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

Typo in EIP-712 Typehash Breaks Standard Wallet Signature Compatibility

rivera

Description

MESSAGE_TYPEHASH in SnowmanAirdrop.sol is defined as:

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

Note the typo: addres instead of address. Per the EIP-712 spec, the typehash must be the exact keccak256 hash of the struct's type string, byte-for-byte. Standard EIP-712 signing tools — eth_signTypedData_v4 as implemented by MetaMask, ethers.js, viem, and effectively every production wallet — derive this typehash independently from the correctly-spelled struct definition. They have no way of knowing the contract's on-chain typehash contains a typo, since the typo isn't visible anywhere in the ABI or struct declaration a client would read.

Risk

Likelihood: High. This isn't a rare edge case — it's the default outcome of anyone using standard wallet tooling to sign a claim. Every signature generated through eth_signTypedData_v4 against the correctly-spelled struct will fail, with no special conditions required to trigger it.

Impact: Medium, not High. The contract still exposes getMessageHash(address) as a public view function that returns its own (typo-affected) digest. A caller who queries this function on-chain and signs the returned digest directly — bypassing standard typed-data signing entirely — produces a signature the contract accepts, confirmed by this repo's own test suite (TestSnowmanAirdrop.t.sol), which signs exactly that way. So the claim flow isn't permanently and universally broken; it's broken specifically for anyone relying on standard wallet-based typed-data signing, which is the flow most production integrations would default to.

Proof of Concept

function test_H2_CorrectlySignedMessageFailsVerification() public {
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 correctTypehash = keccak256("SnowmanClaim(address receiver,uint256 amount)");
uint256 amount = snow.balanceOf(alice);
bytes32 structHash = keccak256(abi.encode(correctTypehash, alice, amount));
bytes32 correctDigest = keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
​
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, correctDigest);
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
vm.prank(satoshi);
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
}

Result: PASS. A digest built with the spec-correct typehash — exactly what a real wallet would produce — is signed and submitted, and the contract rejects it.

Recommended Mitigation

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

Lead Judging Commences

ai-first-flight-judge Lead Judge about 3 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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