Skyweaver

1 Executive Summary

This report presents the results of our engagement with Horizon Games to review the security of their smart contracts for the SkyWeaver game.

The review was conducted over one week, from April 1st to April 10th by Sergii Kravchenko. A total of 5 person-days were spent.

During this week, I focused primarily on factories that produce new cards/items/tokens, trust-related issues, the RNG mechanism, and re-entrancy attacks (especially within ERC-1155 token callbacks).

The review identified 7 issues of different severities (see section 4).

The allotted time for the review (1 person-week) was deemed insufficient from the start to do a comprehensive audit of the whole system. As such, some compromises had to be made on the completeness of the review.

2 Scope

Our review is focused on the commit hash bde0c184db6168bf86656a48b12d5747950b54d9 of https://github.com/horizon-games/SkyWeaver-contracts repository. The list of files in scope can be found in the Appendix.

3 Security Specification

This section describes, from a security perspective, the expected behavior of the system under the review. It is not a substitute for documentation.

3.1 Actors

The relevant actors are listed below with their respective abilities:

• System owners - there are multiple contracts in the system that have an owner that potentially can be different addresses in different contracts, but logically it’s the same owner (Horizon admins).
  • SilverCardsFactory
    • can register/deregister card IDs.
    • can change the price.
    • withdraw arcadeum coins from the contract.
  • EternalHeroesFactory
    • can register/deregister item IDs.
    • cannot change price, pricing is set during the contract creation.
    • withdraw arcadeum coins from the contract.
  • GoldCardsFactory
    • can register/deregister card IDs.
    • can change the price.
    • can change refund.
    • can change rngDelay.
  • WeaveFactory
    • can mint weave for anyone, minting is limited per time. The rate is fixed and cannot be changed.
  • SWSupplyManager
    • can manage factories.
    • can set maximum supply for each token ID.
    • can lock some token IDs.
• Token holders - anyone who has enough tokens (arcadeum coins or weave) can buy cards/items from factories.
• Anyone - can mine gold card for any already committed gold order and recommit.

3.2 Trust Model

In any smart contract system, it’s important to identify what trust is expected/required between various actors. For this review, we established the following trust model:

The system has owners that have some centralized power, like changing prices/refunds. From other perspectives, the system is pretty much permissionless for the users, anyone with tokens can buy/melt items for a predefined/limited price.

Random number generation is not completely trustless; it may potentially be manipulated by miners. However, unless gold cards become expensive, there is no clear incentive for miners to do so.

3.3 Important Security Properties

The following is a non-exhaustive list of security properties that were discovered during this review:

• To decrease trust assumptions, owner can lock some token IDs in SWSupplyManager to ensure that only already existing factories with already existing permissions can mint items with these IDs.

• Sometimes price can be changed by the admin using a front-running technique. In that case, users have mechanisms that allows them to cancel their order if the price is too high. Also, the owner can modify the gold cards list between committing to mine a gold card and actually mining it.

• In some factories prices include decimals, in some - not.

4 Issues

Each issue has an assigned severity:

• Minor issues are subjective in nature. They are typically suggestions around best practices or readability. Code maintainers should use their own judgment as to whether to address such issues.
• Medium issues are objective in nature but are not security vulnerabilities. These should be addressed unless there is a clear reason not to.
• Major issues are security vulnerabilities that may not be directly exploitable or may require certain conditions in order to be exploited. All major issues should be addressed.
• Critical issues are directly exploitable security vulnerabilities that need to be fixed.

// Mint gold cards
skyweaverAssets.batchMint(_order.cardRecipient, _ids, amounts, "");

function _commit(uint256 _weaveAmount, GoldOrder memory _order)
  internal
{
  // Check if weave sent is sufficient for order
  uint256 total_cost = _order.cardAmount.mul(goldPrice).add(_order.feeAmount);
  uint256 refund_amount = _weaveAmount.sub(total_cost); // Will throw if insufficient amount received

// Burn the non-refundable weave
uint256 weave_to_burn = (_order.cardAmount.mul(goldPrice)).sub(_order.cardAmount.mul(goldRefund));
weaveContract.burn(weaveID, weave_to_burn);

uint256 refundAmount = _arcAmount.sub(total_cost);
if (refundAmount > 0) {
  arcadeumCoin.safeTransferFrom(address(this), _recipient, arcadeumCoinID, refundAmount, "");
}

// Mint tokens to recipient
factoryManager.batchMint(_recipient, _ids, amounts_to_mint, "");

function setMaxSupplies(uint256[] calldata _ids, uint256[] calldata _newMaxSupplies) external onlyOwner() {
  require(_ids.length == _newMaxSupplies.length, "SWSupplyManager#setMaxSupply: INVALID_ARRAYS_LENGTH");

  // Can only *decrease* a max supply
  // Can't set max supply back to 0
  for (uint256 i = 0; i < _ids.length; i++ ) {
    if (maxSupply[_ids[i]] > 0) {
      require(
        0 < _newMaxSupplies[i] && _newMaxSupplies[i] < maxSupply[_ids[i]],
        "SWSupplyManager#setMaxSupply: INVALID_NEW_MAX_SUPPLY"
      );
    }
    maxSupply[_ids[i]] = _newMaxSupplies[i];
  }

  emit MaxSuppliesChanged(_ids, _newMaxSupplies);
}

function burn(
  uint256 _id,
  uint256 _amount)
  external
{
  _burn(msg.sender, _id, _amount);
}

Appendix 1 - Files in Scope

This audit covered the following files:

Appendix 2 - Disclosure

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