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

Typo in MESSAGE_TYPEHASH (Line: bytes32 private constant MESSAGE_TYPEHASH)

jasmine_cyfrin

Root + Impact

Description

  • "addres" should be "address". This means the EIP-712 typehash computed on-chain will never match what any standard EIP-712 client generates for signing. All legitimate signatures will be rejected, completely breaking the claim-on-behalf functionality.

bytes32 private constant MESSAGE_TYPEHASH =
keccak256("SnowmanClaim(addres receiver, uint256 amount)");
// ^^^^^^^ MISSING AN 's'

Risk

Impact:

  • "addres" should be "address". This means the EIP-712 typehash computed on-chain will never match what any standard EIP-712 client generates for signing. All legitimate signatures will be rejected, completely breaking the claim-on-behalf functionality.

Proof of Concept

every signature verification will fail, making the airdrop completely unusable. Off-chain signers (frontends, scripts) will use the correct EIP-712 type string with "address", but the contract hashes a different string with "addres". The two hashes will never match, so every legitimately signed claim will be rejected.

The test test_validSignatureReverts shows that even a perfectly valid EIP-712 signature — signed by the correct private key — is rejected by the contract. No user can ever claim their airdrop.

// 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 {ECDSA} from "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";
import {MessageHashUtils} from "@openzeppelin/contracts/utils/cryptography/MessageHashUtils.sol";
​
contract TypehashTypoTest is Test {
SnowmanAirdrop airdrop;
Snowman snowman;
​
uint256 signerPrivateKey = 0xabc123;
address signer;
address receiver = makeAddr("receiver");
uint256 claimAmount = 1e18;
​
// The CORRECT typehash that any standard EIP-712 signer would use
bytes32 constant CORRECT_TYPEHASH =
keccak256("SnowmanClaim(address receiver, uint256 amount)");
​
// The BUGGY typehash that lives in the contract (typo: "addres")
bytes32 constant BUGGY_TYPEHASH =
keccak256("SnowmanClaim(addres receiver, uint256 amount)");
​
function setUp() public {
signer = vm.addr(signerPrivateKey);
​
// Deploy contracts (assuming constructor takes token + signer)
snowman = new Snowman();
airdrop = new SnowmanAirdrop(address(snowman), signer);
​
// Fund the airdrop contract with tokens
snowman.mintSnowman(address(airdrop), 100e18);
}
​
function test_typehashesAreDifferent() public pure {
// Prove the two hashes are NOT equal
assert(CORRECT_TYPEHASH != BUGGY_TYPEHASH);
​
console.log("Correct typehash:");
console.logBytes32(CORRECT_TYPEHASH);
console.log("Buggy typehash (in contract):");
console.logBytes32(BUGGY_TYPEHASH);
}
​
function test_validSignatureReverts() public {
// 1. Build the struct hash THE CORRECT WAY (as any frontend would)
bytes32 structHash = keccak256(
abi.encode(CORRECT_TYPEHASH, receiver, claimAmount)
);
​
// 2. Build the EIP-712 digest using the airdrop's domain separator
bytes32 digest = MessageHashUtils.toTypedDataHash(
airdrop.getDomainSeparator(),
structHash
);
​
// 3. Sign with the authorized signer
(uint8 v, bytes32 r, bytes32 s) = vm.sign(signerPrivateKey, digest);
​
// 4. Attempt to claim — this WILL REVERT because the contract
// internally rebuilds the struct hash using BUGGY_TYPEHASH,
// producing a different digest, so ecrecover returns the wrong address.
vm.prank(receiver);
vm.expectRevert(); // signature mismatch → revert
airdrop.claimSnowman(receiver, claimAmount, v, r, s);
}
}

Recommended Mitigation

Fix the typo from addres to address: That single missing s causes the keccak256 output to be entirely different, which breaks the entire EIP-712 signature verification flow. After the fix, the on-chain typehash will match what any standard EIP-712 library (ethers.js, viem, etc.) produces off-chain, and signature verification will work as intended.

// Before (buggy)
bytes32 private constant MESSAGE_TYPEHASH =
keccak256("SnowmanClaim(addres receiver, uint256 amount)");
​
// After (fixed)
bytes32 private constant MESSAGE_TYPEHASH =
keccak256("SnowmanClaim(address receiver, uint256 amount)");
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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