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

Typo in EIP-712 `MESSAGE_TYPEHASH` Permanently Breaks All Delegation Claims

bugnetter

Description

  • EIP-712 requires the struct type string to exactly match the Solidity types used. Wallets and signing libraries (ethers.js, viem, web3.py) all use canonical type names when computing the EIP-712 digest. The contract's getMessageHash() encodes the struct hash using MESSAGE_TYPEHASH, and _isValidSignature() calls ECDSA.recover on this digest.

  • The MESSAGE_TYPEHASH constant on line 49 of SnowmanAirdrop.sol spells "addres" with one 's' instead of "address". Because keccak256 is collision-resistant, the contract computes a different EIP-712 digest than any standard wallet. Every delegation-path claimSnowman call reverts with SA__InvalidSignature.

// SnowmanAirdrop.sol — line 49
bytes32 private constant MESSAGE_TYPEHASH =
keccak256("SnowmanClaim(addres receiver, uint256 amount)");
// @> "addres" — missing second 's'
// Correct: "SnowmanClaim(address receiver, uint256 amount)"

Risk

Likelihood:

  • The bug is baked into a compile-time constant — it is present from deployment with no preconditions

  • No wallet, SDK, or signing library will produce the typo variant, since they all resolve Solidity type names by their canonical spelling

Impact:

  • 100% of delegation-path claims permanently fail — the delegation feature of SnowmanAirdrop is non-functional at deployment

  • Users who rely on a third party to submit their claim on their behalf cannot receive their Snowman NFTs through this path

Proof of Concept

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
​
import {Test, console} from "forge-std/Test.sol";
import {SnowmanAirdrop} from "../src/SnowmanAirdrop.sol";
import {Snowman} from "../src/Snowman.sol";
import {Snow} from "../src/Snow.sol";
​
contract MockWETH {
mapping(address => uint256) public balanceOf;
function transferFrom(address, address, uint256) external returns (bool) { return true; }
function transfer(address, uint256) external returns (bool) { return true; }
}
​
contract PocCR001_FINAL is Test {
SnowmanAirdrop airdrop;
Snowman snowman;
Snow snow;
​
function setUp() public {
MockWETH weth = new MockWETH();
snow = new Snow(address(weth), 1, address(this));
snowman = new Snowman("svg");
airdrop = new SnowmanAirdrop(bytes32(0), address(snow), address(snowman));
}
​
function test_Baseline_FINAL() public view {
bytes32 typoHash = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
bytes32 correctHash = keccak256("SnowmanClaim(address receiver, uint256 amount)");
assertTrue(typoHash != correctHash, "Hashes must differ");
console.log("Typo hash:");
console.logBytes32(typoHash);
console.log("Correct hash:");
console.logBytes32(correctHash);
}
​
function test_VulnerabilityDemo_FINAL() public {
address user;
uint256 userPk;
(user, userPk) = makeAddrAndKey("claimUser");
​
vm.prank(user);
snow.earnSnow();
uint256 amount = snow.balanceOf(user);
​
bytes32 correctTypehash = keccak256("SnowmanClaim(address receiver, uint256 amount)");
bytes32 structHash = keccak256(abi.encode(correctTypehash, user, 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 digest = keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
(uint8 v, bytes32 r, bytes32 s) = vm.sign(userPk, digest);
​
bytes32 contractDigest = airdrop.getMessageHash(user);
assertTrue(digest != contractDigest, "Digests differ due to typo");
​
bytes32[] memory proof = new bytes32[](0);
vm.prank(user);
snow.approve(address(airdrop), amount);
​
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(user, proof, v, r, s);
}
}

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 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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