President Elector

Summary

The rankCandidatesBySig function allows voters to submit signed messages to rank candidates. However, the current implementation does not adequately prevent replay attacks, enabling a signer to reuse the same signature multiple times across different voting periods, potentially altering past or future vote rankings maliciously.

Vulnerability Details

• Signature Structure:

  • The function encodes the data using the TYPEHASH which includes the orderedCandidates array but does not incorporate the s_voteNumber.

    bytes32 structHash = keccak256(abi.encode(TYPEHASH, orderedCandidates));

    bytes32 hash = _hashTypedDataV4(structHash);

    address signer = ECDSA.recover(hash, signature);

• Replay Potential:

  • Since the s_voteNumber is not part of the signed data, the same signature can be reused for multiple voteNumber instances.

  • An attacker can submit the same signature in different voting rounds, overwriting previous rankings or influencing future elections unintentionally.

Impact

• Vote Manipulation: Attackers can alter voter rankings across multiple voting periods, skewing election results.

• Loss of Trust: The integrity of the election process is undermined, leading to reduced trust in the system.

• Potential for Collusion: Malicious actors can collude to submit multiple fraudulent votes using the same signature, amplifying their influence.

Tools Used

• Manual Code Review: Examined the rankCandidatesBySig function for replay protection mechanisms.

• Static Analysis Tools: Utilized tools like MythX to identify signature-related vulnerabilities.

Recommendations

• Incorporate s_voteNumber into Signed Data:

  • Include the current s_voteNumber within the data structure being signed to bind signatures to specific voting periods.

• Implement Replay Protection:

  • Ensure that each signature can only be used once per voteNumber by tracking used signatures or incorporating unique identifiers.

    mapping(bytes32 => bool) public usedSignatures;

    function rankCandidatesBySig(

    address[] memory orderedCandidates,

    bytes memory signature

    ) external {

    bytes32 structHash = keccak256(abi.encode(TYPEHASH, s_voteNumber, orderedCandidates));

    bytes32 hash = _hashTypedDataV4(structHash);

    address signer = ECDSA.recover(hash, signature);

    require(!usedSignatures[hash], "Signature already used");

    usedSignatures[hash] = true;

    _rankCandidates(orderedCandidates, signer);

    }

• Use Nonces or Unique Identifiers:

  • Integrate a nonce system where each signature includes a unique number that must be incremented with each vote submission.

    mapping(address => uint256) public nonces;

    function rankCandidatesBySig(

    address[] memory orderedCandidates,

    bytes memory signature

    ) external {

    uint256 currentNonce = nonces[msg.sender]++;

    bytes32 structHash = keccak256(abi.encode(TYPEHASH, s_voteNumber, currentNonce, orderedCandidates));

    bytes32 hash = _hashTypedDataV4(structHash);

    address signer = ECDSA.recover(hash, signature);

    _rankCandidates(orderedCandidates, signer);

    }

• Emit Events for Signature-Based Actions:

  • Emit events when signatures are used to facilitate monitoring and auditing.

    event CandidateRankedBySignature(address indexed signer, uint256 voteNumber);

    function rankCandidatesBySig(

    address[] memory orderedCandidates,

    bytes memory signature

    ) external {

    ...

    emit CandidateRankedBySignature(signer, s_voteNumber);

    }

Proof of Concept

• Implement unit tests to verify that signatures cannot be replayed across different voteNumber instances.