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

`SnowmanAirdrop`: malformed EIP-712 typehash (`addres` + spaces) breaks signature compatibility with standard signers

hopeh

Description

SnowmanAirdrop supports claim-on-behalf: a relayer submits a recipient's (v, r, s) EIP-712 signature over the SnowmanClaim struct. For this to interoperate with normal wallets and signing libraries, the EIP-712 type string must match the struct exactly: SnowmanClaim(address receiver,uint256 amount).

The constant used to build the typehash is malformed: address is misspelled as addres, and the string contains stray spaces after the commas. The resulting MESSAGE_TYPEHASH therefore does not equal the EIP-712 type hash that any compliant signer computes from the real struct definition.

// SnowmanAirdrop.sol
bytes32 private constant MESSAGE_TYPEHASH =
keccak256("SnowmanClaim(addres receiver, uint256 amount)"); // @> "addres" + stray spaces
​
// EIP-712 requires exactly:
// keccak256("SnowmanClaim(address receiver,uint256 amount)")

Risk

Likelihood: Medium

  • Any party that signs with a standard EIP-712 library (ethers signTypedData, viem, MetaMask eth_signTypedData_v4, etc.) derives the type hash from the correct struct and produces a digest that does not match the contract's malformed one.

  • Claim-on-behalf is a primary, documented use case ("have someone claim on their behalf using the recipient's v, r, s"), so compliant signers hit this on the normal path.

Impact: Medium

  • Signatures from standard tooling are rejected with SA__InvalidSignature, so the signed / relayed-claim feature is non-functional and non-interoperable for anyone not hand-rolling the exact misspelled string.

  • It also defeats EIP-712's purpose: the human-readable type a wallet shows the signer (addres receiver) is wrong, undermining the typed-data integrity the scheme is meant to guarantee.

Proof of Concept

function test_standardEip712SignatureRejected() public {
// Build the digest the way a compliant signer does, from the CORRECT type string.
bytes32 correctTypeHash = keccak256("SnowmanClaim(address receiver,uint256 amount)");
bytes32 structHash = keccak256(abi.encode(correctTypeHash, receiver, amount));
bytes32 digest = _toTypedDataV4(structHash); // airdrop's EIP-712 domain
(uint8 v, bytes32 r, bytes32 s) = vm.sign(receiverPk, digest);
​
// The contract recomputes with its MALFORMED typehash -> different digest -> reject.
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(receiver, proof, v, r, s);
}

The claim reverts, showing a correctly-produced EIP-712 signature is not accepted.

Recommended Mitigation

Use the exact, correctly-spelled EIP-712 type string — no misspelling and no stray spaces, so the typehash matches what compliant signers compute:

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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