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

Claimed users can reuse their Merkle proof after receiving new Snow

spacerubberducky

Root + Impact

SnowmanAirdrop.claimSnowman() is intended to allow each eligible receiver to claim their Snowman NFT allocation once. The contract even stores claim status in s_hasClaimedSnowman.

The issue is that claimSnowman() sets s_hasClaimedSnowman[receiver] = true, but never checks that mapping before allowing a claim. A receiver who has already claimed can later get the same Snow balance again and reuse the same Merkle proof and signature.

i_snow.safeTransferFrom(receiver, address(this), amount);
​
s_hasClaimedSnowman[receiver] = true;
​
emit SnowmanClaimedSuccessfully(receiver, amount);
​
i_snowman.mintSnowman(receiver, amount);

The getClaimStatus() getter returns true after the first claim, but that status does not prevent another claim.

Risk

Likelihood

Medium. This occurs when a whitelisted receiver claims once, later obtains the same Snow balance again, approves the airdrop contract, and reuses the same proof/signature. The protocol itself allows users to earn Snow weekly during the farming period, so reacquiring Snow is a supported flow.

Impact

Medium. The airdrop is not actually one-time per receiver. A whitelisted user can mint more NFTs than their allocation permits, and the s_hasClaimedSnowman mapping gives a false sense of protection because it is written but never read.

Proof of Concept

Add this test to test/SnowmanFindings.t.sol:

function testClaimStatusDoesNotPreventRepeatClaims() public {
bytes32[] memory proof = _aliceProof();
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
bytes32 digest = airdrop.getMessageHash(alice);
(uint8 v, bytes32 r, bytes32 s) = vm.sign(aliceKey, digest);
​
vm.prank(gasPayer);
airdrop.claimSnowman(alice, proof, v, r, s);
​
assertTrue(airdrop.getClaimStatus(alice));
assertEq(nft.balanceOf(alice), 1);
assertEq(snow.balanceOf(alice), 0);
​
vm.warp(block.timestamp + 1 weeks);
vm.prank(alice);
snow.earnSnow();
​
vm.prank(alice);
snow.approve(address(airdrop), 1);
​
vm.prank(gasPayer);
airdrop.claimSnowman(alice, proof, v, r, s);
​
assertEq(nft.balanceOf(alice), 2);
}

Run:

forge test --match-test testClaimStatusDoesNotPreventRepeatClaims -vv

The test passes. Alice claims once, getClaimStatus(alice) becomes true, Alice earns one new Snow token, and then Alice reuses the same proof/signature to claim a second Snowman NFT.

Recommended Mitigation

Add an SA__AlreadyClaimed() error and check claim status before token transfer and NFT minting.

function claimSnowman(address receiver, bytes32[] calldata merkleProof, uint8 v, bytes32 r, bytes32 s)
external
nonReentrant
{
if (receiver == address(0)) {
revert SA__ZeroAddress();
}
+ if (s_hasClaimedSnowman[receiver]) {
+ revert SA__AlreadyClaimed();
+ }
if (i_snow.balanceOf(receiver) == 0) {
revert SA__ZeroAmount();
}

Also keep setting s_hasClaimedSnowman[receiver] = true before external calls so the state is updated before token transfer and NFT minting.

Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 23 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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