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

EIP-712 Typehash Typo ("addres" instead of "address") Makes Standard Signatures Invalid

harmonygemini8

Description

The SnowmanAirdrop contract uses EIP-712 typed signatures to authorize claims on behalf of a receiver. The domain is correctly defined via EIP712("Snowman Airdrop", "1"), but the MESSAGE_TYPEHASH constant used to build the claim struct hash contains a typo: "addres" instead of "address".

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

Per EIP-712, the typehash must be the keccak256 of the canonical type string "SnowmanClaim(address receiver,uint256 amount)". Because the contract uses the misspelled string, its typehash does not match what a standard, spec-compliant EIP-712 implementation computes.

The contract currently "works" only when the signer reuses the buggy constant through getMessageHash(). Any external integrator (dApp, wallet, standard EIP-712 library) that constructs the digest correctly per the specification will produce a signature that the contract rejects with SA__InvalidSignature.

Risk

Impact: Medium — legitimate claims can never be processed for spec-compliant signers; the airdrop becomes unusable with standard tooling. No funds are directly stolen, but users are locked out of their entitlements.

Likelihood: High — the vulnerability triggers whenever a signer follows the EIP-712 standard, which is the intended usage for a typed-signature scheme.

Proof of Concept

The following Foundry test builds the digest with the correct "address" typehash, signs it with Alice's key, and shows claimSnowman reverts with SA__InvalidSignature:

function testPoc2_StandardEip712SignatureRejected() public {
uint256 amount = snow.balanceOf(alice);
vm.prank(alice);
snow.approve(address(airdrop), amount);
​
bytes32 correctTypehash = keccak256("SnowmanClaim(address receiver, uint256 amount)");
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 structHash = keccak256(abi.encode(correctTypehash, keccak256(abi.encode(alice, amount))));
bytes32 standardDigest = keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
​
assertTrue(standardDigest != airdrop.getMessageHash(alice));
​
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, standardDigest);
vm.expectRevert(abi.encodeWithSelector(SnowmanAirdrop.SA__InvalidSignature.selector));
vm.prank(bob);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
}

Test result: [PASS] testPoc2_StandardEip712SignatureRejected — the spec-compliant signature is rejected.

Recommended Mitigation

Fix the type string to match the EIP-712 standard:

- 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 1 day 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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