immediately upon sale, (4) Removing the separate collectUsdcFromSelling() function entirely since it's no longer needed. This ensures sellers receive payment automatically when their NFT sells, eliminating confusion and preventing funds from being stuck in the contract.

Rewards breakdown

nSLOC

253

This contest has a flat reward structure with the following payouts:

High

100 EXP

Medium

20 EXP

Low

2 EXP

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