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

Multiple Claims of Snowman NFTs Due to Missing Claim Status Check

istels56

Multiple Claims of Snowman NFTs Due to Missing Claim Status Check

Description

The SnowmanAirdrop contract is designed to allow users to claim Snowman NFTs once, by verifying their Snow token balance against a Merkle tree and EIP-712 signature. After a successful claim, the user's Snow tokens are transferred to the airdrop contract, and the s_hasClaimedSnowman mapping is updated to true.

The specific issue is that while the contract tracks whether an address has claimed (s_hasClaimedSnowman[receiver] = true), it never checks this mapping at the beginning of the claim function. Since the signature and Merkle proof are only tied to the user's address and amount, a user who claimed previously can simply acquire the exact same amount of Snow tokens again and reuse their original valid signature and proof to claim the airdrop a second time.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
​
// ... signature and merkle proof verification ...
​
i_snow.safeTransferFrom(receiver, address(this), amount); // send tokens to contract... akin to burning
​
@> // Missing check: require(!s_hasClaimedSnowman[receiver], "Already claimed");
@> s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);
}

Risk

Likelihood:

  • The s_hasClaimedSnowman mapping is written to, but never read for enforcement, leaving the logical gap explicitly in the code.

  • An attacker only needs to regain the exact balance they had during the first claim to reuse their pre-existing signature and Merkle proof, which requires no special on-chain conditions.

Impact:

  • Users can repeatedly claim Snowman NFTs, completely bypassing the one-claim-per-user intended logic.

  • This leads to the inflation of the Snowman NFT supply and unfair drainage of the airdrop pool.

Proof of Concept

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;
​
import {SnowmanAirdrop} from "./SnowmanAirdrop.sol";
import {Snow} from "./Snow.sol";
​
contract ExploitOP003 {
SnowmanAirdrop public airdrop;
Snow public snow;
​
constructor(address _airdropAddress, address _snowAddress) {
airdrop = SnowmanAirdrop(_airdropAddress);
snow = Snow(_snowAddress);
}
​
// Assuming the attacker (e.g., Alice) has already claimed once with amount X
// She can call this function after obtaining X amount of Snow tokens again
function exploitReclaim(
uint256 amount,
bytes32[] calldata merkleProof,
uint8 v,
bytes32 r,
bytes32 s
) external {
// 1. Attacker ensures they have 'amount' of Snow tokens again (e.g., via buySnow or transfer)
// Note: The attacker must approve the airdrop contract to spend the tokens
// 2. Reusing the EXACT SAME valid signature and proof from the first claim
airdrop.claimSnowman(address(this), merkleProof, v, r, s);
​
// 3. Attacker receives NFTs again, despite already having claimed!
}
}

Recommended Mitigation

Add a check at the beginning of the claimSnowman function to ensure the user has not already claimed their airdrop.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}
​
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SA__AlreadyClaimed(); // Note: Add this custom error to the contract
+ }
​
if (!_isValidSignature(receiver, getMessageHash(receiver), v, r, s)) {
revert SA__InvalidSignature();
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 2 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[L-01] Missing Claim Status Check Allows Multiple Claims in SnowmanAirdrop.sol::claimSnowman

# Root + Impact   **Root:** The [`claimSnowman`](https://github.com/CodeHawks-Contests/2025-06-snowman-merkle-airdrop/blob/b63f391444e69240f176a14a577c78cb85e4cf71/src/SnowmanAirdrop.sol#L44) function updates `s_hasClaimedSnowman[receiver] = true` but never checks if the user has already claimed before processing the claim, allowing users to claim multiple times if they acquire more Snow tokens. **Impact:** Users can bypass the intended one-time airdrop limit by claiming, acquiring more Snow tokens, and claiming again, breaking the airdrop distribution model and allowing unlimited NFT minting for eligible users. ## Description * **Normal Behavior:** Airdrop mechanisms should enforce one claim per eligible user to ensure fair distribution and prevent abuse of the reward system. * **Specific Issue:** The function sets the claim status to true after processing but never validates if `s_hasClaimedSnowman[receiver]` is already true at the beginning, allowing users to claim multiple times as long as they have Snow tokens and valid proofs. ## Risk **Likelihood**: Medium * Users need to acquire additional Snow tokens between claims, which requires time and effort * Users must maintain their merkle proof validity across multiple claims * Attack requires understanding of the missing validation check **Impact**: High * **Airdrop Abuse**: Users can claim far more NFTs than intended by the distribution mechanism * **Unfair Distribution**: Some users receive multiple rewards while others may receive none * **Economic Manipulation**: Breaks the intended scarcity and distribution model of the NFT collection ## Proof of Concept Add the following test to TestSnowMan.t.sol  ```Solidity function testMultipleClaimsAllowed() public { // Alice claims her first NFT vm.prank(alice); snow.approve(address(airdrop), 1); bytes32 aliceDigest = airdrop.getMessageHash(alice); (uint8 v, bytes32 r, bytes32 s) = vm.sign(alKey, aliceDigest); vm.prank(alice); airdrop.claimSnowman(alice, AL_PROOF, v, r, s); assert(nft.balanceOf(alice) == 1); assert(airdrop.getClaimStatus(alice) == true); // Alice acquires more Snow tokens (wait for timer and earn again) vm.warp(block.timestamp + 1 weeks); vm.prank(alice); snow.earnSnow(); // Alice can claim AGAIN with new Snow tokens! vm.prank(alice); snow.approve(address(airdrop), 1); bytes32 aliceDigest2 = airdrop.getMessageHash(alice); (uint8 v2, bytes32 r2, bytes32 s2) = vm.sign(alKey, aliceDigest2); vm.prank(alice); airdrop.claimSnowman(alice, AL_PROOF, v2, r2, s2); // Second claim succeeds! assert(nft.balanceOf(alice) == 2); // Alice now has 2 NFTs } ``` ## Recommended Mitigation **Add a claim status check at the beginning of the function** to prevent users from claiming multiple times. ```diff // Add new error + error SA__AlreadyClaimed(); function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s) external nonReentrant { + if (s_hasClaimedSnowman[receiver]) { + revert SA__AlreadyClaimed(); + } + if (receiver == address(0)) { revert SA__ZeroAddress(); } // Rest of function logic... s_hasClaimedSnowman[receiver] = true; } ```

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