Merkle Proof Verification

Overview

The Merkle proof verification detector identifies custom Merkle tree implementations that do not double-hash leaf values before verification. Without double-hashing, the proof is vulnerable to second preimage attacks: an attacker can construct a valid proof for a value that was never included in the original tree by reinterpreting an internal tree node as a leaf.

Custom Merkle implementations have caused $10M+ in whitelist bypasses, unauthorized NFT minting, and airdrop theft. Contracts that roll their own verification instead of using audited libraries carry elevated risk.

Why This Is an Issue

In a standard Merkle tree, internal nodes are computed as hash(left || right). If leaf values are hashed with the same function without distinction (hash(leaf)), an attacker who knows the value of an internal node can submit it as a leaf with a shortened proof. The verifier cannot distinguish between a genuine leaf and an internal node because both are produced by the same hash function.

Double-hashing — hash(hash(leaf)) — ensures that leaf hashes are in a different domain than internal node hashes, preventing this confusion.

How to Resolve

// Before: Vulnerable -- single hash of leaf
function verify(bytes32[] calldata proof, bytes32 root, address account, uint256 amount) public pure returns (bool) {
    bytes32 leaf = keccak256(abi.encodePacked(account, amount));
    return MerkleProof.verify(proof, root, leaf);  // Second preimage risk
}

// After: Fixed -- double hash of leaf
function verify(bytes32[] calldata proof, bytes32 root, address account, uint256 amount) public pure returns (bool) {
    bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(account, amount))));
    return MerkleProof.verify(proof, root, leaf);
}

Examples

Vulnerable

function claimAirdrop(bytes32[] calldata proof, uint256 amount) external {
    bytes32 leaf = keccak256(abi.encodePacked(msg.sender, amount));
    require(MerkleProof.verify(proof, merkleRoot, leaf), "Invalid proof");
    require(!claimed[msg.sender], "Already claimed");
    claimed[msg.sender] = true;
    token.transfer(msg.sender, amount);
}

Fixed

function claimAirdrop(bytes32[] calldata proof, uint256 amount) external {
    // Double-hash prevents second preimage attacks
    bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(msg.sender, amount))));
    require(MerkleProof.verify(proof, merkleRoot, leaf), "Invalid proof");
    require(!claimed[msg.sender], "Already claimed");
    claimed[msg.sender] = true;
    token.transfer(msg.sender, amount);
}

Sample Sigvex Output

[HIGH] merkle-proof-verification
  Potential insecure Merkle proof verification in claimAirdrop
  Location: claimAirdrop @ block 0, instruction 0
  Confidence: 0.70

  Function claimAirdrop appears to implement Merkle proof verification
  (iterative hashing with loop control flow) but does not double-hash
  leaf values before verification.

Detection Methodology

1. Function name matching: Identifies functions with names containing merkle, proof, verify, claim, whitelist, or airdrop.
2. Pattern matching: Detects iterative hashing patterns (multiple KECCAK256 operations with loop control flow) that characterize Merkle proof walking.
3. Double-hash check: Examines the first few blocks for two consecutive KECCAK256 operations in the same block, which indicates the leaf is double-hashed before entering the proof loop.
4. Proxy suppression: Skips proxy contracts where iterative hashing in the fallback/dispatch function is for selector routing, not Merkle verification.

Limitations

False positives: Functions that perform iterative hashing for non-Merkle purposes (e.g., hash chains, commit-reveal schemes) may be flagged if their name matches Merkle-related patterns. Contracts using OpenZeppelin’s MerkleProof library are suppressed. False negatives: Merkle verification performed in a library contract via DELEGATECALL is not analyzed in the calling contract’s context.

Related Detectors

• Hash Collision — detects hash collision risks from abi.encodePacked with dynamic types
• Signature Verification — detects improper signature checks

References

• OpenZeppelin MerkleProof.sol
• CWE-347: Improper Verification of Cryptographic Signature