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

Low/Informational: dead code, single-step collector change, and claim-flow edge cases (typehash typo, balance-bound signatures, unused storage)

foxcryptie

Root + Impact

Description

  • This is a bundle of low-severity and informational issues that don't individually rise to a standalone High/Medium submission, but are worth fixing for correctness and defense-in-depth:

    L-1: MESSAGE_TYPEHASH typo breaks canonical EIP-712 tooling
    SnowmanAirdrop.sol hardcodes "SnowmanClaim(addres receiver, uint256 amount)" — missing the s in address. The contract is internally self-consistent (getMessageHash derives the digest from this same typo'd string, so on-chain verification still works), but any external wallet or SDK that builds typed data from the canonical type string will produce a signature that reverts with SA__InvalidSignature.

solidity
@> bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
​
L-2: Dead custom errors / unreachable guard
Snowman.sol's tokenURI checks ownerOf(tokenId) == address(0) to detect a non-existent token, but OpenZeppelin's ownerOf already reverts with its own error before ever returning address(0) — so ERC721Metadata__URI_QueryFor_NonExistentToken can never be thrown. SM__NotAllowed (declared in the same contract) is also never used anywhere.
​
solidity
function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
@> if (ownerOf(tokenId) == address(0)) { // unreachable: OZ's ownerOf reverts first
revert ERC721Metadata__URI_QueryFor_NonExistentToken();
}
​
L-3: s_claimers is declared and never used
SnowmanAirdrop.sol declares address[] private s_claimers; with a comment describing it as tracking claimer addresses, but no function ever pushes to it or reads it.
​
L-4: Single-step changeCollector
Snow.sol::changeCollector reassigns s_collector immediately with no two-step (propose/accept) confirmation, so a typo'd address or a compromised collector key permanently and irreversibly redirects all future fees (and any ETH swept via collectFee) with no recovery path.
​
L-5: Claim signatures/leaves are balance-bound, causing two related self-inflicted frictions
claimSnowman and getMessageHash both recompute amount from the receiver's live balanceOf rather than a fixed snapshot. Two consequences: (a) a whitelisted address whose balance drifts above its original leaf amount (e.g. someone sends them 1 extra Snow) cannot claim at all until the balance is corrected back; (b) a signature signed for one balance becomes invalid if the balance changes before the claim executes — which also breaks the legitimate third-party delegated-claim flow described in the brief if the receiver's balance moves in between.

Risk

Likelihood:

  • L-1 triggers only when an integrator uses a standards-compliant EIP-712 typed-data builder instead of this contract's own getMessageHash.

  • L-2/L-3 have no runtime trigger — they're static dead code, found by inspection.

  • L-4 requires an admin/collector-key mistake or compromise to matter.

  • L-5 triggers whenever a receiver's Snow balance changes between signing and claim execution, or drifts away from their original leaf amount.

Impact:

  • All items in this bundle are Low/Informational: none allow direct fund loss or unauthorized access on their own. L-1 and L-5 are integration/UX friction; L-2/L-3 are pure code-quality/dead-code; L-4 is a standard single-key operational risk.

Proof of Concept

Only L-1 has a deterministic on-chain-verifiable PoC (a hash mismatch); the rest are static/source-level observations confirmed by direct code inspection (no dynamic PoC applies to dead code or design-only notes).

solidity
function test_typehash_typo() public view {
bytes32 contractTypehash = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
bytes32 canonical = keccak256("SnowmanClaim(address receiver, uint256 amount)");
assertTrue(contractTypehash != canonical, "typo confirmed: canonical EIP-712 tooling would mismatch");
solidity
function test_typehash_typo() public view {
bytes32 contractTypehash = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
bytes32 canonical = keccak256("SnowmanClaim(address receiver, uint256 amount)");
assertTrue(contractTypehash != canonical, "typo confirmed: canonical EIP-712 tooling would mismatch");
}
Run: forge test --match-test test_typehash_typo -vv → PASS, confirming the two type-hashes differ.
​

Recommended Mitigation

- bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
+ bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(address receiver, uint256 amount)");
Fixing the typo means every existing signature scheme must be regenerated consistently — do this once, not as a live migration.
​
- function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
- if (ownerOf(tokenId) == address(0)) {
- revert ERC721Metadata__URI_QueryFor_NonExistentToken();
- }
+ function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
+ ownerOf(tokenId); // reverts on its own via OZ's ERC721NonexistentToken if missing
Remove the unreachable check and the now-fully-unused ERC721Metadata__URI_QueryFor_NonExistentToken/SM__NotAllowed errors, and either wire up s_claimers (e.g. push on a successful claim) or remove it entirely.
​
- function changeCollector(address _newCollector) external onlyCollector {
- if (_newCollector == address(0)) revert S__ZeroAddress();
- s_collector = _newCollector;
- emit NewCollector(_newCollector);
- }
+ address private s_pendingCollector;
+ function proposeCollector(address _newCollector) external onlyCollector {
+ s_pendingCollector = _newCollector;
+ }
+ function acceptCollector() external {
+ if (msg.sender != s_pendingCollector) revert S__NotAllowed();
+ s_collector = s_pendingCollector;
+ emit NewCollector(s_collector);
+ }
For L-5, bind the signed/leaf amount to the Merkle-tree snapshot value rather than live balanceOf — this is the same root fix already recommended for the double-claim finding, and resolves both frictions together.
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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