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

EIP-712 MESSAGE_TYPEHASH contains typo ("addres") and non-canonical spacing, breaking standard wallet signing

p7inc3

Description

Normal behavior: Claims are authorized by an EIP-712 typed-data signature. Standard tooling (MetaMask signTypedData_v4, ethers/viem TypedDataEncoder, hardware wallets) derives the digest from the canonical struct type SnowmanClaim(address receiver,uint256 amount) — exact spelling, no spaces after commas.

Specific issue: The contract's type string is keccak256("SnowmanClaim(addres receiver, uint256 amount)") — it misspells address as addres and adds a space after the comma (non-canonical EIP-712 encoding). Any digest produced by a standards-compliant signer therefore differs from the contract's digest and is rejected with SA__InvalidSignature. The system only works when callers bypass typed-data tooling and manually hash via the contract's getMessageHash() view — so wallet UIs cannot display or sign the claim message, and third-party claim services/SDK integrations fail deterministically.

Root cause:

@> bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)"); // @> "addres" + space @>
// correct EIP-712 encoding: keccak256("SnowmanClaim(address receiver,uint256 amount)")
​
// used for both signing and verification — internally consistent, spec-incompatible:
// line 120: keccak256(abi.encode(MESSAGE_TYPEHASH, SnowmanClaim({receiver, amount})))
// line 80: _isValidSignature(receiver, getMessageHash(receiver), v, r, s)

Because getMessageHash() and _isValidSignature() share the constant, on-chain checks pass with the typo'd hash — the divergence only shows against the standard, which is exactly what every external signer produces.

Risk

Likelihood:

  • The mismatch is deterministic and permanent: the moment anyone signs with standard EIP-712 tooling (any wallet, any SDK, the schema implied by the contract's own SnowmanClaim(address receiver, uint256 amount) struct declaration), the digest differs and the claim reverts SA__InvalidSignature — the PoC reproduces this with zero environmental assumptions.

  • The contract's public struct field names advertise the correct schema, so integrators will naturally construct the canonical type and hit the failure on first use.

Impact:

  • EIP-712 compatibility is broken: wallets cannot sign (or properly display) the claim message; recipients must rely solely on the custom getMessageHash() relay flow, defeating the purpose of typed-data signing (phishing-resistant display of what is being signed).

  • Off-chain signed messages produced by any standard integration are permanently invalid — no on-chain fund loss, but the signature feature is non-functional for its standard use case.

Proof of Concept

The PoC computes the digest two ways against the deployed contract and proves which one is accepted:

  1. Standard path — build the canonical EIP-712 digest exactly as signTypedData would: type SnowmanClaim(address receiver,uint256 amount), domain (name "Snowman Airdrop", version "1", chainid, contract). Sign it → claimSnowman reverts SA__InvalidSignature.

  2. Root-cause pinpoint — build a second digest using the typo'd type string → it equals getMessageHash(alice) byte-for-byte, proving the contract uses the misspelled type.

// forge test --match-test test_F5_typehashTypo_breaksStandardSigning -vv
function test_F5_typehashTypo_breaksStandardSigning() public {
MockWETH weth = new MockWETH();
Snow snow = new Snow(address(weth), 5, makeAddr("collector"));
Snowman nft = new Snowman("data:image/svg+xml;base64,PHN2Zz48L3N2Zz4=");
SnowmanAirdrop airdrop = new SnowmanAirdrop(bytes32(uint256(1)), address(snow), address(nft));
​
(address alice, uint256 alKey) = makeAddrAndKey("alice");
vm.prank(alice);
snow.earnSnow(); // balance > 0 to reach the signature check
​
// 1) Digest exactly as a standard wallet (signTypedData) computes it
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
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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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