Snowman Merkle Airdrop
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Submission Details
Severity: high
Valid

Malformed EIP-712 `MESSAGE_TYPEHASH` breaks signed claims with standard tooling

s4m0y3d

Root + Impact

Description

  • The EIP-712 type hash used to verify claim signatures is built from a malformed type string (a typo plus non-canonical spacing). Any wallet or library that constructs the digest from the correct struct definition produces a different digest, so the contract rejects every standards-compliant signature. The "claim on behalf using v, r, s" feature therefore cannot be used with normal signing tools.

bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
​
​
Two defects in the encodeType string:
1. **Typo:** `addres` instead of `address`.
2. **Non-canonical encoding:** EIP-712 requires no spaces between members — `SnowmanClaim(address receiver,uint256 amount)`.
​
The digest the contract computes in `getMessageHash` / `_hashTypedDataV4` therefore does not match the digest any EIP-712-compliant signer (ethers, viem, MetaMask `signTypedData`) computes from the real struct. `_isValidSignature` recovers a different address and reverts with `SA__InvalidSignature`. The flow only works if the off-chain signer deliberately reproduces this exact buggy type string.

Risk

Likelihood:

  • High (any standard signer hits it).

Impact:

  • Medium (a core advertised feature is broken).

Proof of Concept

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
​
import {Test, console2} 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";
​
/// @notice PoC for M-2: the contract's MESSAGE_TYPEHASH is malformed:
/// keccak256("SnowmanClaim(addres receiver, uint256 amount)")
/// (typo "addres" + non-canonical spaces). A signature produced from the
/// CORRECT, EIP-712-canonical typehash — i.e. what any standard wallet /
/// ethers / viem signTypedData would generate — is therefore rejected.
contract PoC_MalformedTypehash is Test {
Snow snow;
Snowman nft;
SnowmanAirdrop airdrop;
MockWETH weth;
Helper deployer;
​
bytes32 alProofA = 0xf99782cec890699d4947528f9884acaca174602bb028a66d0870534acf241c52;
bytes32 alProofB = 0xbc5a8a0aad4a65155abf53bb707aa6d66b11b220ecb672f7832c05613dba82af;
bytes32 alProofC = 0x971653456742d62534a5d7594745c292dda6a75c69c43a6a6249523f26e0cac1;
bytes32[] AL_PROOF = [alProofA, alProofB, alProofC];
​
address alice;
uint256 alKey;
address satoshi;
​
// The canonical EIP-712 struct type hash a compliant signer would use.
bytes32 constant CORRECT_TYPEHASH = keccak256("SnowmanClaim(address receiver,uint256 amount)");
bytes32 constant DOMAIN_TYPEHASH =
keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)");
​
function setUp() public {
deployer = new Helper();
(airdrop, snow, nft, weth) = deployer.run();
(alice, alKey) = makeAddrAndKey("alice");
satoshi = makeAddr("gas_payer");
}
​
/// Rebuild the EIP-712 digest exactly as a standards-compliant signer would,
/// using the CORRECT type string, against this contract's domain.
function _correctDigest(address receiver, uint256 amount) internal view returns (bytes32) {
bytes32 domainSeparator = keccak256(
abi.encode(
DOMAIN_TYPEHASH,
keccak256(bytes("Snowman Airdrop")), // name set in SnowmanAirdrop constructor
keccak256(bytes("1")), // version
block.chainid,
address(airdrop)
)
);
bytes32 structHash = keccak256(abi.encode(CORRECT_TYPEHASH, receiver, amount));
return keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
}
​
function test_standard_eip712_signature_is_rejected() public {
vm.prank(alice);
snow.approve(address(airdrop), type(uint256).max);
​
uint256 amount = snow.balanceOf(alice); // 1
​
// (A) A correct, EIP-712-canonical signature -> REJECTED by the contract.
bytes32 goodDigest = _correctDigest(alice, amount);
(uint8 v1, bytes32 r1, bytes32 s1) = vm.sign(alKey, goodDigest);
​
vm.prank(satoshi);
vm.expectRevert(SnowmanAirdrop.SA__InvalidSignature.selector);
airdrop.claimSnowman(alice, AL_PROOF, v1, r1, s1);
assertEq(nft.balanceOf(alice), 0, "compliant signature was rejected");
​
// (B) Positive control: only a signature over the contract's OWN buggy
// digest is accepted. The two digests differ purely due to the typehash.
bytes32 buggyDigest = airdrop.getMessageHash(alice);
assertTrue(goodDigest != buggyDigest, "canonical vs malformed digests differ");
​
(uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, buggyDigest);
vm.prank(satoshi);
airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2);
assertEq(nft.balanceOf(alice), 1, "only the malformed-typehash signature works");
​
console2.log("canonical digest :", vm.toString(goodDigest));
console2.log("contract digest :", vm.toString(buggyDigest));
}
}

Recommended Mitigation

Fix the type string so it matches the canonical EIP-712 definition — add the missing `d` and remove the space after the comma:
​
​
- 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 9 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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