A bidder can cancel his bids after the execution of the auctionDemo.claimAuction

[c4-submissions]

Lines of code

https://github.com/code-423n4/2023-10-nextgen/blob/8b518196629faa37eae39736837b24926fd3c07c/smart-contracts/AuctionDemo.sol#L105

Vulnerability details

The auctionDemo contract allows to call the cancelBid or the cancelAllBids even after the execution of the claimAuction if both calls are executed in a block with block.timestamp == minter.getAuctionEndTime(_tokenid). To illustrate it let's assume the following scenario:

• Let's assume (for simplicity) that there is only one bidder, he made 2 bids (1 ether and 2 ether).
• He sends two transactions which he hopes will be included in the same block (the order matters) with block.timestamp == minter.getAuctionEndTime(_tokenid):
  • claimAuction - the bidder can call this method since he is the winner and block.timestamp >= minter.getAuctionEndTime(_tokenid)
  • cancelAllBids - the bidder can call this method since block.timestamp <= minter.getAuctionEndTime(_tokenid) and the claimAution doesn't change the status of the bids.
• If he succeeds then he will get:
  • the NFT as the winner of the auction
  • +1 ether as a refund of the first bid in the claimAution
  • +3 ether in the cancelAllBids since all of his bids can be canceled

So basically the bidder gets the NFT for free and additionally steals 1 ether (in this scenario; it is possible to steal everything with some preconditions) from the auction contract.

The attack above is simplified and therefore is not optimal. To increase the probability of success the bidder can deploy some proxy contract that can:

• make bids
• call the claimAuction
• call the cancelAllBids in the receive if all attack conditions are met:
  • block.timestamp == minter.getAuctionEndTime(_tokenid)
  • it is the refund of the last non-winning bid

In this case, the bidder can send only one transaction, and therefore the probability of success is higher.

Since the timestamp of the next block on Ethereum is quite predictable the probability of this attack is relatively high. If the attack doesn't succeed the bidder will still get the NFT and receive all non-winning bids back so the user doesn't risk losing anything.

It is also worth mentioning that an attacker is not obligated to be a winner to perform this attack. If a bidder is not a winner he can still use the aforementioned proxy contract that will help him to call the cancelAllBids if all conditions are met. But since a bidder can't control the claimAuction call time the probability of success is much lower in this case.

Impact

A malicious user is able to steal ether from the auctionDemo contract with a significant probability.

Proof of Concept

-

Tools Used

Manual Review

Recommended Mitigation Steps

Consider replacing the condition block.timestamp >= minter.getAuctionEndTime(_tokenid) with the strict one block.timestamp > minter.getAuctionEndTime(_tokenid) in the auctionDemo.claimAuction.

Assessed type

Timing

[c4-pre-sort]

141345 marked the issue as duplicate of #1370

[c4-pre-sort]

141345 marked the issue as duplicate of #962

[c4-judge]

alex-ppg marked the issue as duplicate of #1323

[c4-judge]

alex-ppg marked the issue as partial-50