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

EIP-712 `MESSAGE_TYPEHASH` typo breaks signature claims for standard signers

rayair

Root + Impact

Description

  • claimSnowman lets a third party claim on a recipient's behalf using an EIP-712 signature, which means any standard signer (wallet, ethers, viem) must be able to produce a valid signature.

  • The MESSAGE_TYPEHASH string has a typo (addres instead of address, plus a non-canonical space), so the contract's digest differs from the one any compliant signer computes.

```solidity
@> bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
// ^ typo "addres" + non-canonical spacing
```

Risk

Likelihood:

  • Occurs for every signature produced by any EIP-712-compliant tool, since they all use the correct type string address receiver,uint256 amount.

  • Only a custom signer that reproduces the exact buggy string can pass, which no standard wallet does.

Impact:

  • The "claim on behalf via v, r, s" feature is unusable with standard tooling — valid signatures are rejected.

  • Core advertised functionality is broken for all conforming integrations.

Proof of Concept

Self-contained Foundry test. Run: `forge test --match-test test_M2_StandardEip712SignatureRejected -vvv`
​
```solidity
// 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";
​
contract M2 is Test {
Snow snow; Snowman nft; SnowmanAirdrop airdrop; MockWETH weth;
address alice; uint256 aliceKey;
​
function setUp() public {
(alice, aliceKey) = makeAddrAndKey("alice");
weth = new MockWETH();
snow = new Snow(address(weth), 1, makeAddr("collector"));
nft = new Snowman("ipfs://svg");
airdrop = new SnowmanAirdrop(bytes32(uint256(1)), address(snow), address(nft));
// give alice a non-zero balance so getMessageHash does not revert
vm.deal(alice, 1e18);
vm.prank(alice);
snow.buySnow{value: 1e18}(1);
}
​
function test_M2_StandardEip712SignatureRejected() public {
// Build the CORRECT EIP-712 digest, exactly as ethers/viem/a wallet would.
bytes32 DOMAIN_TYPEHASH =
keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)");
bytes32 domainSep = keccak256(abi.encode(
DOMAIN_TYPEHASH, keccak256(bytes("Snowman Airdrop")), keccak256(bytes("1")),
block.chainid, address(airdrop)
));
bytes32 correctTypehash = keccak256("SnowmanClaim(address receiver,uint256 amount)");
bytes32 structHash = keccak256(abi.encode(correctTypehash, alice, uint256(1)));
bytes32 correctDigest = keccak256(abi.encodePacked(hex"1901", domainSep, structHash));
​
// The contract's own digest (built from the typo'd typehash) differs:
assertTrue(correctDigest != airdrop.getMessageHash(alice));
​
// So a standard, correctly-produced signature is REJECTED by the contract:
bytes32[] memory emptyProof;
(uint8 v, bytes32 r, bytes32 s) = vm.sign(aliceKey, correctDigest);
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(alice, emptyProof, v, r, s);
}
}
```
​
**What it proves:** a signature created with the canonical EIP-712 type string (what every real wallet/library uses) does not match the contract's digest and is rejected with `SA__InvalidSignature`. The "claim on behalf" feature cannot be used by any standard signer.

Recommended Mitigation

``diff
- bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
+ bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(address receiver,uint256 amount)");
```
​
**Why this fixes it:** the type string now matches the EIP-712 canonical encoding, so the digest computed on-chain equals the one any standard signer produces, and valid signatures verify.
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 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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