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

[M-1]Incorrect Bid Increment Calculation

chargingfoxsec

[M-1]Incorrect Bid Increment Calculation

Description

  • The placeBid function calculates the minimum required bid amount based on the previous bid and an increment percentage

  • The calculation (previousBidAmount / 100) * (100 + S_MIN_BID_INCREMENT_PERCENTAGE) suffers from precision loss due to integer division, resulting in lower required bid amounts than intended

function placeBid(uint256 tokenId) external payable isListed(tokenId) {
// ...existing code...
@> requiredAmount = (previousBidAmount / 100) * (100 + S_MIN_BID_INCREMENT_PERCENTAGE);
require(msg.value >= requiredAmount, "Bid not high enough");
// ...existing code...
}

Risk

Likelihood: HIGH

  • Occurs on every bid calculation where previousBidAmount is not a multiple of 100

  • Integer division always rounds down in Solidity

Impact: MEDIUM

  • Required bid increments will be lower than the intended 5%

  • For a bid of 123 wei, next required bid would be 105 wei instead of 129 wei

Proof of Concept

contract BidTest {
function testIncorrectBidCalculation() public pure {
uint256 previousBid = 123;
// Current calculation
uint256 incorrect = (previousBid / 100) * 105; // = 105
// Correct calculation
uint256 correct = (previousBid * 105) / 100; // = 129
assert(incorrect < correct); // Will pass, showing the issue
}
}

Recommended Mitigation

function placeBid(uint256 tokenId) external payable isListed(tokenId) {
// ...existing code...
- requiredAmount = (previousBidAmount / 100) * (100 + S_MIN_BID_INCREMENT_PERCENTAGE);
+ requiredAmount = (previousBidAmount * (100 + S_MIN_BID_INCREMENT_PERCENTAGE)) / 100;
require(msg.value >= requiredAmount, "Bid not high enough");
// ...existing code...
}

Change the order of operations to perform multiplication before division to avoid precision loss in the bid increment calculation.

Updates

Lead Judging Commences

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

BidBeasts Marketplace: Integer Division Precision Loss

Integer division in requiredAmount truncates fractions, allowing bids slightly lower than intended.

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