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

Incorrect EIP-712 MESSAGE_TYPEHASH causes all wallet-signed claims to revert

blockend

Root + Impact

Description

SnowmanAirdrop uses EIP-712 signing so claimants sign a SnowmanClaim(receiver, amount) message off-chain, verified on-chain in claimSnowman. For wallets to produce valid signatures, MESSAGE_TYPEHASH must exactly match the spec-derived type string: "SnowmanClaim(address receiver,uint256 amount)".

The constant is malformed — addres typo and an extra space after the comma — so it hashes to a different typehash than any spec-compliant wallet or library generates.

bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
//> @> "addres" should be "address"; no space after the comma

Risk

Likelihood:

  • Every claimant signing via a standard EIP-712 wallet (MetaMask, Rabby, WalletConnect) or library (ethers.js, viem) produces a digest from the correct type string, which never matches MESSAGE_TYPEHASH — ecrecover returns a signer that doesn't equal receiver on every claim attempt.

  • This is unconditional on the intended code path, affecting all users who don't separately call getMessageHash() and blind-sign the raw digest outside their wallet's UI.

Impact:

  • claimSnowman always reverts with SA__InvalidSignature for spec-compliant signatures — full DoS on the contract's core function.

  • The only accepted signature requires bypassing wallet UX to blind-sign a raw digest, defeating EIP-712's anti-phishing purpose.

Proof of Concept

testEIP712TypehashMismatch_RejectsSpecCompliantSignature rebuilds the digest with the spec-correct type string and shows the contract rejects it. testContractOnlyAcceptsMalformedTypehash shows the contract only accepts its own broken getMessageHash() output. Same signer and data in both — only the typehash differs, flipping revert to success.

function testEIP712TypehashMismatch_RejectsSpecCompliantSignature() public {
vm.prank(alice);
snow.approve(address(airdrop), 1);
uint256 amount = snow.balanceOf(alice);
​
bytes32 CORRECT_TYPEHASH = keccak256("SnowmanClaim(address receiver,uint256 amount)");
bytes32 correctStructHash = keccak256(abi.encode(CORRECT_TYPEHASH, alice, 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 correctDigest = keccak256(abi.encodePacked("\x19\x01", domainSeparator, correctStructHash));
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, correctDigest);
​
vm.prank(satoshi);
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s);
}
​
function testContractOnlyAcceptsMalformedTypehash() public {
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
bytes32 brokenDigest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, brokenDigest);
​
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s); // succeeds
assert(nft.balanceOf(alice) == 1);
}

Recommended Mitigation

Fix the typehash to match the exact ABI type name and comma spacing every wallet generates.

- 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 about 7 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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