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

Malformed `MESSAGE_TYPEHASH` makes every standard EIP-712 wallet signature invalid, so no user can ever claim a Snowman

zodmastro

Root + Impact

Description

  • SnowmanAirdrop::claimSnowman requires an EIP-712 signature from the receiver over the SnowmanClaim struct, and no code path skips that check, not even when the receiver claims for themselves.

  • The type string in MESSAGE_TYPEHASH is not the canonical EIP-712 encoding: it misspells address as addres and adds a space after the comma. Wallets derive the type string themselves from the typed-data JSON, always joining members with , and no space, and addres is not a valid Solidity type, so no wallet can produce a signature matching this typehash.

bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
//@> canonical form must be "SnowmanClaim(address receiver,uint256 amount)"

Risk

Likelihood:

  • Every claim runs through _isValidSignature, so this is hit on 100% of claim attempts, by every user, from deployment onward.

  • Any signer using eth_signTypedData_v4 (MetaMask, ethers, viem, Ledger, WalletConnect) produces a digest built from the canonical type string, which can never equal this contract's digest.

Impact:

  • The protocol's core function is unusable: claimSnowman reverts with SA__InvalidSignature for every signature produced by a standard wallet, so no Snowman NFT can be claimed through the intended flow and the airdrop is permanently bricked.

  • The only workaround is blind-signing the raw 32-byte digest from getMessageHash with eth_sign, which MetaMask disables by default and other wallets block or warn against, and which destroys the readable-signing guarantee EIP-712 exists to provide. personal_sign does not work either, since it prepends the \x19Ethereum Signed Message:\n32 prefix.

Proof of Concept

Save as test/Sub2.t.sol and run forge test --match-path test/Sub2.t.sol -vv. The test builds the digest exactly as a compliant wallet would, signs it with the recipient's own key, and shows the contract rejects it.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
​
import {Test} 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";
import {Helper} from "../script/Helper.s.sol";
​
contract Sub2BrokenTypehash is Test {
Snow snow;
Snowman nft;
SnowmanAirdrop airdrop;
MockWETH weth;
​
address alice;
uint256 alKey;
​
bytes32[] AL_PROOF = [
bytes32(0xf99782cec890699d4947528f9884acaca174602bb028a66d0870534acf241c52),
bytes32(0xbc5a8a0aad4a65155abf53bb707aa6d66b11b220ecb672f7832c05613dba82af),
bytes32(0x971653456742d62534a5d7594745c292dda6a75c69c43a6a6249523f26e0cac1)
];
​
function setUp() public {
Helper helper = new Helper();
(airdrop, snow, nft, weth) = helper.run();
(alice, alKey) = makeAddrAndKey("alice");
}
​
function test_CompliantWalletSignatureIsRejected() public {
// Build the digest exactly as eth_signTypedData_v4 would for
// SnowmanClaim{address receiver; uint256 amount}
bytes32 canonicalTypehash = keccak256("SnowmanClaim(address receiver,uint256 amount)");
bytes32 domainSeparator = keccak256(
abi.encode(
keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
keccak256("Snowman Airdrop"),
keccak256("1"),
block.chainid,
address(airdrop)
)
);
bytes32 structHash = keccak256(abi.encode(canonicalTypehash, alice, snow.balanceOf(alice)));
bytes32 walletDigest = keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
​
// The contract's digest can never equal a compliant wallet's digest.
assertTrue(walletDigest != airdrop.getMessageHash(alice));
​
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
(uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, walletDigest);
​
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(alice, AL_PROOF, v, r, s); // claiming is impossible
}
}

Output:

Ran 1 test for test/Sub2.t.sol:Sub2BrokenTypehash
[PASS] test_CompliantWalletSignatureIsRejected() (gas: 98686)
Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 8.75ms (653.19µs CPU time)

Recommended Mitigation

Use the canonical EIP-712 type string.

- 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 3 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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