Snowman Merkle Airdrop
AI First Flight #10
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Submission Details
Severity: high
Valid

EIP-712 Typehash Contains Typo Causing Signature Incompatibility

odinshell

Description

The SnowmanAirdrop contract implements EIP-712 typed structured data signing to allow users to authorize claims via off-chain signatures. The signature verification should allow gas-less claims where a third party can submit the claim transaction on behalf of the user.

The MESSAGE_TYPEHASH constant contains a typo - "addres" instead of "address" - which causes the contract's type hash to differ from what standard EIP-712 signing tools would generate. This makes signatures generated by compliant wallets (MetaMask, ethers.js, etc.) incompatible with the contract's verification.

// src/SnowmanAirdrop.sol:49
bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
// ^^^^^^^ @> TYPO: "addres" should be "address"

The typehash is used in getMessageHash() to construct the EIP-712 digest:

// src/SnowmanAirdrop.sol:112-122
function getMessageHash(address receiver) public view returns (bytes32) {
// ...
return _hashTypedDataV4(
keccak256(abi.encode(MESSAGE_TYPEHASH, SnowmanClaim({receiver: receiver, amount: amount}))) // @> Uses wrong typehash
);
}

Risk

Likelihood: Medium

  • Any user attempting to sign with standard EIP-712 compliant tools will generate incompatible signatures

  • The typo is subtle and easy to miss during development and testing

  • Tests pass because they sign the digest returned by getMessageHash() which already includes the typo

Impact: High

  • Users cannot claim airdrops using standard wallet signing flows

  • Integration with front-end applications using ethers.js or web3.js will fail

  • MetaMask and other wallets will produce signatures the contract rejects

  • The airdrop claim functionality is effectively broken for production use

Proof of Concept

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
​
import {Test, console2} from "forge-std/Test.sol";
​
contract TypehashTest is Test {
function testTypehashMismatch() public pure {
// What the contract uses (WRONG - typo)
bytes32 wrongHash = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
​
// What EIP-712 compliant signers would use (CORRECT)
bytes32 correctHash = keccak256("SnowmanClaim(address receiver, uint256 amount)");
​
console2.log("Contract typehash: ", vm.toString(wrongHash));
console2.log("Correct typehash: ", vm.toString(correctHash));
​
// These produce different hashes - signatures will be incompatible
assert(wrongHash != correctHash);
}
}

Why Tests Pass Despite This Bug:

The test file uses Foundry's vm.sign() which signs whatever digest is returned by getMessageHash():

// test/TestSnowmanAirdrop.t.sol:82-85
bytes32 alDigest = airdrop.getMessageHash(alice); // Returns digest with WRONG typehash
(uint8 alV, bytes32 alR, bytes32 alS) = vm.sign(alKey, alDigest); // Signs the wrong digest
// Verification passes because both use the same wrong typehash

In production, external signing tools would compute:

keccak256("SnowmanClaim(address receiver, uint256 amount)") // Correct

But the contract expects:

keccak256("SnowmanClaim(addres receiver, uint256 amount)") // Typo

Recommended Mitigation

Fix the typo in the typehash constant:

// src/SnowmanAirdrop.sol:49
- bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
+ bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(address receiver, uint256 amount)");
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 9 days ago
Submission Judgement Published
Validated
Assigned finding tags:

[H-02] Unconsistent `MESSAGE_TYPEHASH` with standart EIP-712 declaration on contract `SnowmanAirdrop`

# Root + Impact ## Description * Little typo on `MESSAGE_TYPEHASH` Declaration on `SnowmanAirdrop` contract ```Solidity // src/SnowmanAirdrop.sol 49: bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)"); ``` **Impact**: * `function claimSnowman` never be `TRUE` condition ## Proof of Concept Applying this function at the end of /test/TestSnowmanAirdrop.t.sol to know what the correct and wrong digest output HASH. Ran with command: `forge test --match-test testFrontendSignatureVerification -vvvv` ```Solidity function testFrontendSignatureVerification() public { // Setup Alice for the test vm.startPrank(alice); snow.approve(address(airdrop), 1); vm.stopPrank(); // Simulate frontend using the correct format bytes32 FRONTEND_MESSAGE_TYPEHASH = keccak256("SnowmanClaim(address receiver, uint256 amount)"); // Domain separator used by frontend (per EIP-712) bytes32 DOMAIN_SEPARATOR = keccak256( abi.encode( keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"), keccak256("Snowman Airdrop"), keccak256("1"), block.chainid, address(airdrop) ) ); // Get Alice's token amount uint256 amount = snow.balanceOf(alice); // Frontend creates hash using the correct format bytes32 structHash = keccak256( abi.encode( FRONTEND_MESSAGE_TYPEHASH, alice, amount ) ); // Frontend creates the final digest (per EIP-712) bytes32 frontendDigest = keccak256( abi.encodePacked( "\x19\x01", DOMAIN_SEPARATOR, structHash ) ); // Alice signs the digest created by the frontend (uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, frontendDigest); // Digest created by the contract (with typo) bytes32 contractDigest = airdrop.getMessageHash(alice); // Display both digests for comparison console2.log("Frontend Digest (correct format):"); console2.logBytes32(frontendDigest); console2.log("Contract Digest (with typo):"); console2.logBytes32(contractDigest); // Compare the digests - they should differ due to the typo assertFalse( frontendDigest == contractDigest, "Digests should differ due to typo in MESSAGE_TYPEHASH" ); // Attempt to claim with the signature - should fail vm.prank(satoshi); vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector); airdrop.claimSnowman(alice, AL_PROOF, v, r, s); assertEq(nft.balanceOf(alice), 0); } ``` ## Recommended Mitigation on contract `SnowmanAirdrop` Line 49 applying this: ```diff - bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)"); + bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(address receiver, uint256 amount)"); ```

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