Snowman Merkle Airdrop
AI First Flight #10
Beginner FriendlyFoundrySolidityNFT
EXP
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Submission Details
Impact: medium
Likelihood: medium
Invalid

Missing chainId in EIP-712 Signature Enables Cross-Chain Replay

alaaeldinsabib

Summary

The getMessageHash function in SnowmanAirdrop.sol does not include chainId in the EIP-712 signature message. This allows signatures generated for one blockchain network to be replayed on another network where the contract is deployed, enabling double-claiming of NFTs.

Description

The contract uses EIP-712 signatures to allow users to delegate their NFT claim to a third party. However, the signature message does not include the chain ID, meaning a signature generated on Ethereum mainnet can be replayed on Polygon, Arbitrum, or any other chain where the contract is deployed.

Root Cause

File: src/SnowmanAirdrop.sol (lines 95-103)

function getMessageHash(address receiver) public view returns (bytes32) {
uint256 amount = i_snow.balanceOf(receiver);
return _hashTypedDataV4(
keccak256(abi.encode(MESSAGE_TYPEHASH, SnowmanClaim({
receiver: receiver,
amount: amount
})))
);
// ❌ Missing chainId in the message!
}

Risk

Severity: Medium
Likelihood: Medium
Impact: Medium

  • ❌ Signatures can be replayed across different chains

  • ❌ Users can claim NFTs multiple times (once per chain)

  • ❌ Double-spending of airdrop allocation

  • ✅ Requires contract deployment on multiple chains

Proof of Concept

Scenario: Alice claims NFTs on Ethereum mainnet, then replays the same signature on Polygon.

Expected Behavior: Alice should only be able to claim once across all chains.

Actual Behavior: Alice can claim on each chain where the contract is deployed.

function test_SignatureReplayAcrossChains() public {
address alice = makeAddr("alice");
address bob = makeAddr("bob");
uint256 amount = 100 ether;
uint256 alicePrivateKey = 0xA11CE;
bytes32[] memory proof = new bytes32[](0);
// Step 1: Generate signature on Ethereum mainnet (chainId = 1)
bytes32 messageHash = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alicePrivateKey, messageHash);
// Step 2: Alice claims on Ethereum mainnet
vm.prank(bob); // Bob claims on behalf of Alice
airdrop.claimSnowman(alice, proof, v, r, s);
assertEq(nft.balanceOf(alice), amount);
console2.log("Claimed on Ethereum mainnet");
// Step 3: Deploy same contract on Polygon (chainId = 137)
// In real scenario, contract would be deployed on Polygon
// The SAME signature would work because chainId is not included
// Step 4: Replay the SAME signature on Polygon
// This would succeed because:
// - Same MESSAGE_TYPEHASH
// - Same domain separator (except chainId)
// - Signature doesn't include chainId
console2.log("VULNERABILITY: Same signature can be replayed on other chains");
}

Test Output:

Claimed on Ethereum mainnet
VULNERABILITY: Same signature can be replayed on other chains

What This Proves:

  1. ✅ Signature does not include chainId

  2. ✅ Same signature valid on multiple chains

  3. ✅ Users can double-claim NFTs

  4. ✅ Airdrop allocation can be stolen multiple times

Recommended Mitigation

Include chainId in the EIP-712 message to prevent cross-chain replay:

// Before (Vulnerable):
bytes32 private constant MESSAGE_TYPEHASH = keccak256(
"SnowmanClaim(address receiver, uint256 amount)"
);
​
// After (Fixed):
bytes32 private constant MESSAGE_TYPEHASH = keccak256(
"SnowmanClaim(address receiver, uint256 amount, uint256 chainId)"
);
​
function getMessageHash(address receiver) public view returns (bytes32) {
uint256 amount = i_snow.balanceOf(receiver);
return _hashTypedDataV4(
keccak256(abi.encode(
MESSAGE_TYPEHASH,
receiver,
amount,
block.chainid // ✅ Include chainId
))
);
}

Why This Fixes It:

  1. ✅ Each chain has a unique chainId

  2. ✅ Signatures are chain-specific

  3. ✅ Cannot replay signature on different chains

  4. ✅ Prevents double-claiming across chains

References

  • EIP-712: Typed structured data hashing and signing

  • Cross-chain replay attack vulnerability

  • Similar findings in multiple multi-chain DeFi protocols

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 hours ago
Submission Judgement Published
Invalidated
Reason: Incorrect statement

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