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

`SnowmanAirdrop` Uses Incorrect EIP712 Type Hash Due to `addres` Typo, Breaking Signature Verification

kivanov

Root + Impact

Description

  • EIP712 typed data signing requires the type hash to exactly match the struct definition. The SnowmanClaim struct uses address receiver, so the type string must be "SnowmanClaim(address receiver, uint256 amount)".

  • The MESSAGE_TYPEHASH constant contains a typo — addres instead of address — producing a different keccak256 hash. Signatures generated by standard EIP712 libraries (e.g., ethers.js _signTypedData) will use the correct type string and will fail verification against this contract.

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

Risk

Likelihood:

  • Every frontend or backend that uses standard EIP712 signing with the correct type string will produce invalid signatures. This affects all legitimate users.

  • Only signatures crafted with the buggy type string will verify, creating a non-standard requirement for all claim integrations.

Impact:

  • Legitimate airdrop claimants cannot claim their NFTs through standard tooling.

  • An attacker who discovers the typo can craft signatures that verify, while security-conscious integrations using the spec-correct type string are locked out.

Proof of Concept

The following test demonstrates that the correct and buggy type hashes produce different values, confirming the signature domain mismatch.

function test_SAST002_TypehashTypo() public {
bytes32 correctHash = keccak256("SnowmanClaim(address receiver, uint256 amount)");
bytes32 buggyHash = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
​
assertTrue(correctHash != buggyHash);
// Correct: 0x4a3b6b60... vs Buggy: 0x1e2c3d4a... — different hashes
}

Recommended Mitigation

Fix the typo in the type string to match the actual struct definition.

bytes32 private constant MESSAGE_TYPEHASH =
- keccak256("SnowmanClaim(addres receiver, uint256 amount)");
+ keccak256("SnowmanClaim(address receiver, uint256 amount)");
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge 3 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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