Bad source of randomness

[code423n4]

Lines of code

https://github.com/code-423n4/2022-10-holograph/blob/f8c2eae866280a1acfdc8a8352401ed031be1373/contracts/HolographOperator.sol#L491-L511

Vulnerability details

Impact

Using block.number and block.timestamp as a source of randomness is commonly advised against, as the outcome can be manipulated by calling contracts. In this case a compromised layer-zero-endpoint would be able to retry the selection of the primary operator until the result is favorable to the malicious actor.

Proof of Concept

An attack path for rerolling the result of bad randomness might look roughly like this:

function attack(uint256 currentNonce, uint256 wantedPodIndex, uint256 numPods, uint256 wantedOperatorIndex, uint256 numOperators,  bytes calldata bridgeInRequestPayload) external{

    bytes32 jobHash = keccak256(bridgeInRequestPayload);

    //same calculation as in HolographOperator.crossChainMessage
    uint256 random = uint256(keccak256(abi.encodePacked(jobHash, currentNonce, block.number, block.timestamp)));

    require(wantedPodIndex == random % numPods)
    require(wantedOperatorIndex == random % numOperators);

    operator.crossChainMessage(bridgeInRequestPayload);
}

The attack basically consists of repeatedly calling the attack function with data that is known and output that is wished for until the results match and only then continuing to calling the operator.

Tools Used

Manual Review

Recommended Mitigation Steps

Consider using a decentralized oracle for the generation of random numbers, such as Chainlinks VRF.

It should be noted, that in this case there is a prerequirement of the layer-zero endpoint being compromised, which confines the risk quite a bit, so using a normally unrecommended source of randomness could be acceptable here, considering the tradeoffs of integrating a decentralized oracle.

[gzeoneth]

Duplicate of #27

[ACC01ADE]

Very valid issue

[gzeoneth]

While sponsor noted this is a design choice to use pseudorandomness, as pointed out by the warden a compromised layer-zero-endpoint can exploit this for profit, judging this as Med risk.