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

Security Vulnerability Report: EIP-712 Typehash Typo in SnowmanAirdrop

r7al38

Root + Impact

Root Cause: A simple spelling mistake in the MESSAGE_TYPEHASH constant breaks EIP‑712 compliance.
Impact: All valid signatures generated by wallets (MetaMask, ethers.js) are rejected, making the claimSnowman function unusable.

Description

  • Under normal behavior, the contract uses EIP‑712 structured signatures to verify that a user authorizes the claim. The frontend builds a typehash following the exact Solidity struct definition, signs it, and sends the signature to the contract.

    The problem is that the contract defines its typehash with a typo – addres instead of address. Because the typehash does not match the one used off‑chain, every genuine signature fails verification, and the transaction reverts with SA__InvalidSignature.

// @> Line where the typo exists
bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
// ^^^^^^ should be "address"

Risk

Likelihood:High

  • The typo is hard‑coded; it will affect every claim attempt.

  • Any user interacting through a standard wallet will always produce a signature with the correct typehash, so the failure is guaranteed on each call.

Impact:High

  • The core functionality of the contract – claiming Snowman NFTs – is completely broken.

  • Users cannot receive their NFTs, leading to loss of promised assets and complete loss of trust in the project.

Proof of Concept

The following Foundry test demonstrates that a valid off‑chain signature is rejected by the contract:

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.24;
​
import "forge-std/Test.sol";
import "../src/SnowmanAirdrop.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
​
// Mock ERC20 token
contract MockSnow is ERC20 {
constructor() ERC20("Mock Snow", "MSNOW") {
_mint(msg.sender, 1_000_000 * 10**18); // all tokens go to the deployer (test contract)
}
}
​
// Mock NFT contract
contract MockSnowman {
function mintSnowman(address, uint256) external {}
}
​
contract SnowmanAirdropPoC is Test {
SnowmanAirdrop public airdrop;
MockSnow public snow;
MockSnowman public snowman;
​
address public user = address(0x123);
uint256 public userPrivateKey = 0x123456;
​
function setUp() public {
// 1. Deploy mock tokens
snow = new MockSnow();
snowman = new MockSnowman();
​
// 2. Deploy the target airdrop contract (needs snow + snowman addresses)
airdrop = new SnowmanAirdrop(keccak256("mock"), address(snow), address(snowman));
​
// 3. Transfer some tokens from the test contract to the user
// (the test contract owns all minted tokens)
snow.transfer(user, 1000 * 10**18);
​
// 4. User approves the airdrop contract to spend their tokens
vm.startPrank(user);
snow.approve(address(airdrop), type(uint256).max);
vm.stopPrank();
}
​
function test_ValidSignatureFailsBecauseOfTypo() public {
uint256 amount = snow.balanceOf(user);
require(amount > 0, "User has no Snow tokens");
​
// Correct typehash (used by wallets / ethers.js)
bytes32 correctTypeHash = keccak256(
"SnowmanClaim(address receiver,uint256 amount)"
);
​
// Struct hash with the correct typehash
bytes32 structHash = keccak256(
abi.encode(
correctTypeHash,
SnowmanAirdrop.SnowmanClaim({receiver: user, amount: amount})
)
);
​
// Domain separator (must match the one used in the contract)
bytes32 domainSeparator = keccak256(
abi.encode(
keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
keccak256(bytes("Snowman Airdrop")),
keccak256(bytes("1")),
block.chainid,
address(airdrop)
)
);
​
// Final digest: \x19\x01 ‖ domainSeparator ‖ structHash
bytes32 digest = keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
​
// Sign the digest with the user's private key
(uint8 v, bytes32 r, bytes32 s) = vm.sign(userPrivateKey, digest);
​
// Attempt to claim – the contract will use its buggy typehash and reject the signature
bytes32[] memory emptyProof = new bytes32[](0);
​
vm.prank(user);
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(user, emptyProof, v, r, s);
​
// If we reach here, the test passed (valid signature was rejected)
}
}
​

Result: The test passes, proving that a correct signature is rejected because of the typo.


Recommended Mitigation

Fix the spelling of addres to address in the typehash string, then re‑deploy the contract.

- 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 7 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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