Inconsistent validation logic prevents bidding at the seller-defined minimum price

Description

• For this auction, there's a BidBeastsNFTMarketPlace::Listing.minPrice set by the seller, which, as the name suggests, is the minimum price at which the seller is willing to sell their NFT.

• But the contract enforces that the first bid must be strictly greater than this minPrice, preventing bids exactly at minPrice (line 151 in BidBeastsNFTMarketPlace::placeBid):

  if (previousBidAmount == 0) {

  ​

  requiredAmount = listing.minPrice;

  @> require(msg.value > requiredAmount, "First bid must be > min price");
  
  

• Plus, it can be seen that both BidBeastsNFTMarketPlace::settleAuction and BidBeastsNFTMarketPlace::takeHighestBid functions checks if the highest bid is greater than or equal to (>=) the minPrice, indicating the seller's intent to accept bids at minPrice (lines 186 and 196 respectively):

  function settleAuction(uint256 tokenId) external isListed(tokenId) {

  ...

  @> require(bids[tokenId].amount >= listing.minPrice, "Highest bid did not meet min price");

  ...

  }

  ​

  function takeHighestBid(uint256 tokenId) external isListed(tokenId) isSeller(tokenId, msg.sender) {

  ...

  @> require(bid.amount >= listings[tokenId].minPrice, "Highest bid is below min price");

  ...

  }
  
  

• This discrepancy forces bidders to overpay by at least 1 wei, creating unnecessary friction.

Risk

Likelihood: Medium

• Occurs when a bidder tries to bid exactly at minPrice for the first bid.

Impact: Low

• Bidders can't bid at the exact minimum price

• No funds at risk, just a minimal overpayment of 1 wei

Proof of Concept

• Add the following test test_cannotBidAtMinPrice in the test file:

  function test_cannotBidAtMinPrice() public {

  // Mint and list NFT

  _mintNFT();

  _listNFT(); // MIN_PRICE = 1 ether

  console.log("Minimum price set at:", MIN_PRICE);

  ​

  // Attempt to place a bid exactly at min price

  vm.prank(BIDDER_1);

  vm.expectRevert("First bid must be > min price");

  market.placeBid{value: MIN_PRICE}(TOKEN_ID);

  ​

  // Place a valid first bid above min price

  uint256 validBid = MIN_PRICE + 1 wei;

  vm.prank(BIDDER_1);

  market.placeBid{value: validBid}(TOKEN_ID);

  ​

  (, uint256 amount) = market.bids(TOKEN_ID);

  console.log();

  console.log("Valid first bid placed:", amount);

  }
  
  

• Run the above test using the command:

  forge test --mt test_cannotBidAtMinPrice -vv
  
  

• The output we get:

  Ran 1 test for test/BidBeastsMarketPlaceTest.t.sol:BidBeastsNFTMarketTest

  [PASS] test_cannotBidAtMinPrice() (gas: 309520)

  Logs:

  Minimum price set at: 1000000000000000000

  ​

  Valid first bid placed: 1000000000000000001

  ​

  Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 1.14ms (197.40µs CPU time)

Recommended Mitigation

Change the strict greater-than (>) to greater-than-or-equal-to (>=) in the placeBid function: