NextGenRandomizerNXT uses the PREVRANDAO opcode as source of randomness

[c4-submissions]

Lines of code

https://github.com/code-423n4/2023-10-nextgen/blob/8b518196629faa37eae39736837b24926fd3c07c/hardhat/smart-contracts/XRandoms.sol#L35-L43

Vulnerability details

Summary

The NextGenRandomizerNXT contract uses block.prevrandao as the source of randomness this value is not truly random, and validators who are chosen to propose new blocks will be able to know the value of the PREVRANDAO opcode.

Vulnerability Details

As the NextGenRandomizerNXT uses the block.prevrandao together with deterministic things like block.number to calculate the random hash. The result of minting an NFT can be deterministically calculated by validators. This creates unfair market conditions.

Here we can see that the flow of generating a random hash with the NextGenRandomizerNXT contract uses the PREVRANDAO opcode as the source of randomness:

function calculateTokenHash(uint256 _collectionID, uint256 _mintIndex, uint256 _saltfun_o) public {
    require(msg.sender == gencore);
    bytes32 hash = keccak256(abi.encodePacked(_mintIndex, blockhash(block.number - 1), randoms.randomNumber(), randoms.randomWord()));
    gencoreContract.setTokenHash(_collectionID, _mintIndex, hash);
}

function randomNumber() public view returns (uint256){
    uint256 randomNum = uint(keccak256(abi.encodePacked(block.prevrandao, blockhash(block.number - 1), block.timestamp))) % 1000;
    return randomNum;
}

string[100] memory wordsList = ["Acai", "Ackee", "Apple", ...];

function randomWord() public view returns (string memory) {
    uint256 randomNum = uint(keccak256(abi.encodePacked(block.prevrandao, blockhash(block.number - 1), block.timestamp))) % 100;
    return getWord(randomNum);
}

Impact

Validators are able to already know which NFT they would get if they mint now. This creates very unfair market conditions, as validators are able to cheat and depending on the NFT collection steal funds by doing so. It also destroys the rarity variables of the artist.

Tools Used

Manual Review

Recommendations

Do not use the PREVRANDAO opcode as a source of randomness, if the outcome of the random hash is not trivial.

Assessed type

MEV

[c4-pre-sort]

141345 marked the issue as duplicate of #163

[c4-judge]

alex-ppg marked the issue as duplicate of #1901

[c4-judge]

alex-ppg marked the issue as unsatisfactory:
Invalid

[c4-judge]

alex-ppg marked the issue as unsatisfactory:
Invalid