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

High: Typo in MESSAGE_TYPEHASH breaks EIP-712 signature verification causing Airdrop DoS

lovelymoon554

Normal behavior requires standard EIP-712 type hash formatting where address is used as the primitive data type for signature verification.
The contract contains a typographical error in MESSAGE_TYPEHASH where addres is written instead of address, creating a type hash mismatch between client signatures and on-chain verification.

Normal behavior requires standard EIP-712 type hash formatting where address is used as the primitive data type for signature verification. The contract contains a typographical error in MESSAGE_TYPEHASH where addres is written instead of address, creating a type hash mismatch between client signatures and on-chain verification.

// Root cause in SnowmanAirdrop.sol:
//@> bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");

Risk

Likelihood:

  • Reason 1 Users generating signatures off-chain via standard SDKs (Ethers.js/Viem) build EIP-712 digests using the canonical address type string.

  • Reason 2 Every execution of claimSnowman validates signatures against getMessageHash(), triggering validation failure on every invocation.

Impact:

  • Impact 1 Complete Denial of Service (DoS) for all eligible airdrop recipients trying to claim via signatures.

  • Impact 2 Inability to execute contract core functionality without updating and redeploying the contract.

Proof of Concept

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
​
import {Test} from "forge-std/Test.sol";
import {SnowmanAirdrop} from "../src/SnowmanAirdrop.sol";
​
contract SnowmanAirdropPoCTest is Test {
SnowmanAirdrop public airdrop;
uint256 internal userPrivateKey = 0xA11CE;
address internal user = vm.addr(userPrivateKey);
​
bytes32 public constant CORRECT_TYPEHASH = keccak256("SnowmanClaim(address receiver,uint256 amount)");
​
function setUp() public {
airdrop = new SnowmanAirdrop(bytes32(0), address(0x1), address(0x2));
}
​
function test_signature_fails_due_to_typo() public view {
uint256 amount = 100 ether;
bytes32 correctStructHash = keccak256(abi.encode(CORRECT_TYPEHASH, user, amount));
bytes32 correctDigest = keccak256(abi.encodePacked("\x19\x01", airdrop.getSnowAddress(), correctStructHash));
bytes32 contractDigest = airdrop.getMessageHash(user);
assertTrue(correctDigest != contractDigest, "Digest mismatch due to typo");
}
}
​
​
PoC Explanation:
1. The test sets up an EIP-712 typed signature off-chain using the correct standard type string ("SnowmanClaim(address receiver,uint256 amount)").
2. It then calls the contract's `getMessageHash()` view function to generate the on-chain expected digest.
3. The assertion proves that `correctDigest` and `contractDigest` do not match. As a result, any valid off-chain signature will be rejected by `_isValidSignature()`, proving the DoS vulnerability.

Recommended Mitigation

- bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(addres receiver, uint256 amount)");
+ bytes32 private constant MESSAGE_TYPEHASH = keccak256("SnowmanClaim(address receiver, uint256 amount)");
​
Mitigation Explanation:
Correct the typo in the string passed to `keccak256` for `MESSAGE_TYPEHASH` by spelling `address` with two 's' characters instead of one (`addres`). This ensures the on-chain struct hash aligns with standard off-chain EIP-712 message signing protocols.
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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