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

EIP-712 `MESSAGE_TYPEHASH` contains the typo `addres` — standards-compliant wallet signatures never verify

0xcr0w

Root + Impact

Description

  • EIP-712 requires the on-chain typeHash to equal keccak256 of the canonical struct type string the wallet hashes off-chain. For struct SnowmanClaim { address receiver; uint256 amount; } the canonical string is exactly "SnowmanClaim(address receiver,uint256 amount)".

  • The on-chain constant misspells address as addres and adds an EIP-712-illegal space after the comma, so MetaMask/viem/ethers produce digests against a different typehash. The on-chain recovery never matches the wallet-signed digest.

// src/SnowmanAirdrop.sol
@> bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
// ^^^^^^ typo
// ^ illegal whitespace

Risk

Likelihood:

  • Reason 1: Every standards-compliant off-chain signer (MetaMask eth_signTypedData_v4, viem, ethers, ledger) hashes the struct definition SnowmanClaim(address receiver,uint256 amount) — the contract hashes a different string.

  • Reason 2: Wallets reject signing if you ask them to sign the typo'd type, because addres is not a valid Solidity type — so the protocol cannot even instruct users to "sign this broken type."

Impact:

  • Impact 1: 100% of standard claim attempts revert with SA__InvalidSignature — full DoS of the airdrop.

  • Impact 2: If the team works around it with a custom raw-keccak signer, users must blind-sign opaque digests outside the wallet's typed-data UX — phishing surface.

Proof of Concept

The PoC builds two parallel digests: one with the canonical EIP-712 typehash (which every conformant wallet produces) and one with the on-chain typo'd typehash. We sign Alice's claim with the canonical typehash — i.e., we simulate exactly what eth_signTypedData_v4 would emit. When that signature is submitted to claimSnowman, the contract recomputes the digest using its broken typehash, recovers a different signer, and reverts. The expected revert proves the two paths can never agree, so no real wallet user can ever pass the signature gate.

function test_standardEIP712SignatureFails() public {
bytes32 CORRECT = keccak256("SnowmanClaim(address receiver,uint256 amount)");
bytes32 ONCHAIN = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
assertTrue(CORRECT != ONCHAIN);
​
bytes32 structHash = keccak256(abi.encode(CORRECT, alice, snow.balanceOf(alice)));
bytes32 digest = _toTypedDataHash(domainSeparator, structHash);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alicePk, digest);
​
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(alice, proof, v, r, s);
}

Recommended Mitigation

The fix is purely the string contents — restoring the canonical EIP-712 type encoding: the type name address spelled correctly and no whitespace inside the parameter list (per EIP-712 §"Definition of encodeType"). This change must ship before any signatures are distributed to users, because old signatures and new signatures hash to different digests and existing test-suite signatures (if any) will all need regeneration.

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

Follow-up: regenerate every test signature, document that claimants should use standard eth_signTypedData_v4, and add a CI check that the type string round-trips through keccak256 to a known-correct constant.

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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