Snowman Merkle Airdrop
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Submission Details
Severity: high
Valid

[H-03] EIP-712 typehash deviates from canonical format ("addres" + space) — breaks all standard-wallet signatures

cthan

[H-03] EIP-712 typehash deviates from canonical format ("addres" + space) — breaks all standard-wallet signatures

Description

SnowmanAirdrop.MESSAGE_TYPEHASH is computed from a typestring that diverges from the EIP-712 canonical format in two ways:

// src/SnowmanAirdrop.sol:49
bytes32 private constant MESSAGE_TYPEHASH = keccak256(
"SnowmanClaim(addres receiver, uint256 amount)"
);

EIP-712 specifies the canonical form "TypeName(type1 name1,type2 name2)" — exact field types, no whitespace except the single space between type and name. The contract violates this in two places:

  1. Spelling: addres instead of address (missing 's')

  2. Spacing: , uint256 (extra space after comma) instead of ,uint256

Either deviation alone produces a different keccak256 digest than the canonical typehash. Both together definitely do.

When a user signs a Snowman claim through MetaMask, ethers.js, viem, Rabby, or a hardware wallet (Ledger/Trezor), the wallet computes the digest using the canonical typestring "SnowmanClaim(address receiver,uint256 amount)". That digest does NOT match the contract's broken MESSAGE_TYPEHASH. The on-chain ECDSA.tryRecover (line 107) returns an address different from receiver, the equality check at line 108 fails, and claimSnowman reverts with SA__InvalidSignature.

The only way to produce a signature the contract will accept is to manually construct the typed-data digest in custom code that mirrors the typo and the extra space — which no standard wallet does.

Risk

  • Likelihood: High — the bug fires deterministically on every claim attempt that uses standard EIP-712 tooling, which is the documented "claim on behalf" UX.

  • Impact: High — the protocol's headline "claim on behalf" feature (per README: "Recipients can either claim a Snowman NFT themselves, or have someone claim on their behalf using the recipient's v, r, s signatures") is broken for every user with a standard wallet.

  • Risk = Likelihood × Impact = High

Impact

Every recipient of the airdrop who attempts to delegate their claim through any standard wallet — MetaMask, Rabby, ethers.js, viem, hardware wallet — produces signatures that revert on-chain. The "claim on behalf" feature is functionally dead for all production-tooling users. The only working path requires hand-rolling signing code that mirrors the contract's broken typestring, which excludes virtually all UX integrations (gasless claims, account abstraction, third-party submitters, dApp frontends).

Proof of Concept

Full PoC at .audit/poc/PoC_H-03.t.sol. The key test function (the second test in the file is a control proving the bug is isolated to the typehash):

function test_CanonicalSignature_IsRejected() public {
// ARRANGE: alice has Snow (minted via Helper); approval set in setUp.
uint256 amount = snow.balanceOf(alice);
assertGt(amount, 0, "alice must have Snow for the signature check to be reached");
​
// Compute the digest the way a standard wallet would — using the CANONICAL typehash
// ("SnowmanClaim(address receiver,uint256 amount)" — no typo, no space).
bytes32 canonicalDigest = _buildDigest(CANONICAL_MESSAGE_TYPEHASH, alice, amount);
​
// Alice signs with her private key (standard wallet behaviour).
(uint8 v, bytes32 r, bytes32 s) = vm.sign(aliceKey, canonicalDigest);
​
// ACT + ASSERT: claim must revert because the contract computes a DIFFERENT digest
// (using the broken typehash) and recovery yields a wrong address.
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(alice, aliceProof, v, r, s);
​
assertEq(nft.balanceOf(alice), 0, "alice should not have received any NFT");
}

Test passes inside hardened audit container (forge exit 0). The canonical EIP-712 signature — exactly what every standard wallet produces — is rejected. The companion control test in the file proves a signature using the broken typestring IS accepted, isolating the bug to the typehash constant.

Recommended Mitigation

Fix the typestring to canonical EIP-712 form — both the typo and the extra space:

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

After the fix, all EIP-712-compliant wallets produce signatures the contract accepts. Add a unit test that signs a claim using vm.sign against an independently-computed canonical typehash and asserts on-chain recovery matches receiver — this catches future regressions if the typestring is edited.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 6 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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