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

EIP-712 MESSAGE_TYPEHASH is malformed (typo 'addres' + spaces), so signatures from standard wallets are rejected and eligible users cannot claim

bytethebuilder

Root + Impact

Description

The protocol authenticates claims with an EIP-712 signature, but the struct type string used to build MESSAGE_TYPEHASH is malformed:

@> bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");

Two problems vs the EIP-712 spec:

  1. Typo: addres instead of address (the actual struct field type is address).

  2. Whitespace: EIP-712 encodeType requires NO spaces - it must be SnowmanClaim(address receiver,uint256 amount). This string has a space after ( ... , between members.

EIP-712-compliant signers (MetaMask eth_signTypedData_v4, ethers/viem signTypedData, etc.) derive the type hash from the correct canonical encoding SnowmanClaim(address receiver,uint256 amount). That produces a different MESSAGE_TYPEHASH, hence a different structHash and a different final digest than the one getMessageHash() builds on-chain. So a signature a normal user creates with their wallet does NOT recover to receiver in _isValidSignature, and claimSnowman reverts with SA__InvalidSignature.

The contract is only self-consistent if the user signs the contract's own non-standard digest by hand (as the test does via vm.sign(key, airdrop.getMessageHash(...))). Real recipients using standard wallet tooling cannot produce a valid signature, so they cannot claim their airdrop - and since claimSnowman is the only way to receive a Snowman, the airdrop is effectively unclaimable through normal flows.

Risk

Likelihood: High - every recipient using a standard EIP-712 wallet/library (the expected flow, and the whole point of EIP-712) is affected.

Impact: Medium - core functionality (claiming) is broken for normal users; a legitimate, properly-intended signature is rejected, denying eligible users their airdrop (protocol DoS / failed delivery of value).

Proof of Concept

A signature over the correct EIP-712 digest (what a wallet produces) is rejected by claimSnowman. Runnable Foundry test (add to TestSnowmanAirdrop.t.sol):

function test_PoC_standardEip712SignatureIsRejected() public {
uint256 amount = snow.balanceOf(alice); // 1
​
// What a compliant wallet/library computes (correct type string, no typo, no spaces):
bytes32 CORRECT_TYPEHASH = keccak256("SnowmanClaim(address receiver,uint256 amount)");
bytes32 structHash = keccak256(abi.encode(CORRECT_TYPEHASH, alice, amount));
​
// Rebuild the contract's EIP-712 domain separator: EIP712("Snowman Airdrop", "1")
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)
)
);
bytes32 correctDigest = keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
​
// Alice signs the CORRECT digest, exactly as her wallet would
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, correctDigest);
​
vm.prank(alice);
snow.approve(address(airdrop), amount);
​
// The contract uses the malformed typehash internally, so it rejects the valid signature
vm.prank(satoshi);
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
}

Run forge test --mt test_PoC_standardEip712SignatureIsRejected -vv; it passes - a correctly-formed EIP-712 signature is rejected, so wallet users cannot claim.

Recommended Mitigation

Fix the type string to the exact EIP-712 canonical encoding (correct type, no spaces) so the on-chain typehash matches what wallets/libraries produce:

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

With the corrected typehash, getMessageHash() reconstructs the same digest standard EIP-712 signers produce, so legitimately-signed claims verify and recipients can claim. (Add a unit test that signs via the canonical EIP-712 encoding - not the contract's own helper - to catch this class of mismatch.)

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 4 hours 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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