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

`MESSAGE_TYPEHASH` is not a valid EIP-712 `encodeType` string ("addres" typo plus spaces after commas), so no signature produced by a spec-compliant wallet can ever be accepted and the claim-on-behalf flow is unusable off-chain

friedsocks233

Description

The contract inherits OpenZeppelin's EIP712 and hashes the SnowmanClaim struct so that a recipient can sign a claim with a standard wallet and a third party can submit it.

The type string used to build the typehash is malformed in two independent ways: the type is spelled addres rather than address, and EIP-712 forbids whitespace in encodeType. The canonical string for this struct is SnowmanClaim(address receiver,uint256 amount). A compliant signer derives the typehash from the struct definition and therefore produces a different digest from the one the contract checks.

// src/SnowmanAirdrop.sol
struct SnowmanClaim {
address receiver;
uint256 amount;
}
...
@> bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
// canonical: keccak256("SnowmanClaim(address receiver,uint256 amount)")

Risk

Likelihood:

  • On every attempt to sign a claim through eth_signTypedData_v4, which is the standard path in every mainstream wallet and in ethers/viem. The derived typehash never matches, so _isValidSignature recovers a different address and the claim reverts with SA__InvalidSignature.

Impact:

  • The advertised "a third party can claim on behalf of a recipient" flow cannot be driven by standard tooling. A recipient must instead blind-sign the raw 32-byte digest returned by getMessageHash, which wallets present as an unreadable hex blob with a security warning — defeating the entire purpose of using EIP-712.

  • No loss of funds and no on-chain inconsistency: the contract hashes consistently with itself, so claims still succeed if the signer signs the raw digest. This is a functionality and standards-compliance defect, which is why it is rated Low.

Proof of Concept

Paste the harness and test function(s) into test/PoC.t.sol, placing test functions inside contract PoC. Any additional helper contracts go outside contract PoC. Run forge test --match-path test/PoC.t.sol -vv.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
import {Test, console2} from "forge-std/Test.sol";
import {Snow} from "../src/Snow.sol";
import {Snowman} from "../src/Snowman.sol";
import {SnowmanAirdrop} from "../src/SnowmanAirdrop.sol";
import {MockWETH} from "../src/mock/MockWETH.sol";
contract PoC is Test {
Snow snow;
Snowman nft;
MockWETH weth;
address collector = makeAddr("collector");
address attacker = makeAddr("attacker");
address victim = makeAddr("victim");
function setUp() public {
weth = new MockWETH();
snow = new Snow(address(weth), 1, collector);
nft = new Snowman("ipfs://snowman");
}
function _leaf(address who, uint256 amount) internal pure returns (bytes32) {
return keccak256(bytes.concat(keccak256(abi.encode(who, amount))));
}
// Insert the test function below here.
}

A signature over the correctly-encoded EIP-712 typed data is rejected.

function test_SpecCompliantEip712SignatureIsRejected() public {
(address receiver, uint256 key) = makeAddrAndKey("receiver2");
uint256 amount = 1;
vm.deal(receiver, 10 ether);
vm.prank(receiver);
snow.buySnow{value: 1 ether}(amount);
SnowmanAirdrop airdrop = new SnowmanAirdrop(_leaf(receiver, amount), address(snow), address(nft));
bytes32[] memory emptyProof = new bytes32[](0);
vm.prank(receiver);
snow.approve(address(airdrop), type(uint256).max);
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 correctTypehash = keccak256("SnowmanClaim(address receiver,uint256 amount)");
bytes32 structHash = keccak256(abi.encode(correctTypehash, receiver, amount));
bytes32 correctDigest = keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
(uint8 v, bytes32 r, bytes32 s) = vm.sign(key, correctDigest);
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(receiver, emptyProof, v, r, s);
assertTrue(
correctTypehash != keccak256("SnowmanClaim(addres receiver, uint256 amount)"),
"contract's typehash differs from the EIP-712 spec encoding"
);
}

Recommended Mitigation

- 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 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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