This is not a walkthrough of every contract or code of the challenge. I am sharing my notes and resources I have used to complete this challenge, as well as some lessons I think are useful to take away after completing the challenge. I highly recommend you finish the challenge yourself first and only use this as additional content.

• GameAsset and AssetHolder seem like normal ERC721 and ERC1155 contracts

• Looking at AssetWrapper contracts wrap function we see it immediately calls _wrap which mints an ERC1155 token to assetOwner parameter without any check that it is the owner of the ERC721 token, so anyone can mint the ERC1155.

• Another major problem is the fact that _wrap calls ERC1155s _mint function which can be maliciously used via Reentrancy, because the _mint function calls an external function onERC1155Received to check that a contract can receive the ERC1155 token.

• Wrapping tokens into ERC1155 can be dangerous because there are several functions from which we can reenter a contract if there is no ReentrancyGuard implemented

  • The vulnerable ERC1155 functions are safeTransferFrom(), safeBatchTransferFrom(), _mint() and _mintBatch() (more on this in Resources section).

  • This is because all of these functions implement an external function call to msg.sender contract, calling the onERC1155Received() function, from where an attacker can reenter.

• The attack contract needs to call wrap for the first NFT with assetOwner = address(this) and implement onERC1155Received to then again call wrap for the other NFT with the same assetOwner and then call unwrap on both NFTs to trap them in the wrapper contract

• Challenge link

• Github contracts

• Attack contract

• Test file with solution

• Where to find solidity reentrancy attacks (RareSkills)