LarvaLabs Meebits Contest - jvaqa's results

Handle

jvaqa

Vulnerability details

Impact

Beebots.TradeValid() Will Erroneously Return True When Maker Is Set To Address(0) and makerIds Are Set To The TokenIds of Unminted Beebot NFTs

Beebots.verify() Returns True No Matter What Signature Is Given When Signer Is Set To address(0). This means that BeeBots.tradeValid() will erroneously return true when maker is set to address(0). Finally, before an NFT has even been minted at all, it is assumed to have an owner of address(0) due to the idToOwner mapping being initialized to zero for all uninitailized slots, so an attacker can call tradeValid() with maker set to address(0) and makerIds set to the tokenIds of any unminted nftIds, and tradeValid() will erroneously return true.

Proof of Concept

(1) Beebots.verify() Returns True No Matter What Signature Is Given When Signer Is Set To address(0) (1a) BeeBots.verify() does not check to ensure that signer is not address(0). (1b) The reason that this is a problem is that ecrecover fails silently if the signature does not match, and simply returns zero. (1c) So if an attacker passes in address(0) as the signer, then verify will return true no matter what signature is provided, since ecrecover will return address(0), and the signer is address(0), so verify will pass. (1d) This means that BeeBots.tradeValid() will erroneously return true when maker is set to address(0). (2) Before an NFT has even been minted at all, it is assumed to have an owner of address(0) due to the idToOwner mapping being initialized to zero for all uninitailized slots (2a) Solidity inializes all mappings to 0 for all slots that have not yet been set. (2b) So for any nft id that has not yet been minted, the corresponding owner in the mapping BeeBots.idToOwner is address(0), even though that nft should not even exist. (2c) This means that an attacker can call tradeValid() with maker set to address(0) and makerIds set to any unminted nftIds, and tradeValid() will erroneously return true.

Recommended Mitigation Steps

(1) Add this check to Beebots.verify(): require(signer != address(0), "Cannot verify signatures from 0x0");

(2) Add this check to Beebots.tradeValid(): require(maker != address(0), "Maker 0x0 not allowed");