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

Malformed EIP-712 MESSAGE_TYPEHASH in SnowmanAirdrop breaks signature verification for standard signers

duranovskyy

Root + Impact

SnowmanAirdrop's MESSAGE_TYPEHASH is not a valid EIP-712 type string (typo addres and a stray space), so the digest the contract verifies against differs from the one a standard EIP-712 wallet signs. Signatures from the normal signing flow are rejected and legitimate claims revert.

Description

  • Claims are authorized by an EIP-712 signature over SnowmanClaim(address receiver,uint256 amount).

  • The typehash string is malformed, so the contract's digest is not the one a standard signer (e.g. eth_signTypedData_v4) produces. Such a signature fails _isValidSignature and the claim reverts with SA__InvalidSignature.

@> bytes32 private constant MESSAGE_TYPEHASH =
@> keccak256("SnowmanClaim(addres receiver, uint256 amount)");
// "addres" typo + space after the comma;
// canonical EIP-712 form is "SnowmanClaim(address receiver,uint256 amount)"

Risk

Likelihood:

  • Any claimer signing through a standard EIP-712 wallet signs over the canonical type string, producing a digest that never matches the contract's malformed one, so verification fails for them every time.

Impact:

  • Legitimate claimers cannot pass signature verification; claimSnowman reverts and the airdrop's core function is unusable through the intended signing flow.

Proof of Concept

A signature over the canonical EIP-712 digest (what a standard wallet produces) is rejected by the contract. Full test: test/snow/AirdropTypehash.t.sol — result [PASS].

function test_standardSignatureIsRejected() public {
snow.mint(claimer, AMOUNT);
vm.prank(claimer);
snow.approve(address(airdrop), type(uint256).max);
​
// digest a standard EIP-712 wallet would sign (canonical type string)
bytes32 domainSep = 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(
keccak256("SnowmanClaim(address receiver,uint256 amount)"),
claimer,
AMOUNT
));
bytes32 digest = keccak256(abi.encodePacked("\x19\x01", domainSep, structHash));
(uint8 v, bytes32 r, bytes32 s) = vm.sign(CLAIMER_PK, digest);
​
bytes32[] memory proof = new bytes32[](0);
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(claimer, proof, v, r, s);
}

Recommended Mitigation

Replace the malformed type string with the canonical EIP-712 form. With the correct string, the digest the contract builds in getMessageHash matches the digest a standard wallet produces via eth_signTypedData_v4, so legitimate signatures verify and claims succeed.

- 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 1 hour 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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