Tag Archives: nft

Picaroons Contract Review

The Picaroons is a new NFT collection by Alex Lucas, with support from Pranksy. Holders of certain NFTs by Alex were able to mint up to 2 tokens, then during public mint anyone could mint 2 tokens (and previous minters could mint 2 more). The 10k collection quickly sold out during public mint, and is now only available on secondary marketplaces.

An interesting thing about this contract is the extra effort into preventing bots from minting directly from the contract. They maintained an allow list of addresses in their web app instead of putting the snapshot into the contract, and minting required a signature generated by their web app. Let’s dig into the contract code

Mint

    function mint(bytes32 hash, bytes memory signature, uint256 tokenQuantity, uint256 maxAmountAllowed) external payable {
        require(saleLive, "SALE_CLOSED");
        // Is the signature valid
        require(matchAddresSigner(hash, signature), "DIRECT_MINT_DISALLOWED");
        // Verify if the keccak256 hash matches the hash generated by the hashTransaction function
        require(hashTransaction(msg.sender, tokenQuantity, maxAmountAllowed) == hash, "HASH_FAIL");
        // Out of stock check
        require(totalSupply() + tokenQuantity <= TOTAL_SUPPLY, "OUT_OF_STOCK");
        // Is the address allow more mint more tokens?
        require(amountMintedList[msg.sender] + tokenQuantity <= maxAmountAllowed, "EXCEED_ALLOC");
        require(PRICE * tokenQuantity <= msg.value, "INSUFFICIENT_ETH");

        for(uint256 i = 0; i < tokenQuantity; i++) {
            amountMintedList[msg.sender]++;
            _safeMint(msg.sender, totalSupply() + 1);
        }
       
    }

The mint function requires the following:

  1. Sale isn’t closed
  2. The hash argument was signed (signature) by a known signing address
  3. The hash argument can be verified with the other function arguments
  4. There are tokens available to mint
  5. You have not minted too much
  6. You sent enough ETH

Once those checks pass, it’s a simple mint loop, incrementing a counter for how many you have minted. The interesting parts are requirements 2 & 3.

Match Address Signer

    address private _signerAddress = 0xF5F6C1B8F13F2A41Ce8474AAD3Dd5050364eef1f;

    ...
        
    function matchAddresSigner(bytes32 hash, bytes memory signature) private view returns(bool) {
        return _signerAddress == hash.recover(signature);
    }

The _signerAddress is defined as a static variable. In order to generate a verifiable signature for the hash, the private key associated with this signing address must exist in their web app. Because this address is static, with no way to change it, they hopefully had some good app and server security to prevent any leaking of the private key. Assuming no one else has access to the private key, the signature can only be generated by their web app, and therefore no one, including bots, could mint directly from the contract. All minting actions had to go through their web app, which can be protected from bots using more traditional methods.

How did they secure their web app from bots?

  • They required a captcha before a signature was generated, which prevents most web bots.
  • Before the public mint, only addresses on an allow list based on NFT ownership were able to mint.

This allow list was generated from a snapshot ~2 weeks before minting was enabled. But unlike many allow list snapshots, it wasn’t put into the contract. Instead they must have made a database for their web app to check against, which was only used during the pre-sale.

How did they make a signature in a web app that could be verified within the contract?

Libraries like web3.js provide a way to sign data in javascript using a private key. The library also provides message hashing. So by using a library like web3.js in a server-side web app, you can do the following:

  1. Encode function arguments into a message
  2. Hash that message
  3. Sign the hash

The hash and signature can then be returned from the web app to the browser, then passed to the contract, thereby preventing direct contract minting.

Hash Transaction

This function reconstructs the hashed message that was signed. It does this by encoding the arguments, then hashing them with solidity’s standard hashing algorithm, keccak256.

    function hashTransaction(address sender, uint256 qty, uint256 maxAmountAllowed) private pure returns(bytes32) {
          bytes32 hash = keccak256(abi.encodePacked(
            "\x19Ethereum Signed Message:\n32",
            keccak256(abi.encodePacked(sender, qty, maxAmountAllowed)))
          );
          return hash;
    }

That’s about it for the minting. Let’s see what Slither says…

Slither Analysis

Slither is a python tool for static analysis of Solidity contracts. You can use it to get a quick summary of the contract code, and then look for any deeper issues.

$ slither 0x545f0a45Ba06C7C5b1a5Fb0b29008462ceEA07b7 --print human-summary

No significant issues found. Nearly all the issues are in the dependency contracts, which are pretty standard, and the only issues Slither finds for ThePicaroons contract are cosmetic.

Gas Efficiency

The contract uses ERC721Enumerable. While it’s a somewhat common extension to ERC721, and it adds some useful functionality, it definitely increases gas costs for minting and transfers. Azuki did a deep analysis of this and created ERC721A to solve this gas issue without losing key functionality. Using ERC721A would have been a nice improvement to the Picaroons contract, but the contract is otherwise very simple and efficient.

Provenance Hash

One final detail in the contract is the use of a NFT Provenance Hash. This can be used to verify that the reveal after mint was defined before minting began, and not manipulated after mint but before reveal. The fourth transaction on the contract, right after public mint was enabled, was to set the provenance hash to e4b58ca66f8bf42902acff422cb634ff8c6d012627a71f195071c725558670b9. While they don’t give a nice breakdown on their website for verifying the metadata of the tokens, like BAYC does, it’s still a good thing to do and provides that verification capability to anyone that wants to do the work.

    /// Set the smart-contract proof as an hash of all NFT metadata hashes
    /// @param hash of all NFTs metadata hashes
    function setProvenanceHash(string calldata hash) external onlyOwner notLocked {
        proof = hash;
    }

Dead mfers Contract Review

Dead mfers is a collection by sartoodles, who is an active member of the unofficial mfers community, and has created a few mfers derivative collections. Owners of Dead mfers receive free airdrops regularly on the polygon network, and more benefits will be unlocked when the collection is 90% minted. There is also a video game in the works. Let’s look at the contract code

Mint Random

The mint function used for Dead mfers is mintRandom:

    function mintRandom() payable public
    {
        require(mintRandomEnabled == true, 'Random minting is not enabled for this contract');
        require(msg.value == mintFee, 'Eth sent does not match the mint fee');
        
        _splitPayment();
        
        uint256 tokenID = _drawRandomIndex();
        
        _safeMint(msg.sender, tokenID);
    }

This requires that random minting is enabled, and the fee passed in is correct, in this case 0.0069 ETH. _safeMint is a standard ERC721 function from OpenZeppelin, so let’s look at the middle 2 functions: _splitPayment and _drawRandomIndex.

Split Payment

    function _splitPayment() internal
    {
        if(msg.value != 0){
            uint256 splitValue = msg.value / 10;
            uint256 remainingValue = msg.value - splitValue;
            
            payable(shareAddress).transfer(splitValue);
            payable(owner()).transfer(remainingValue);
        }
    }

This function sends 10% of the mint fee to shareAddress, and the remaining 90% to the contract owner. The shareAddress is set when this contract is initialized by the factory contract, which is covered in more detail below. If you look at any of the Mint Random transactions, you can see that 10% of every mint fee is sent to 0xe28564784a0f57554d8beec807e8609b40a97241, aka autominter.eth. This is how AutoMinter makes money by providing tools to NFT creators.

Sample transaction showing 10% of mint fee going to autominter.eth

Draw Random Index

    function _drawRandomIndex() internal returns (uint256 index) {
        //RNG
        uint256 i = uint(keccak256(abi.encodePacked(block.timestamp))) % remaining;

        // if there's a cache at cache[i] then use it
        // otherwise use i itself
        index = cache[i] == 0 ? i : cache[i];

        // grab a number from the tail
        cache[i] = cache[remaining - 1] == 0 ? remaining - 1 : cache[remaining - 1];
        
        // store the position of moved token in cache to be looked up (add 1 to avoid 0, remove when recovering)
        cachePosition[cache[i]] = i + 1;
        
        remaining = remaining - 1;
    }

This function gets a pseudo random number between 0 and remaining, then returns that number (index). The rest is mostly to coordinate between other minting functions like _drawIndex that are not used for Dead mfers, so that the same token isn’t minted twice.

That’s about it – the code is relatively minimal without much going on. One downside to this contract is that you can only mint a single token at a time. I suspect that if it allowed batch minting of multiple tokens in a single transaction, like in Backgroundmfers or 3DMutantMfers, then a lot more Dead mfers would be minted.

I could conclude the review here, but there’s some interesting additional details to cover. If you’re looking at the code on etherscan, you may notice it says Minimal Proxy Contract for 0x72668b08926a69ae9c926aeb572559efc7f42cd6. What does this mean?

AutoMinter Proxy

The Dead mfers contract is not a custom contract. Instead, it is a minimal proxy for a contract by AutoMinter. This means that the Dead mfers contract forwards all the contract interactions to the autominter contract code, while the state data that records your actions and NFT ownership is maintained within the Dead mfers contract. By using this minimal proxy pattern, sartoodles was able to re-use someone else’s code, and save ETH with a cheaper contract deployment.

When deploying a minimal proxy contract, the main thing is to provide initialization parameters. This is what customizes the contract for your use vs someone else’s. In other words, many other contracts could be minimal proxies for the same autominter contract, but each minimal proxy has been initialized differently. For Dead mfers, here are the first 5 initialization parameters:

To find these, start on the contract page. There is a More Info section in the upper right that looks like this:

In the middle row, it shows “… at txn” and then a transaction hash. Click on that transaction hash to see the transaction details page, which looks like this:

You can see when the transaction happened, the Contract that was interacted with (the AutoMinterFactory covered below), the transaction Value of 0.025 ETH, and lots of other details. At the bottom, click on the Click to see More link. That displays the Input Data, which shows that a create function was called.

What you see is the binary data passed in to the create function. At the bottom, click Decode Input Data to see a human readable table of parameters, such as the name_ being Dead mfers. This create function actually belongs to a different contract, AutoMinterFactory. Below is a short video showing the contract navigation to find all this information, as well as the AutoMinterFactory contract described next.

Auto Minter Factory

The AutoMinterFactory contract does the actual deployment and initialization of the minimal proxy contract used by Dead mfers. As part of that initialization, it sets some of its own parameters on the contract, such as the shareAddress mentioned above.

The create function on the factory contract is relatively clear: (note that the actual create function used by the latest AutoMinterFactory might be different than below, because it’s an upgradeable contract behind a proxy contract):

    function create(string memory name_,
        string memory symbol_,
        string memory baseURI_,
        string memory appId_,
        uint256 mintFee_,
        uint256 size_,
        bool mintSelectionEnabled_,
        bool mintRandomEnabled_,
        address whiteListSignerAddress_,
        uint256 mintLimit_,
        uint256 royaltyBasis_,
        string memory placeholderImage_) payable public
    {
        require(msg.value >= fee, 'Must pass the correct fee to the contract');
        
        address payable clone = payable(ClonesUpgradeable.clone(erc721Implementation));

        AutoMinterERC721(clone).initialize(name_,
            symbol_,
            baseURI_,
            msg.sender,
            mintFee_,
            size_,
            mintSelectionEnabled_,
            mintRandomEnabled_,
            whiteListSignerAddress_,
            mintLimit_,
            royaltyBasis_,
            placeholderImage_
        );
        
        emit ContractDeployed(appId_, appId_, clone, msg.sender);
    }

The erc721Implementation address points to the current deployed version of AutoMinterERC721, so create clones this contract then initializes it.

To recap, the contract deployment worked like this:

  1. The create function is called on a proxy contract that forwards to the AutoMinterFactory implementation contract (a proxy is used so that the factory contract can be upgraded in the future)
  2. The AutoMinterFactory implementation contract clones and initializes a minimal proxy contract for the current version of the AutoMinterERC721 implementation contract
  3. The minimal proxy contract is initialized with the parameters from the create function in step 1
  4. All calls to the minimal proxy contract (for Dead mfers) are forwarded to the AutoMinterERC721 implementation contract

None of the above is bad or dangerous; it’s necessary complexity to provide re-usable & customizable contract code. The result is a simple & cheap contract.

Upgradeable

You may also notice that one of the first lines of code says the contract is upgradeable:

contract AutoMinterERC721 is Initializable, ERC721Upgradeable, OwnableUpgradeable

If the Dead mfers contract was a regular proxy contract, then this line above would make it possible to deploy a new version of the AutoMinterERC721 contract. Upgradeable NFT contracts are not necessarily bad, but it is a warning sign, because the implementation contract could be changed at any time, potentially introducing insecure code. In the case of Dead mfers, because it’s only a minimal proxy, the implementation contract cannot be upgraded.

Slither Analysis

Slither is a python tool for static analysis of Solidity contracts. You can use it to get a quick summary of the contract code, and then look for any deeper issues.

$ slither 0x50092539a224953d82995c9d950e042da4556283 --print human-summary

In this case, some of library contracts are marked as Complex, while the main contract AutoMinterERC721 is not, which is good. But Slither also says there are 3 high severity issues. Let’s see what those are:

$ slither 0x50092539a224953d82995c9d950e042da4556283
AutoMinterERC721._drawRandomIndex() (contracts/AutoMinterERC721.sol#114-129) uses a weak PRNG: "i = uint256(keccak256(bytes)(abi.encodePacked(block.timestamp))) % remaining (contracts/AutoMinterERC721.sol#116)"

OwnableUpgradeable.__gap (@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol#82) shadows:
	- ContextUpgradeable.__gap (@openzeppelin/contracts-upgradeable/utils/ContextUpgradeable.sol#31)
ERC721Upgradeable.__gap (@openzeppelin/contracts-upgradeable/token/ERC721/ERC721Upgradeable.sol#431) shadows:
	- ERC165Upgradeable.__gap (@openzeppelin/contracts-upgradeable/utils/introspection/ERC165Upgradeable.sol#36)
	- ContextUpgradeable.__gap (@openzeppelin/contracts-upgradeable/utils/ContextUpgradeable.sol#31)

The first issue says that _drawRandomIndex, called by the mintRandom function, uses a weak PRNG, which stands for pseudorandom number generator. The random index for a token is generated by doing a modulo on the current block timestamp, which apparently could be manipulated by an ethereum blockchain miner, to reorder the block to get a different random index. This may be a problem in other situations, but in this case, it doesn’t really matter which random token you get when you call mintRandom.

The second and third issues are about state variable shadowing in some of the OpenZeppelin upgradeable contract dependencies. This means the __gap variable is used in multiple contracts but is not explicitly assigned. This appears to be a design choice in upgradeable contracts, to explicitly leave room for new state variables in future versions of a contract. So this high severity issue isn’t actually a problem here either.

All the other issues Slither detects either don’t really matter or don’t apply to how the Dead mfers contract is used.

Conclusion

Overall this is a simple contract. It is unfortunately limited to minting one at a time, and it doesn’t look like AutoMinter supports batch minting. This is one reason why many other projects will deploy their own custom contracts, in addition to custom code for different minting options. So if you’re ok with single minting, AutoMinter might be a great option for creating your own NFT collection. And be sure to mint your own Dead mfer to receive some free airdrops, or if you want to play the upcoming video game, recently teased on @Sartoodles.

3D Mutant Mfers

3D Mutant Mfers Contract Review

3DMutantMfers is a new NFT collection by @scott_visuals. He is also the artist and creator of 3DMfers and Cosmo Creatures. For this mint, the team wanted to reward holders of 3DMfers and Cosmo Creatures with free mints, while still keeping public mints very affordable at 0.029 ETH. The idea with the free mint was that you would be able to get 1 3DMutantMfer for each 3DMfer or Cosmo Creature you held. Unfortunately, there was a bug in the contract function that caused the free claim to fail for most people. Let’s dig in to the contract code.

Public Mint

Let’s start with the public mint function, which works great and is very simple.

  function mint( uint numberOfTokens ) external payable mintCompliance( numberOfTokens ) {
    require( msg.value >= price * numberOfTokens, "Ether value sent is not correct" );

    _mintLoop( msg.sender, numberOfTokens );    
  }

The mint function takes in the number of tokens you want to mint, ensures that you’ve sent the correct amount of ETH (0.029 * numberOfTokens), then calls _mintLoop with msg.sender (your wallet address).

Mint Compliance

Before you can mint, there’s a number of conditions that are checked by mintCompliance.

  modifier mintCompliance( uint numberOfTokens ) {
    require( isActive,                            "Sale must be active to mint 3DMutantMfers" );
    require( numberOfTokens <= maxOrder,          "Can only mint 20 tokens at a time" );
    require( numberOfTokens > 0,                  "Token Mint Count must be > 0" );

    uint256 supply = _owners.length;
    require( supply + numberOfTokens <= MAX_SUPPLY, "Purchase would exceed max supply of 3DMutantMfers" );
    
    uint256 mintedCount = addressMintedBalance[msg.sender];
    require(mintedCount + numberOfTokens <= MAX_PER_WALLET, "Max NFT per address exceeded");

    _;
  }

The mintCompliance modifier function is also used by the freeClaimMint function covered below. It checks for the following conditions:

  • Minting is active
  • You are minting at least one token but not more than 20
  • You can’t exceed the total supply of 4444 tokens
  • You can’t mint more than 1000 tokens into 1 wallet

All of these numbers can be changed by the contract owner, with the restriction that total supply cannot be set lower than the number of tokens. The functions that allow these modifications are below:

 function setActive(bool isActive_) external onlyOwner {
    if( isActive != isActive_ )
      isActive = isActive_;
  }

  function setMaxOrder(uint maxOrder_) external onlyOwner {
    if( maxOrder != maxOrder_ )
      maxOrder = maxOrder_;
  }

  function setPrice(uint price_ ) external onlyOwner {
    if( price != price_ )
      price = price_;
  }

  function setMaxSupply(uint maxSupply_ ) external onlyOwner {
    if( MAX_SUPPLY != maxSupply_ ){
      require(maxSupply_ >= _owners.length, "Specified supply is lower than current balance" );
      MAX_SUPPLY = maxSupply_;
    }
  }

  // Update Max Tokens A Wallet can mint
  function setMaxPerWallet(uint maxPerWallet_ ) external onlyOwner {
    if( MAX_PER_WALLET != maxPerWallet_ ){
      MAX_PER_WALLET = maxPerWallet_;
    }
  }

Mint Loop

  function _mintLoop(address _receiver, uint256 numberOfTokens) internal {
    uint256 supply = _owners.length;

    for (uint256 i = 0; i < numberOfTokens; i++) {
      addressMintedBalance[_receiver]++;
      _safeMint( _receiver, supply++, "" );
    }
  }

This function simply iterates through the number of tokens to mint, increments that number of tokens for your wallet (_receiver) then mints each token. This contract uses ERC721B, which is an implementation of ERC721 optimized to reduce gas when minting multiple tokens. Because this is relatively new code, and not from OpenZeppelin (the defacto standard library), it’s possible there’s some issues that haven’t been identified yet. However, the code in each function is relatively simple and very similar to standard ERC721 code.

Free Claim Mint

Let’s look at the freeClaimMint, which had an issue where most transactions attempts failed with “Warning! Error encountered during contract execution [Out of gas]“.

function freeClaimMint( uint numberOfTokens, bytes memory signature, string[] memory contract1TokenIds, string[] memory contract2TokenIds ) external mintCompliance( numberOfTokens ) {
    
    require(verifySender(signature, contract1TokenIds, contract2TokenIds), "Invalid Access");

    // Check to make sure there are token ids
    require(contract1TokenIds.length > 0 || contract2TokenIds.length > 0, "Empty Token IDs");

    uint totalTokenIds = contract1TokenIds.length + contract2TokenIds.length;
    require(totalTokenIds == numberOfTokens, "Token IDs and Mint Count mismatch");

    // Lets make sure we are not claiming for already claimed tokens of contract 1
    bool isValidTokenIds = true;
    for (uint i = 0; isValidTokenIds && i < contract1TokenIds.length; i++) {
      for (uint j = 0; isValidTokenIds && j < contract1ClaimedTokensCount; j++) {
        string memory contractClaimedToken = contract1ClaimedTokens[j];
        string memory tokenToClaim = contract1TokenIds[i];

        if (keccak256(bytes(tokenToClaim)) == keccak256(bytes(contractClaimedToken))) {
          isValidTokenIds = false;
        }
      } 
    } 
    require(isValidTokenIds, "Cosmo Creatures Token ID passed is already claimed");

    // Lets make sure we are not claiming for already claimed tokens of contract 2
    for (uint i = 0; isValidTokenIds && i < contract2TokenIds.length; i++) {
      for (uint j = 0; isValidTokenIds && j < contract2ClaimedTokensCount; j++) {
        string memory contractClaimedToken = contract2ClaimedTokens[j];
        string memory tokenToClaim = contract2TokenIds[i];

        if (keccak256(bytes(tokenToClaim)) == keccak256(bytes(contractClaimedToken))) {
          isValidTokenIds = false;
        }
      } 
    } 
    require(isValidTokenIds, "3D Mfrs Token ID passed is already claimed");


    for (uint i = 0; i < contract1TokenIds.length; i++) {
      contract1ClaimedTokensCount++;
      contract1ClaimedTokens.push(contract1TokenIds[i]);
    }
    
    for (uint i = 0; i < contract2TokenIds.length; i++) {
      contract2ClaimedTokensCount++;
      contract2ClaimedTokens.push(contract2TokenIds[i]);
    }

    _mintLoop( msg.sender, numberOfTokens );
  }

As you can see, this is quite a complicated function. You can compare it to the BackgroundMfers minting functions to see how much simpler their implementation is for each free claim function. In this case, the intention was to provide a single function that would mint 1 or more tokens depending on how many you owned from 3DMfers and Cosmo Creatures. But for most people that tried this, the transaction failed, losing them gas. The team is very supportive of their community, though, and they very quickly responded by:

  1. announcing the problem
  2. removing the free claim button from their website
  3. refunding gas fees to everyone that had a failed transaction
  4. creating a claim form and then airdropping 3DMutantMfers, at their own expense

Over 1.3 ETH is a somewhat expensive bug to pay for, but it builds a lot of good will, and makes it clear this is a quality team that’s not out for a quick cash grab. So what went wrong? I’m reminded of the Zen of Python: explicit is better than implicit. In the BackgroundMfers minting functions, they made everything very explicit:

  • each contract has its own minting function (mfers, dadmfers1, dadmfers2)
  • you have to select precisely which tokens from the external contract to mint with

For 3DMutantMfers, the team tried to make a very simple user experience, requiring a lot of implicit/hidden complexity that didn’t quite work. Let’s look at the code more.

Verify Signature

The very first line, after mintCompliance, is verifySignature, which checks that the function arguments have been signed by a known signerAddress.

  function verifySender(bytes memory signature, string[] memory contract1TokenIds, string[] memory contract2TokenIds) internal view returns (bool) {

    string memory contract1TokensString = "";
    string memory contract2TokensString = "";

    for (uint i = 0; i < contract1TokenIds.length; i++) {
      contract1TokensString = string(abi.encodePacked(contract1TokensString, contract1TokenIds[i], i < contract1TokenIds.length - 1 ? "," : ""));
    }
    
    for (uint i = 0; i < contract2TokenIds.length; i++) {
      contract2TokensString = string(abi.encodePacked(contract2TokensString, contract2TokenIds[i], i < contract2TokenIds.length - 1 ? "," : ""));
    }

    bytes32 hash = ECDSA.toEthSignedMessageHash(keccak256(abi.encodePacked(msg.sender, contract1TokensString, contract2TokensString)));
    return ECDSA.recover(hash, signature) == signerAddress;
  }

This means the website must be generating these signatures, hopefully on a backend server where the private key for the signerAddress is kept secure. There is a function to change the signer address, which could be used in case the key is compromised, or just as good practice key rotation.

  function setSignerAddress(address _newSignerAddress) external onlyOwner {
    signerAddress = _newSignerAddress;
  }

Token Validation

Next the function tries to validate the following:

  • There is at least 1 external contract token to check (3DMfers or Cosmo Creatures)
  • The number of tokens to mint matches the number of tokens that you own
  • The tokens to mint haven’t been claimed already

For the last point, this is done for each contract, and I’ve copied the first block below.

    // Lets make sure we are not claiming for already claimed tokens of contract 1
    bool isValidTokenIds = true;
    for (uint i = 0; isValidTokenIds && i < contract1TokenIds.length; i++) {
      for (uint j = 0; isValidTokenIds && j < contract1ClaimedTokensCount; j++) {
        string memory contractClaimedToken = contract1ClaimedTokens[j];
        string memory tokenToClaim = contract1TokenIds[i];

        if (keccak256(bytes(tokenToClaim)) == keccak256(bytes(contractClaimedToken))) {
          isValidTokenIds = false;
        }
      } 
    } 
    require(isValidTokenIds, "Cosmo Creatures Token ID passed is already claimed");

Minting

Finally, there’s 2 loops, 1 for each contract, to increment a counter and record which contract tokens have been claimed. I only included 1 loop below for brevity. Once the loops are completed, there’s a call to _mintLoop to finally mint the tokens.

    for (uint i = 0; i < contract2TokenIds.length; i++) {
      contract2ClaimedTokensCount++;
      contract2ClaimedTokens.push(contract2TokenIds[i]);
    }

    _mintLoop( msg.sender, numberOfTokens );

This final part seems simple enough, but it turns out this where the bug is. Let’s see what we can learn from slither.

Slither Analysis

Slither is a python tool for static analysis of Solidity contracts. You can use it to get a quick summary of the contract code, and then look for any deeper issues.

$ slither 0x09f589e03381b767939ce118a209d474cc6d52fc --print human-summary

As we’ve seen above, there’s definitely complex code. One less important issue identified is that there’s no check for a zero address when setting the signerAddress. However, signerAddress is only used for the free claim, which shouldn’t be used anyway due to the bug. Almost all other issues are for the 11 dependency contracts, and none are significant. Except when you look at all the analysis, slither does identify the likely cause for the out of gas error:

$ slither 0x09f589e03381b767939ce118a209d474cc6d52fc
MutantMfers.freeClaimMint(uint256,bytes,string[],string[]) (MutantMfers.sol#1401-1450) has costly operations inside a loop:
- contract1ClaimedTokensCount ++ (MutantMfers.sol#1440)
MutantMfers.freeClaimMint(uint256,bytes,string[],string[]) (MutantMfers.sol#1401-1450) has costly operations inside a loop:
- contract2ClaimedTokensCount ++ (MutantMfers.sol#1445)

Magmar, the creator of the BackgroundMfers contract, explained the issue in the Discord:

The issue here is out of gas. The problem is the free mint contract has two variable-gas for loops and is screwing with Metamask’s infura gas estimation fees. These functions were likely tested with low quantities but the gas usage will increase as minting increases, costing more and more gas, with a higher likelihood of failing. They will likely work if you gas up your transaction heavily, but you will spend 0.02+ for a free mint.

In other words, for users that had many 3DMfers and/or Cosmo Creatures, their wallet underestimated the gas fee required, causing the transaction to fail. Some people in the discord said they successfully got free claims by increasing the gas fee, which is a more advanced user behavior. Perhaps if a Counter was used instead, everything would have worked great – lower gas fees and better wallet gas estimation. This is an excellent example of how different Solidity and the EVM are different from most other programming languages and environments, where incrementing a counter is simple and very cheap.

Conclusion

So what could have been done differently? As stated above, if contract1ClaimedTokensCount and contract2ClaimedTokensCount were Counters, then maybe everything would have worked as intended. Also, it’s possible the counters were not even necessary. In all the instances that one of the counters is used, the corresponding array length (such as contract2ClaimedTokens.length) could have been used instead. Alternatively (or in addition to) they could have done the following:

  • separate claiming functions for 3DMfers and Cosmo Creatures
  • let users choose how many free tokens to mint, and which tokens to claim with
  • address any user experience changes on the website, such as an option to mint with all tokens from a contract (some people own quite a few Cosmo Creatures, so a “claim with all” button could be helpful)

The user experience would be a little different, but I doubt free minters would complain. And two more things that every contract creator should do:

  1. Run slither on your contract
  2. Test every public function in many ways, with many inputs

Given the situation, what went right?

  • the team quickly notified everyone and removed the free claim button from the website
  • they refunded all the failed transactions
  • they provided a simple form for claiming free tokens, then airdropped them, paying the gas fee themselves
  • the public mint function worked great at a very affordable price
  • art was revealed 24 hours later with ~25% of the collection minted, and many happy owners
BackgroundMfers Banner

Dadmfers Background Mfers Contract Review

dadmfers is the first mfers derivative project, and they just recently released BackgroundMfers, a series of dadmfers inspired banner images. This required a new contract, with some additional complexity because of the number of mint options. The contract provides 5 separate minting functions for getting your background mfers NFTs:

  1. Public mint for anyone
  2. Mfer mint, for anyone that has mfers, at a reduced price
  3. Whitelist mint, free anyone on their list
  4. Dadmfers v1 mint for free
  5. Dadmfers v2 mint for free

Why some many minting options? It does add complexity, but there are good reasons, such as more favorable pricing for mfers holders, or free mints for dadmfers holders, while still allowing anyone to participate with the public option. There are two dadmfers options, v1 & v2, because the original v1 contract for dadmfers had very high gas fees. This turned a lot of people away, but the team quickly responded with a much more gas optimized contract. However, since the original contract had already been deployed, a new one was needed, because smart contracts cannot be upgraded once deployed (unless you use a proxy contract). So a v2 gas optimized contract was deployed, and everyone that minted on v1 was given free airdrops for v2. Ok, enough backstory, lets look at the background mfers contract code, available on etherscan.

  function mintPublic(uint256 _mintAmount) public payable mintCompliance(_mintAmount) {
    require(msg.value >= publicCost * _mintAmount, "Not enough eth sent!");
    require(_mintAmount < maxMintAmountPlusOne, "trying to mint too many!");
    _mintLoop(msg.sender, _mintAmount);
  }

Public Mint

This mintPublic function is very simple at first glance, but has a lot of dependencies to go into. It takes a _mintAmount, checks it with mintCompliance, then does 2 more checks before minting. Let’s look at mintCompliance since it’s also used by the other mint functions.

mintCompliance

  modifier mintCompliance(uint256 _mintAmount) {
    require(_mintAmount > 0, "Invalid mint amount!");
    require(supply.current() + _mintAmount < maxSupplyPlusOne, "Max supply exceeded!");
    require (saleIsActive, "Public sale inactive");
    _;
  }

Here we can see 3 requirements:

  1. _mintAmount must be a positive integer
  2. You can’t mint more than is available
  3. Minting must be active

These have some implications:

  • There is a max supply
  • Minting can be de-activated or re-activated

The max supply is defined by uint256 public maxSupplyPlusOne = 10_001 at the top of the contract. However, there’s also the following function at the bottom of the contract, which allows the contract owner to lower the supply.

  function lowerSupply(uint256 newSupply) public onlyOwner {
      if (newSupply < maxSupplyPlusOne) {
          maxSupplyPlusOne = newSupply;
      }
  }

Supply lowering can be a good thing to do if the collection doesn’t mint out in a certain period of time. By lowering the supply, you can preserve the current rarities and NFT values, potentially making the existing NFTs more valuable. Dadmfers v2 also lowered the supply after some time passed and it hadn’t sold out.

The contract owner can also disable or enable minting with the setSale function, which sets the salesIsActive variable.

  function setSale(bool newState) public onlyOwner {
    saleIsActive = newState;
  }

_mintLoop

  function _mintLoop(address _receiver, uint256 _mintAmount) internal {
    for (uint256 i = 0; i < _mintAmount; i++) {
      supply.increment();
      _safeMint(_receiver, supply.current());
    }
  }

This function is called with msg.sender (i.e. your wallet address) and the amount you want to mint. It does a simple for loop to increment the used supply, then mints a token. _safeMint is a standard function in OpenZeppelin’s ERC721, so we won’t go into that here.

Here’s a sample transaction for mintPublic, which transfers 1 token for 0.0169 Ether.

Mfers Mint

  function mintWithMfers(uint256 [] memory nftIds) public payable mintCompliance(nftIds.length) {
    require(msg.value >= mferCost * nftIds.length, "Not enough eth sent!");
    
    for (uint256 i = 0; i < nftIds.length; i++) {
      require(mfersContract.ownerOf(nftIds[i]) == msg.sender, "You must own all the mfers!");
      require(usedMferIds[nftIds[i]] == false, "One of the mfer IDs has already been used!");
      supply.increment();
      _safeMint(msg.sender, supply.current());
      usedMferIds[nftIds[i]] = true;
    }
  }

For this minting function, it expects a list of mfers tokens. These come from the selectors on the website. After checking mintCompliance and amount of eth sent, it loops through the tokens. If any token is not owned by you, or has already been used to mint, this function will fail. But if you are the owner of all the mfers tokens, it will do a mint for each one, and record the mfers token as used.

If you’re wondering why you have to check token ownership in the contract, when it’s already done on the website, that’s because smart contracts can be called directly, without going through the website. For example, you could go to the Write Contract section on etherscan, find the mintWithMfers function, and enter values directly there. This is something that all smart contract developers need to be aware of – you can’t assume people will only interact with the contract through your website.

Let’s dig into require(mfersContract.ownerOf(nftIds[i]) == msg.sender a bit more. mfersContract is defined at the top of the contract as nftInterface mfersContract = nftInterface(0x79FCDEF22feeD20eDDacbB2587640e45491b757f);.

So what’s nftInterface?

interface nftInterface {
    function ownerOf(uint256 tokenId) external view returns (address owner);
    function balanceOf(address owner) external view returns (uint256);
    function totalSupply() external view returns (uint256);
}

This is defined outside the contract, and provides a way to interact with another contract, by defining some functions that the contract should support. So mfersContract is an interface to the smart contract behind mfers (which you can see at this address on etherscan: 0x79FCDEF22feeD20eDDacbB2587640e45491b757f), and the background mfers contract is calling the mfers contract to check token ownership. These 3 functions (ownerOf, balanceOf, and totalSupply) are all standard ERC721 functions.

Here’s a sample transaction for mintWithMfers, which transfers 1 token for 0.0069 Ether.

Dadmfer v1 & v2 Mint

  function mintWithDadmfersV1(uint256 [] memory nftIds) public mintCompliance(nftIds.length) {
    for (uint256 i = 0; i < nftIds.length; i++) {
      require(dadmfersV1Contract.ownerOf(nftIds[i]) == msg.sender, "You must own all the dadmfer V1s!");
      require(usedV1Ids[nftIds[i]] == false, "One of the dadmfer IDs has already been used!");
      supply.increment();
      _safeMint(msg.sender, supply.current());
      usedV1Ids[nftIds[i]] = true;
    }
  }

  function mintWithDadmfersV2(uint256 [] memory nftIds) public mintCompliance(nftIds.length) {
    for (uint256 i = 0; i < nftIds.length; i++) {
      require(dadmfersV2Contract.ownerOf(nftIds[i]) == msg.sender, "You must own all the dadmfer V2s!");
      require(usedV2Ids[nftIds[i]] == false, "One of the dadmfer IDs has already been used!");
      supply.increment();
      _safeMint(msg.sender, supply.current());
      usedV2Ids[nftIds[i]] = true;
    }
  }

Both of these functions are very similar to each other and the mintWithMfers function. The main difference is dadmfersV1Contract vs dadmfersV2Contract. Just like with mfersContract, these are interfaces to the dadmfers contracts. The difference with mintWithMfers is that the initial requirement check for eth sent is gone, because these are free mints.

Here’s an example transaction for mintWithDadmfersV2, getting 3 tokens for 0 Ether.

Slither Analysis

Slither is a python tool for static analysis of Solidity contracts. You can use it to get a quick summary of the contract code, and then look for any deeper issues.

$ slither 0xc0a5393aA132DE6a66369Fe6e490cAc768991Ea5 --print human-summary

This fits with what we’ve seen above, the BackgroundMfers contract is complex code for minting ERC721 NFTs. The 11 medium issues are very technical to describe, but reduce down to “not a problem”. The contract does not implement onERC721Received, so there’s no real reentrency concerns, and the other issues look more like syntax & style choices in foreignNftsForWallet, which is a read-only function that is not used for minting.

Conclusion

While somewhat complicated, the background mfers contract looks quite safe for minting, and appears to be gas optimized. It allows batch minting to receive multiple tokens, which usually saves in gas fees. And there’s nothing in the minting functions that seems unnecessary. If you have any questions about the project, you can ask in the dadmfers discord; everyone is very friendly. As of publish time, they have not revealed the images yet, but when they do, you’ll be able to see the background mfers on looksrare or opensea.

I didn’t cover the whitelistMint because most people won’t be using that one. There’s also a complicated looking function foreignNftsForWallet that returns all the token IDs owned by a wallet, for a given contract. You can test this for yourself in the Read Contract section on etherscan, if you know a wallet address that owns one or more of mfers, dadmfers v1 or v2. For example, the address 0x4873f1768e1833fa6fb720b183715c7f57ecf953 is the wallet of the contract creator, so if you enter that and input 1, you can see it owns token 908 for dadmfers v2. Use input 0 for dadmfers v1 or input 2 for mfers.